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Old 03-19-2010, 12:29 PM   #1
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LT vs P-Metric

I got into a friend's F150 (virutally identical to mine) the other day and immediately noticed a "firmer" ride. Since the trucks are essentially the same, I asked whether he had LT tires on the truck. Answer was yes. So, an immediately noticable difference in ride quality; as in not as good as the P-Metrics on my truck. I have been thinking of changing to LTs when my Rugged Trails wear out (50M now and expect at least 60M miles). Those of you who have made this switch, do you notice better stability when towing? That would be my motivation for switching. We are towing a TT that weighs just over 5,000 pounds and a tongue weight of maybe 550. Would you make the change from P to LT?
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Old 03-19-2010, 12:48 PM   #2
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And here is another thought? What about Load Range C rather than E? It looks to be perhaps a compromise in the middle between P-Metric and Load Range E.
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Old 03-19-2010, 01:08 PM   #3
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And here is another thought? What about Load Range C rather than E? It looks to be perhaps a compromise in the middle between P-Metric and Load Range E.
Bob, P-metric refers to the tire size not the load rating. If you are running Range C now and go up to D or E you will probably notice a big difference in handling while towing, and I mean better. Yeah the ride is stiffer when running around town. My friend at work made the switch on his Chevy truck for towing his boat and couldn't belive the improved handling.
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Old 03-19-2010, 04:11 PM   #4
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Jeeper, I think you answered my question about the benefit. I have P275/65R18 SL's on there right now; only rated to 35 PSI. LT275/65R18 Load Range C would give me 50 PSI. And LT275/65R18 Load Range E would go up to 80 PSI. It seems that either of the LT's are likely to provide more stability under towing conditions because of the stiffer sidewall and increased pressure. Thanks for the feedback on your friends truck.
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Old 03-19-2010, 04:15 PM   #5
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So, Jeeper, you are right in that I mistakenly equated P-Metric with SL load rating (although I think almost all P-Metric tires are SL, some are XL). I really should have been comparing Load Ratings SL, C, and E.
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Old 03-19-2010, 05:43 PM   #6
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I have also been contemplating changing the 275/60/20's that came on my 05 Ram1500. The load rating on these is 2601 lbs @ 44 psi verses the GAWR of 3900lbs for the truck (133% of the axle rating). I haven't seen a tire load rating chart, but I'm guessing that a 20" LT tire is probably rated aroung 3500 lbs @ 80 psi. At what point does it become overkill?

I was also considering changing out to 17" LT tires and wheels as they're a little smaller diameter which would increase the rear ratio a bit which would be a good thing.

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Old 03-19-2010, 07:23 PM   #7
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I have also been contemplating changing the 275/60/20's that came on my 05 Ram1500. The load rating on these is 2601 lbs @ 44 psi verses the GAWR of 3900lbs for the truck (133% of the axle rating). I haven't seen a tire load rating chart, but I'm guessing that a 20" LT tire is probably rated aroung 3500 lbs @ 80 psi. At what point does it become overkill?

I was also considering changing out to 17" LT tires and wheels as they're a little smaller diameter which would increase the rear ratio a bit which would be a good thing.

Dave
Dave, 17" is rim size not tire size. Your rolling diameter would remain close to the same and therfore would not have much effect as related to gear ratio. What you would be doing is increasing your sidewall height. Personally I am not crazy about anything over 17" rim size on a vehicle especially one used for towing for many reasons. I think you would be much better served with the 17" rims. The 20" tires and rims are made to "look" good not perform as a truck tire.

Bob, Class C rating is really a "car" tire and they have lighter sidewalls for a smoother ride. This is one reason I don't really like SUVs for towing as the designers goal with an SUV is to carry a family in "car like" comfort and to do so they put the lesser load rating tires on, and put shocks and springs with different valving and rates under them than you will find in a truck. Not to say that many SUVs don't have the power but people towing with them really need to make sure they change their tires up to a Class D or even E for the stiffer sidewalls and make sure their shocks and springs are up to the task. In many cases a set of the Firestone air bags can make a big difference, but I digress.

If you move up to a stiffer sidewall be aware that you only need to inflate them to the amount of load so even with a Class E that may be rated for 80lbs you may only need 55 to carry your load and thus you won't have such a harsh ride, you will however still have the stiffer sidewall and carcas which only improves your stability in towing.

With all that said I should probably change the tires on our Liberty for towing our boat now.....
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Old 03-19-2010, 09:03 PM   #8
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If your truck came with load range "C" tires and you are planning to change out to load range "E" Make sure your rims are compatible. load range "E" are rated for 10,000 and are 10 ply tires and have an 80 lbs max pressure rating, the valve steams should be changed as will. I believe at an 80 psi rating they are concidered high pressure. Standard valve steams will not hold up to 80 psi. On 3/4 ton trucks a load range "E" is standard equipment. A load range "E" tire is the best way to go when towing large campers, control is all part of handling...trust me I know....
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Old 03-19-2010, 10:18 PM   #9
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Dave, 17" is rim size not tire size.
Jeeper, you're kidding me, right?

The tire circumference of a LT 265/70R17E tire is less than the circumference of the P275/60/20 tires that I have now. It would be the same thing as changing the rear ratio from the current 3.92 to a 4.10 ratio.

I believe that the Ram tow package includes 3.55 ratio with the 17" tires and a 3.92 ratio with the 20" tires. The final ratio, and engine RPM are the same.

By the way, the F250/350's have an option for 275/65SR20E tires. You need something to fill those huge tire openings in the fenders they give you now!

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Old 03-19-2010, 10:34 PM   #10
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I have an Excel spreadsheet that calculates the circumference of a tire. All you have to do is enter the size. I use it when I am changing size/rim on a vehicle to assess the difference in circumference. For example, my VW has 17" summer tires and 16" winter tires. the winter tires are narrower but have a higher profile so the circumference is the same. If anyone wants a copy of this spreadsheet, PM me with your email address and I'll send it to you.

Dave, I agree with Jeeper that 17 or 18 inch tires are much better for load than a 20. Depending on the size and aspect ratio of the 17" tire, you may very well have a smaller circumference with a 17. Be aware that this will affect your speedometer and odometer accuracy. This can often be corrected by purchasing an "electronic tuner" than can adjust and compensate for the difference in diameter.
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