Join Date: Oct 2003
Bronco Info: 1995 F150 SAS 35s
I wish that people still read this - we will see I guess. I was reading the initial post and also I checked a few of the calculators out. There is one large flaw that I see. It seems everyone is using their nominal tire height to base these numbers. That is not the actual height of the tire, thus the circumference is not accurate. The number that needs to be used is tire revs per mile, which can be calculated from rollout. Using the numbers from the first post, a stock tire turns 698 times per mile, when tire specs for a 235-75-15 usually hover more around 715. Also, 35 in the first post turned 576 times per mile, while manufacturer specs are normally right at 600. If you notice, the error in those two numbers goes in the opposite direction. By the way, to deduce numbers from the first post numbers I multiplied inches traveled x gear ratio / 12 (to get into feet) and then divided 5280 by that number.
Now if you use rollout/rev per mile the numbers come out quite different.
3.55s and 235s = 24.96 inches per turn of the driveshaft.
4.10 and "35"s = 25.75 inches per turn of the driveshaft
so while yes, this does mean that 4.10s and 35s achieve slightly more ovedrive than stock, it is VERY slight. Changing the overdrive from .71 to .70 would make more difference (25.24) A "normal" 35 inch tire with 4.10s is the closest you can get to stock. This does not take into account additional rotational mass or additional air resistance. I suppose if you wanted to gear lower than stock (similar to going to 3.73s with stock tires) you would want 4.56s (or don't forget the 4.27 ratio). Also, if you call 31s stock, then 4.10s and 35s would leave you geared slightly deeper than you were.
So in my case, with a 35 and 4.10s, at 60 mph, I would be turning right at 1750 rpms.
(1750 turns of the motor will turn my dshaft 2460 times (.71 OD in e40d) which with 4.10 gears will turn my wheels 600 times (2460/4.10) and 600 turns of my 35 = one mile.)
All this stems from the belief that many on this site seem to have that 4.56 and 35s = stock. I am very aware that it is not a bad combo that gives good power and might even affect mpg around town, but it is not stock. I am happy power wise with my truck with 35s and stock gears, so the boost from 4.10s = bonus. On the highway, you will be hard pressed to convince me that 4.56 will get better fuel mileage than 4.10 (with 35s) The engine (my engine in my truck anyways) has adequate power, so 4.56s would just turn higher rpms, which ask any over the road trucker = lower mpgs. Also at 70 and 75 with my setup you would be looking at just over 2000 and just under 2200 rpms, respectively. Sorry for the long windedness, I just hate seeing inaccurate info being regurgitated as the gospel truth.