What tire size?

[Chillbilly]

I wanted to get back with everyone who responded and say thank you. First, I want to make clear my intentions. I DO want to keep the same size tire as my original equipment, P235/70R17, because of all the reasons mentioned in the responses. My main reason is for stiffer sidewalls. When I put the trailer tongue weight on the truck, and a grill, screen room and chairs in the truck bed, the original tires really squat. After I hook up the equalizer hitch, they look better, but I'm told I should go to a stiffer sidewall. Keep in mind that I am well within the limits of my truck. However, I can't find the same size tire in an LT series. So, tire stores are trying to sell me something compatible. I'm actually getting more confused. I have studied the charts at Tirerack.com. They don't have any LT tires in the 235 size either. Does anyone know who makes an LT235/70R17 tire? Another thing is the load rating. I'm finding it difficult to find anything other than load range E in a 10-ply rating. I know I probably would be better off with a C tire, but can't find any. If anyone can point me in the right direction, I would really, really appreciate it.

[Glenn5995]

As you have found out, as far as I can tell, no one makes a LT235/70R17 size tire. A LT245/70R17 is available from several tire manufacturer's. It will be slightly larger (width & height) than the P235/70R17, but it is about as close in size to the original tire size that you can get in a LT tire.

[Chillbilly]

Hard to believe, but I called my local Ford dealer, Ted Russell Ford in Knoxville, and they stated that they did NOT have an LT equivalent tire for the F150 to replace the P rated tires. They did try to convince me that their P rated tires were actually the same as LT rated tires, and quote "used on SUVs and light trucks". They claim that the "XL" designation for extra load is equivalent to the LT rating. I did some research on the Hankook Dynapro AS tires which came with my F150, and found that many have complained about the weak sidewalls. I have a dilemma which not even Ford can address. I don't want to change tire sizes. If I change to a LT245 as suggested by Glen5995, it will increase my tire sidewall height from 6.487 to 6.763, increasing my tire size by approximately 1/2 inch, which would make my speedometer off by about 2 mph too slow. I can live with that, but I will also have some loss of power as the rear end ratio is slightly changed. Will this be noticeable? What other differences are there?

[Sgt. Schultz]

Things to consider when dropping a chunk of change on new tires:
Can your rims support the PSI required to carry the load with the new tires
Will your TPMS support the tire
Will the change effect your Speedometer or ODO
What is the weight difference for the new tires and what effect will that have on the steering / load cap of the truck (up to 20 lbs/tire)
Are their OEM restrictions that would deter a change in tire size on OEM rims
From what you are saying it would appear that the tire/rim manufacturers see an issue with putting anything greater than an XL tire on your current set up
I suggest looking at moving to the OEM tire/rim combination that is listed for your truck that meet your requirement(s) rather than just trying to putting something on that dosent appear designed for your truck & that could cost you in the long run
Just my 2 cents

[acadianbob]

Originally Posted by Chillbilly

If I change to a LT245 as suggested by Glen5995, it will increase my tire sidewall height from 6.487 to 6.763, increasing my tire size by approximately 1/2 inch, which would make my speedometer off by about 2 mph too slow. I can live with that, but I will also have some loss of power as the rear end ratio is slightly changed. Will this be noticeable? What other differences are there?

I think you are getting it. You are correct that you would see about 2% difference. It would be like your 3.73 rear axle ratio became 3.66. I don't think you would need to worry about that change. If you want LT tires, I think you can do that. 2% of 60 mph is only 1 mph. Not a big deal.

FWIW, I have found a BIG difference in tire brands. My truck came with BG Goodrich; which were fine. I replaced them with Nokian tires. Both are P tires not LT. The Nokians are considerably heavier and stiffer. I only say this as you might find an acceptable P tire. Good luck!

[fajitas21]

I agree with the brands thing. I've got P - metric Eagle LS/2 Tires on my truck and they performed much better than the Dunlops before them did. Not knocking any brand or supporting another, just seems that not all tires are made alike, even within the same classifcation.

In your case, in my opinion, I'd be fine with the 245's and not sweat the speed difference. The additional strain on the truck is extremely minimal as well, so I also would sweat that. It's unlikely to rub with that small a change, so I would sweat that.

