Trailmaster Install
BF Guru
Joined: Sep 2009
Posts: 5,143
From: GTA, Ontario, CANADA
regardless of what speed you're gonna be driving @, you still need to change the gear ratio from factory, 'cause those 3.42's or 3.73's (depending on what option ordered) is not going to let the engine spool up to it's power band easily enough.
i have a formula that is used to determine what gears should be selected, depending on tire diameter, cruising speed, and engine RPM.
......................where did i put it now?........................gonna see if i can find it.
i have a formula that is used to determine what gears should be selected, depending on tire diameter, cruising speed, and engine RPM.
......................where did i put it now?........................gonna see if i can find it.
Starting Member
Joined: Oct 2009
Posts: 193
Seems like keeping stock differential gearing would make the engine work harder to get up to speed, but would cruise at a given speed in a lower rpm.
We'll use the diameter calculated in the post above and assume 3rd gear (1:1) for simplicity's sake.
Stock tires at 2000 (engine) rpm:
2000rpm on the driveshaft divided by 3.42 turns of the driveshaft per turn of the wheels.
2000/3.42=584.8 wheel rpm
28.878*pi=90.68" circumference
584.8*90.68"=53,027.9 inches per minute
(insert generic inches per minute to mph conversion here)
50.2mph
Then you solve for engine rpm after inserting the new tire radius.. (this involves me mashing buttons on a graphing calc).
Engine rpm at the same speed is very close to 1750rpm.
Based on this, I believe it's safe to say that city driving mileage would decrease, but highway mileage would increase.
However, if torque is your main concern then yes, change your diff gear to something lower (lower meaning higher number).
We'll use the diameter calculated in the post above and assume 3rd gear (1:1) for simplicity's sake.
Stock tires at 2000 (engine) rpm:
2000rpm on the driveshaft divided by 3.42 turns of the driveshaft per turn of the wheels.
2000/3.42=584.8 wheel rpm
28.878*pi=90.68" circumference
584.8*90.68"=53,027.9 inches per minute
(insert generic inches per minute to mph conversion here)
50.2mph
Then you solve for engine rpm after inserting the new tire radius.. (this involves me mashing buttons on a graphing calc).
Engine rpm at the same speed is very close to 1750rpm.
Based on this, I believe it's safe to say that city driving mileage would decrease, but highway mileage would increase.
However, if torque is your main concern then yes, change your diff gear to something lower (lower meaning higher number).
BF Guru
Joined: Sep 2009
Posts: 5,143
From: GTA, Ontario, CANADA
ok James. but if 3.91's equal your stock 3.42's, then your "need' for 33's on the truck is gonna be @ the same disadvantage as it was when it was stock.
more so actually, since those 33's weigh a hell of alot more than the factory 235/75/R15's.
seems to me, you'd be better off getting 4.10's or 4.30's to make it move. and since you said you're not gonna be doing hiway driving...............what's a little higher rev'ing when you're driving so little each year.
BTW, i did find my calculations sheet, but you guys obviously found it soon than i could get back on here.
