Join Date: Apr 1999
Location: Midland County,TX, USA
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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.
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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.
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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]