I'd make sure you are happy with the tires before doing it, IE reading reviews and whatnot of people with similar setups doing similar things.

I have towed a couple hundred miles with my new truck and these P-metric 275/60/R20 and I was a bit skeptical at first, but they are doing a good enough job for me. I just needed to air them up to around 40PSI (max is 44 for most P tires), and rim can handle I think 50PSI.

I'd sure like to have more plys tho, and will be doing the exact same situation you are doing when the time comes. However, I've pretty much found my replacement tire, the Michelin MS/2 LTX. My father-in-law has been running those for a couple years, and my wife's Equinox have them too (the crossover version), and those are some nice boots on a vehicle.

[Chillbilly]

Thanks again. For better understanding, this is a NEW truck which came with Hankook Dynapro AS P235/70R17 tires, which seem to squat when a load is put on them. I was advised that the Hankooks have very weak sidewalls and they should be replaced with something stiffer. As I understand it, the only way to be guaranteed a stiffer sidewall is to move to an LT rated tire. This is my only requirement - not a larger tire. I don't want to replace the tires for any other reason. I'm really surprised that no one has had this same experience, as the F150 is a popular tow vehicle for light-weight trailers. And, I'm even more surprised that FORD has not addressed the issue. But I'm still looking for answers. If anyone has a recommendation, I would appreciate it. Come on FORD, chime in here with some input !!!!

[fajitas21]

One final thought Chillbilly. My truck is a few months old as well, using P rated tires. The Silverado 1500 is a popular Tow Vehicle as well, and same issue with the P rated tires also.

Have you actually pulled anything yet? Did it not perform up to the task? My thoughts are to save yourself $1000 in tires and just use these till they are done, then get the ones you want. Most tire blowouts are due to wrong pressure (usually running way underinflated), and you're right...it's unlikely F150's would ship with a tire that is known to blow out while towing, but it's not unheard of (Bridgestone Ford Explorers comes to mind)....

In the end tho, I'd LOVE to have the Michelin's I want on my truck right now, but these Eagles have actually done fine, and I need to spend that $1000 is more needed ways.

[MtnGuy]

Originally Posted by Chillbilly

For better understanding, this is a NEW truck which came with Hankook Dynapro AS P235/70R17 tires, which seem to squat when a load is put on them.

Chillbilly, Nankook tires came on my 2006 F150, also. I have the Dynapro AT P235/75R17s instead. I have 30,000 miles on the tires, with about 12,000 miles of that towing my Trailmanor and Surveyor. The tires have held up well, and still have a lot of tread left. I plan on keeping these tires until they wear out, then put LT tires on my truck. My trailer tows sweet, although I am hoping that the slightly stiffer sidewall on LT tires will make my trailer tow even better.

Like fajitas21 suggested, try your Nankooks and see how they do. I am not 1 to spend $1000 to replace something that is not worn out and works reasonable well.

[Chillbilly]

Yes, I have towed my Flagstaff 829FKSS travel trailer half a dozen times over the past year. It tows great and the F150 handles it superbly. I just don't like the look of the tires when a load is put on them - they squat to the point of looking under pressure. Because of the way they look, I make absolutely sure the pressure is correct every time. Many have told me that it is natural for them to look this way. I have no complaints except for the way they look. But I am getting ready to take a 3800 mile, 3 month long trip and I would like to be more confident in the tires. Maybe I should forget about the look and wait for them to wear out. I'm just astonished that there are no LT rated replacement tires in this size. I think FORD could have done better engineering and planning. A Ford service manager told me that the F150 came with P rated tires because it wasn't really meant to do any heavy work. And, he would not recommend a replacement tire size because he (Ford) could not take responsibility for the change in the speedometer, etc. This makes no sense what-so-ever to me. It's a TRUCK with the largest towing capability and payload for this size vehicle of all 3 major brands. What were they thinking? Thanks to all who have responded. I do take all of your comments into consideration as I know that experience is the best medicine. I hope to see you on the road.

