Wheel size and gearing - Diesel Forum - TheDieselStop.com
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post #1 of 2 (permalink) Old 08-03-2005, 02:33 AM Thread Starter
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Wheel size and gearing

I am considering a F250 CC SRW SB 4x2 or 4x4 and was wondering how big an effect the different 17, 18, and 20 inch wheel sizes has on the actual gearing of the trucks (rpm to mph)?

The rear end choices are 3.73, 4.10, and 4.30, and if my calculations are correct, going from 3.73 to 4.10 is about a 9% lowering of the gearing.

The wheel/tire choices are as follows for the 4x2 and/or 4x4, with the tire diameter and percent change shown (maybe I need to use the circumference instead, but you get the idea):
LT265/70Rx17 31.6 = stock tire size on F250 4x2 and 4x4.
LT275/65Rx18 32.1 1.5%
LT275/70Rx18 33.2 4.9%
LT275/65Rx20 34.1 7.8%

So, if my calculations are correct, it would seem that if you get the 20 inch wheel option you end up with a 7.8% higher gearing than with the base 17 inch wheels, which would not be good for towing as it would be like having 3.44 gearing with the 17 inch wheels (i.e. less rpm at any given mph if in same gear), and you probably should get the 4.10 rear which would lower the gearing back down to around the equivalent of 3.78 gearing with the 17 inch wheels.

Even the LT275/70Rx18 tires have an issue like this with the 4.9% increased ratio which would be equivalent to running a 3.55 rear with the 17 inch wheels.

If I knew the transmission gear ratios I could figure the actual rpm at a given gear and speed and then figure the rpm change with the different size wheels. But just to grab some random numbers, if you are doing 2000 rpm at 60 mph with the 17 inch wheels, and then put on the 20 inch wheels, and my assumptions are correct, then the rpm should drop by 9% or 180 rpm to 1820 rpm. Not a big deal I guess, just need to make sure the speedometer and odometer are recalibrated.

Any comments?

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post #2 of 2 (permalink) Old 08-03-2005, 03:28 PM
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Re: Wheel size and gearing

Assumption: You want to order a new truck, and you want to know which factory options to order.

The rear end choices are 3.73, 4.10, and 4.30, and if my calculations are correct, going from 3.73 to 4.10 is about a 9% lowering of the gearing.

[/ QUOTE ]

The only rear end ratio available on a 2006 F-250 or F-350 SRW PSD is 3.73.

If you go with the Dooley with a PSD, then you have the choice of 3.73, 4.10, or 4.30 rear end ratio, but the only tire size available is LT245/75R17.

The math is fairly easy if you convert all tires to tire revs/mile. Then the percentage difference in the revs/mile will be the percentage difference in the speedo, odo, tripmeter, and "feels like" rear end ratio.

LT245/75R17 = 664 tire revs/mile
LT265/70R17 = 662
LT275/65R18 = 652
LT275/70R18 = 627
LT275/65R20 = 605

So going from the stock LT245/75R17 to the optional LT275/65R20 means:

664 minus 605 = 59
divided by 664 = 8.9 percent

So assuming your speedo/odo is accurate with the factory setting, your engine RPM will be 8.9 percent less at the same speed with the tall tires than with the stock tires. Or your actual speed will be 8.9 percent faster at the same engine RPM. And your rear end ratio will "feel like" 8.9 percent longer legs.

So your 3.73 with the 20" wheels will feel like a 3.40 (3.73 minus 8.9 percent). Not as good for towing, but better for unloaded cruising. Apparently, Ford thinks if you opt for the bling bling of the 20" wheels, you care more about looks and image than about getting any work done. So they make the bling bling option available. [img]/ubbthreads/images/graemlins/shocked.gif[/img]

The tranny ratio in overdrive is 0.71 with the automagic tranny, or 0.72 with the hand shaker. If you use the revs/mile trick, you don't care about that trivia. [img]/ubbthreads/images/graemlins/warmsmile.gif[/img]
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