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Old 03-19-2010, 10:38 PM   #11
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Dave, you are exactly right. Just put your 2 different tire sizes into my spreadsheet, the 17" size is -4.2% smaller than your 20". Your rear axle ratio "change" is exactly right too. 3.92 becomes 4.10.
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Old 03-19-2010, 10:51 PM   #12
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I agree with Jeeper that 17 or 18 inch tires are much better for load than a 20.

Sorry guys, but I can't agree. All of them will have the same load rating within a 100 lbs, but the 20's will have the shorter, stiffer sidewall and will have the largest contact patch. The biggest deterent is the cost!

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Old 03-19-2010, 11:29 PM   #13
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Until you hit a pothole or road hazard and the short sidewall cannot protect your rim. Ruined tire, bent rim, . . . .
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Old 03-19-2010, 11:51 PM   #14
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Those 20" E rated will still have a 5" sidewall and at 80 psi, the wheel will be safe. A lot more other things may hurt!

Now, as for the wheels with 235/45r18's on our Mazda 6, they would be a little less round!

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Old 03-19-2010, 11:57 PM   #15
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In Ford's '09 Towing Guide they state, "Reduce GCWR and Maximum Towing Weight by 500 lbs. on models with 18" or 20" wheels."
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Old 03-19-2010, 11:59 PM   #16
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Until you hit a pothole or road hazard and the short sidewall cannot protect your rim. Ruined tire, bent rim, . . . .
You called this one right Bob.

This is a good reason why you will never see 20" rims on offroad vehicles unless they are running 40" tires. You want that sidewall. Also "low profile" rims and tires are worthless on any vehicle that is 4-wheel drive and really negate the 4x4 altogether. I saw lots of 4x4 SUVs in the ditch last winter in the snow(one on our hill in our neighborhood, the only way in or out) with the low profile tires, while other vehicles had no problems.

Also with many of the low profile rims you cannot use tire chains. Forbidden by the manufacurers of the wheels.

I don't see any semis or trucks used for heavy duty use running down the road with low profile tires. There is a reason for that. Besides the cost
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Old 03-20-2010, 12:02 AM   #17
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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!

Dave
Dave what I was refering to is the "17" and the "20" at the end of the size is the size Rim that the tires fit.... Reguardless of the rim size you need to buy the correct size tire. Yes you can change sizes and that does change the Ratio but it is not preferable to actually changing the gearing as you end up out of the power band of the engine. Depending on which way you go, you either loose gas mileage or power.

It is a HUGE problem with the Jeep crowd who is always trying to stuff bigger tires under their rigs but then are bummed that they have lost so much horse power. They then try stupid stop gap measures to try and get it back like cold air intake kits and re-chiping the computer when what they should be doing is re-gearing the differentials.
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Old 03-21-2010, 09:12 AM   #18
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Why would one change what the manufacture has design the truck to do with the equipment that came on it, why not buy truck with the correct equipment that is needed to do the job.
I bought a 2008 Chevy pick new, it is a heavy duty 1/2 ton rated @ 10,500 lbs. The frame on the truck is nothing like the standard 1/2 ton and nothing like the 3/4 ton it has it's own design.
The truck came equip with:
6.0 gas engine
heavy duty trans (4) speed
4:10 rear
tires: P265/70R17 standard load range, Goodyear Wrangler AT/S - 2,535 lbs
Heavy duty tow package
factory brake controller
At the dealer I question the tires that came on it from the factory, they the dealer call GM and as the question to make sure that it had the correct tires on it for the trucks weight rating, and they are the correct ones.
Now it was suggested to me by the mechanics that when these tires wear out to have load range "E" installed, But they did say to install the same size tire but in a 10 ply. This would give it a stiffer ride but better control.
I must say I have not any problems so far, but I am going to change the tires to load range "E"
As far as large rims and low profile tires, they look ok on Honda's and golf carts but when it comes to a truck that has to do work like towing you could be asking for trouble, the lower the beed of the tire sets to the ground and you hit a pothole the more of a chance the beed will break away from the rim there is no room for the side walls to absorb the shock and very little flexability when you are carrying heavy loads, so something has to give. On a personnal note 20" feet as they are called on the street with low profile tires is called pimped out and that's fine if you like the look. As stated above put the correct tires on, this way you will not change what the truck was design to do......Just my view.
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Old 03-21-2010, 10:15 AM   #19
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Guys, I'm referring to factory optioned larger tire/wheel packages. Although the aspect ratios are lower (60 or 65 instead to 70) they are also a little wider therefore the sidewall height is about the same as the factory 17" tire. I wasn't talking about larger diameter rims with 35 or 40 series tires on them!

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Old 03-21-2010, 10:28 AM   #20
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The way I see it.

Low profile tire have the advantage that there is less side wall flex per number of plies. That is to say the side to side flex, squirm, of the tire is less than a high profile tire. All this makes the steering feel more connected to the road, more sporty if you will. The low number of plies will make for a smoother ride and a more car like feel and more vulnerable to potholes. All this speaks against stability when towing as we want to reduce the squirm to make the rig track better. Less side to side.

My truck will spend 90% of it's mileage NOT towing so I am happy to have a more car like ride. Ideally I should have a different set of wheels/tires for towing than for my daily ride. If I were a contractor hauling stuff and navigating construction sites I would rather have "truck" tires. So in summary, I'll keep my 20" wheels and my car tires for the commute and if the fancy strikes me and I have a spare $1000 dollars I'll get my towing wheels & tires in addition to my snows and my summers.
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