[Glenn5995]

Chillbilly,

Here is another option for you to consider. The Hankook Dynapro AS P235/70R17 XL has an advertised diameter of 30.0". If you don't mind going to an All Terrain tire, BFGoodrich makes their All Terrain T/A KO in a LT225/70R17, which has an advertised diameter of 29.6". It is a LT tire and the BFG All Terrain KOs have a 3-ply sidewall.

It would be slightly smaller in width and diameter than your current tires, but by going smaller, it would help (slightly - probably not noticeable) your towing performance.

Just another option.

Personally, before spending money on new tires, I would load it up like you would for regular camping trip and then take it to the scales. As long as the weights are within the capacity of the tires and it tows well, I would just make sure they are properly inflated for the weight, wear them out and then buy what you want.

Good luck with whatever you decide.

[Chillbilly]

That's really an aggressive looking tire. I think I would prefer more of a street tire. Thanks for the input. By the way, I lived in Glendale for about 8 years and loved Arizona!

[Chillbilly]

I wanted to end this thread by including the final response from Ford.

"Thank you for taking the time to contact us with your inquiry. Please be advised that on pages 31-32 of the Owner Guide, states that:

Your vehicle is equipped with tires designed to provide a safe ride and handling capability.

'Only use replacement tires and wheels that are the same size, load index, speed rating and type (such as P-metric versus LT-metric or all-season versus all-terrain) as those originally provided by Ford. The recommended tire and wheel size may be found on either the Safety Compliance Certification Label or the Tire Label which is located on the B-Pillar or edge of the driver’s door. If this information is not found on these labels then you should contact your authorized dealer as soon as possible. Use of any tire or wheel not recommended by Ford can affect the safety and performance of your vehicle, which could result in an increased risk of loss of vehicle control, vehicle rollover, personal injury and death. Additionally the use of non-recommended tires and wheels could cause steering, suspension, axle or transfer case/power transfer unit failure."

I'm now trying to find the most recommended P rated tire with the XL (eXtra Load) designation.

[acadianbob]

Pay attention the the Load Index. I don't believe it is the same for all tires of the same size. I would go for a tire with the best Load Index; as long as you like the tire/dealer/etc.

[fajitas21]

Good info, thanks for the follow up ChillBilly, this has been an informative thread.

[Chillbilly]

Doing my research, I came across an article which I just have to pass on. It is long, but it contains a wealth of information.

Tires for Towable RVs and Tow Vehicles

by Warren Petkovsek
Let’s talk about tires today. Tires are not only essential for any vehicle to operate on or off the road; they are also the most critical safety concern for any motorist including and especially RVers. The scope of this article will be limited to towable RVs and tow vehicles and will not include motor homes. That could easily be a complete article so we’ll save that for another day.
Many serious and even fatal accidents have been directly attributed to catastrophic and sudden tire failure otherwise known as blowouts. Even tire failures that didn’t result in accidents have caused a lot of damage and ruined more than one RV vacation. You owe it to yourself and your family to prevent tire failures. You can easily accomplish this by educating yourself and becoming a “tire expert” like me. Well, like it or not, school’s in session. Let’s make “tire class” as informative and painless as possible. Heck, it may even be fun. Here we go.
Proper tire inflation is critical to the performance and safety of every vehicle, especially RVs. Sometimes we think, “If a little is good, then a lot is better”. This is not true with regard to tire pressure. If we overinflate our tires the tread will crown slightly in the center and will wear out quicker there. This crowning will also reduce the contact patch or the amount of rubber on the road and result in less wet traction; a very dangerous condition. Overinflated tires also cause the vehicle to ride much rougher and who wants that?
Underinflated tires are also dangerous because they run hotter due to increased sidewall flex and friction due to rolling resistance. The rolling resistance also reduces fuel mileage. Under inflation is also a major cause of tire failure. The tire doesn’t actually carry the load; the air inside the tire does. Let’s talk some more about a tire’s load capacity.
There’s a lot more to selecting replacement tires than looking at the size stamped on the sidewall of the old tires. It is vital that we match the tire with the weight it has to carry. Let’s suppose that the rear tires of your tow vehicle weigh 3,200 pounds each when loaded up to travel and the trailer is loaded and hitched. You would only know this if you weigh each wheel of your trailer and tow vehicle. You really need to do this! If you have single rear wheels and load range “E” tires then the tires can carry that weight if inflated to the recommended 80 psi air pressure. But what if your rear axle weighs only 4,400 pounds or 2,200 pounds per wheel? Then your “E” rated tires can carry that load at 50 psi and not be overloaded or underinflated. If this is your normal load you may want to use “D” rated tires which can carry 2,800 – 3,000 pounds at their maximum recommended pressure of 65 psi. If your vehicle came equipped with “D” rated tires and you want to increase your safety margin by upgrading to “E” rated tires that’s OK too. Just check with your tire professional to be sure that your wheels can handle the additional load and air pressure.
Tires come in all sizes tread designs, methods of construction and load carrying capacities and we need to know the proper tire for both our trailers and our tow vehicles. With regard to size and tread design – some folks purchase tires with regard to nothing but appearance. Let’s look at the big, radically lifted 4X4 with huge tires that have an aggressive mud tread. Chances are that this is a show truck that will never leave the pavement, but I digress. Let’s stay on the subject of tires.
Oversize tires with a larger than stock diameter will change the overall gearing of your tow vehicle and adversely affect its towing performance. Many off-roaders use larger diameter tires to better navigate over rocks and obstructions, but anyone that uses larger diameter tires must also change the gear ratios in their truck’s axles in order to maintain proper gearing and performance of their trucks. Larger tires may also cause clearance problems thus restricting steering and even suspension travel on your tow vehicle.
Generally speaking, you just can’t go wrong by replacing tires with the original equipment size tire that came from the factory on your vehicle or trailer. You can find the recommended tire size and inflation pressure for a vehicle on a sticker located on the driver’s side door jamb. On a towable RV this sticker is located outside on the left side near the front. Oh, you can put a slightly wider tire on your tow vehicle as long as you keep the original tire diameter. Example: You can replace a tire size P235/75 (any wheel diameter) with a size P255/70 tire. The new tire will only be 70% as tall as it is wide, but it is also slightly larger so things even out and the tire diameter stays the same. If you have any doubts about this be sure to speak to your tire professional before you make a decision. This tire width upgrade only applies to tow vehicles. There is no reason to install larger or wider tires on any trailer. That would serve no purpose.
Let’s talk about tread patterns. Very aggressive or knobby mud treads are OK if you need them, but in most cases that is rare indeed. These types of treads are noisy on the road, they don’t generally wear as long as less aggressive treads and they will also reduce your fuel mileage. That noise you hear is the knobby tread making impact with the pavement which equates to rolling resistance which, in turn, lowers fuel mileage. The large void spaces in the knobby tread also reduce the contact patch or the amount of rubber in contact with the road which degrades wet on-road traction and accelerates tread wear.
Highway treads are much less aggressive than knobby mud treads and they more resemble the tires found on passenger cars. The advantages of this type of tread are longer tread life, better fuel economy and a larger contact patch. This is the type of tire tread found on eighteen-wheelers because these trucks will (realistically) never leave the pavement. If you operate your vehicle solely on paved roads then this is the best and safest tire for you. This is also the only tread design that is suitable for your trailer. A more aggressive tread on a trailer would serve no practical purpose.
The final type of tread design is the all terrain tread. This is a good compromise between aggressive treads and highway treads. This design provides good wet traction on paved roads, they look good and they provide enough off-road traction for most of us. Actually, if you need more than an all-terrain tire for off-road traction then you better have a winch on the front of your truck.
Regarding the fancy aftermarket wheels or “mags”; I’m generally opposed to them. My concern is that the aftermarket wheel may not be rated for the same load or air pressure as your stock wheels. If you’ve just got to change wheels then make sure the tire guy proves to you that the new wheels are as adequate as the old ones.
Many towable RVs come equipped with ST or “special trailer” tires. These tires are to be used on trailers and never on anything else. They generally have a lower speed rating than regular tires, but the rubber compound in ST tires is designed to retard deterioration of the rubber due to ultraviolet light. I’ve used these before with good results, but I’ve done a lot of research and I believe that a LT or “light truck” tire could accomplish the same thing by carrying the same load, lasting just as long and be more readily available at tire stores. My new fifth wheel weighs 16,000 pounds and it came equipped with LT tires so I guess I’ll have the opportunity to prove out my research. I’m very confident in this.
Earlier I mentioned that ultraviolet sunlight is harmful to the rubber compounds in tires. This is much more critical for our trailers than with our cars and tow vehicles. The reason is that trailers are stored for extended periods and the tires are exposed to sunlight for quite some time. The vehicles that we drive are used often enough that the tires roll and thus warm up on a regular basis releasing anti-aging chemicals in the rubber. This is why we should cover our trailer tires to protect them from sunlight whenever the rig is parked. I even cover mine in a campsite. There are tire covers available at RV dealers and specialty RV stores such as Camping World and they are cheaper and easier to store and carry than, say, plywood.
As if we weren’t already in “information overload”, there is even more vital data that is molded into the sidewalls of tires. As I mentioned in a previous article, tires have a shelf life. Statistics show that most tire failures occur during the sixth year of tire age – NOT service. If you have tires that are over six years old then you are on borrowed time. The bad part is that the tires you buy may have been sitting on a shelf or in some warehouse for several years and then they are sold as new tires to the unsuspecting public. We can avoid this by checking the manufacture date of the tire. The last four numbers of the DOT code that is molded on the tire’s sidewall will tell us this date. My new fifth-wheel came with tires that were marked “3107”. This means that the tires were manufactured during the thirty-first week of 2007 or in June of the year that the trailer was delivered to me. That’s good! If any new vehicle that you want to buy has tires that are more than three years old, I believe that you should insist on the dealer either replacing the tires or giving you a tire allowance so that you can have them replaced. I mean, why give up half of your tire life? I’ll elaborate with some examples.
I was told all this information about tire age years ago, but I was somewhat skeptical. Even after eight years of use the tires on my first travel trailer still looked new. I carefully checked air pressure before and sometimes during each trip and I weighed each wheel to ensure that the tires weren’t overloaded. But one of them did fail and the blowout was truly spectacular. I just happened to be looking in the mirror when the unfortunate event occurred and there was no doubt that the tire was holding air at the time. I know this because the explosion was truly of epic proportions. I was very lucky that the disintegrating tire didn’t cause damage to the trailer which is often the case with RVs. Two other RVing couples that are both friends of ours each bought used fifth-wheels and both experienced multiple blowouts; one on their first trip and the other shortly after. They had no clue how old the tires were or even that tire age mattered. The really bad thing about multiple tire failures on the road is that you have to replace them with what is available wherever you are. You may not have many choices and you may not get the price you want not to mention the inconvenience.
‘One more thing. We all have to level our rigs in campsites by driving up on boards. It is very important that the boards are wide enough to support the entire tread of the tire. This means that none of the tire tread can hang over the edge of the board. If any of the tire hangs off of the board damage to the belts may occur. Remember, there is a lot of weight on the tires and that can cause damage. I use 2” X 12” boards for this purpose and my tires are always completely supported.
In conclusion, If you or anyone you know has ever experienced a blowout then one or more of the following will be true: (1) The tow vehicle or trailer may have been purchased used and the owner has no idea of the chronological age of the tires. (2) The owner has no idea if the tire is properly inflated and (3) he has no idea how much weight the tire was carrying. Remember, an overloaded, over age or underinflated tire is just a blowout waiting to happen. Please remember to weigh your rig, check your tires and be safe on the road.
Happy trails!

[dstars99]

For what it is worth:
You can go to a 255/65/17 and be within .1 inch in tire diameter. Quick search shows both cooper and kuhmo both make load range c tires.

[Chillbilly]

Yes, I have been thinking of that size. The extra 3/4 inch width shouldn't make a difference, except for a little more weight. I'll look at the Coopers and Kumos. Thanks.

[Chillbilly]

I came up with a chart to help me determine which tire sizes I could substitute for my original equipment tires. Consider my tire size of P235/70R17. The first 3 numbers are the section width in millimeters, the width of the tire. Column A of the chart lists some of the more popular tire widths made. I have converted the millimeters to inches in the second column because it is easier for me to visualize. The next 2 numbers of the tire size are the profile or aspect ratio, the tires sidewall height as a percentage of the width. In other words, using my tire size above, the sidewall height is 70% of 235. This tells me that it has a sidewall height of 164.5 millimeters, or 6.476 inches. Row 1 of the chart lists the more popular tire profiles. The last 2 numbers in the tire size are the rim diameter size in inches. Why one part of the tire size is in millimeters and another in inches is questionable. The rest of the chart is a listing of the sidewall heights of the different combinations of tires. The diameter of my tire can be obtained by adding the rim diameter size (17) and 2 tire sidewall heights (6.476 plus 6.476). This gives a tire diameter of 29.95 inches. My tire is specified as being 30 inches in diameter. If I wanted to find an equivalent tire size, I simply need to find another size which has the closest sidewall height. Looking at the chart, I have a couple to pick from; a 205/80 and a 275/60. The 205/80 would be slightly smaller in diameter (6.457 vs. 6.476), less than 1/16 inch. The 275/60 would be slightly larger in diameter (6.496 vs. 6.476), less than 1/16 inch. However, the 205/80 would only be 8.071 inches wide compared to my original 9.252 inches wide tire. The 275/60 is wider than my original tire, 10.827 inches. So, one tire is 1.181 (approx. 1-3/16) thinner and the other is 1.575 (approx. 1-9/16) wider. Now, I have to make a determination as to which I would prefer. The wider tire would give me more surface traction area, but would be slightly heavier, and I would need to check for clearance. A more important factor is the load range. The substitute tire MUST have an equal or greater load range as the original tire. This information can also be found on the tire. The higher the tire's load index number, the greater its load carrying capacity. I hope this chart can help you to determine what tire sizes you can use. Chart is in my next posting. You may have to reformat the column spacing. Or email me and I will send you the spreadsheet.

[Chillbilly]

W inches 45 50 55 60 65 70 75 80 85 90
145 5.709 2.569 2.855 3.140 3.425 3.711 3.996 4.282 4.567 4.853 5.138
155 6.102 2.746 3.051 3.356 3.661 3.966 4.271 4.577 4.882 5.187 5.492
165 6.496 2.923 3.248 3.573 3.898 4.222 4.547 4.872 5.197 5.522 5.846
175 6.890 3.101 3.445 3.790 4.134 4.479 4.823 5.168 5.512 5.857 6.201
185 7.283 3.277 3.642 4.006 4.370 4.734 5.098 5.462 5.826 6.191 6.555
195 7.677 3.455 3.839 4.222 4.606 4.990 5.374 5.758 6.142 6.525 6.909
205 8.071 3.632 4.036 4.439 4.843 5.246 5.650 6.053 6.457 6.860 7.264
215 8.465 3.809 4.233 4.656 5.079 5.502 5.926 6.349 6.772 7.195 7.619
225 8.858 3.986 4.429 4.872 5.315 5.758 6.201 6.644 7.086 7.529 7.972
235 9.252 4.163 4.626 5.089 5.551 6.014 6.476 6.939 7.402 7.864 8.327
245 9.646 4.341 4.823 5.305 5.788 6.270 6.752 7.235 7.717 8.199 8.681
255 10.039 4.518 5.020 5.521 6.023 6.525 7.027 7.529 8.031 8.533 9.035
265 10.433 4.695 5.217 5.738 6.260 6.781 7.303 7.825 8.346 8.868 9.390
275 10.827 4.872 5.414 5.955 6.496 7.038 7.579 8.120 8.662 9.203 9.744
285 11.220 5.049 5.610 6.171 6.732 7.293 7.854 8.415 8.976 9.537 10.098
295 11.614 5.226 5.807 6.388 6.968 7.549 8.130 8.711 9.291 9.872 10.453
305 12.008 5.404 6.004 6.604 7.205 7.805 8.406 9.006 9.606 10.207 10.807