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Aggressive Gearing To Match Those Tires

G

gandolphxx

Charter Member
Full Member
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Jan 24, 2002
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Houston, TX
As you change the diameter of your tires up from stock you effectively change your gear ratio. You may want to check out these articles and consider a change to get more power in the mud that those big tires tempt you to play in.

from http://www.superlift.com
When taller tires are installed on a truck but the axle ratios stay the same; the effective gear ratio is reduced. This means the engine is forced to operate below its power band, and performance and fuel economy suffer as a result. In order to restore the effective gear ratio (and the truck?s performance), you?ll need to have the axle gears swapped to lower (numerically higher) ratios.
Click to expand...

gear ratio facts

4wheelparts tables

Offroad hints

4x4 101 <-- must read for mud crawlers.

About differentials <-- this is great stuff for everbody.

All kind of calculators <-- saves a lot of searching for formulas.

Bottom line, you can have your cake and eat it to if you have the right gear ratio. ;D

NB: your overdrive allows you to have some very aggressive gears and still not kill the MPG. You will have to figure out how to trick the computer after the install.

NB: If you tires are way out of range from stock, you may get ABS warning lights unless you fix the computer.
 
gandolphxx: Thanks for the pointers to some great sites! I had considered a gear swap, but most of the 4-wheeling I do here in AZ is rock crawling. When you're moving slowly (not flogging through mud) it's easy enough to force the motor to stay in its powerband by just downshifting, and the t-case in these things is fantastic (2.73:1 4 Low I think...?)

In any case, I'm not going to go much larger than stock (not planning on a lift) so I think the gearing will be fine. I had not thought about the ABS issue though... you don't think that a small change (1-2 inch total OD) will cause a problem?

It will also throw off the speedo a bit (I think), but the variation isn't enough to concern me. I'm used to more heavily modified older cars that end up as much as 10 mph off at highway speed.

Again, thanks for the thoughtful info!
 
Crexis said:
gandolphxx: ?
In any case, I'm not going to go much larger than stock (not planning on a lift) so I think the gearing will be fine. ?I had not thought about the ABS issue though... you don't think that a small change (1-2 inch total OD) will cause a problem?

It will also throw off the speedo a bit (I think), but the variation isn't enough to concern me. ?I'm used to more heavily modified older cars that end up as much as 10 mph off at highway speed.

Again, thanks for the thoughtful info!
Click to expand...

I had a mismatch, 1", for a while, some nasty weather, and my ABS decided to take a few walks on the wild side. Nothing real bad, but enough to let you know it didn't like it. Bigger differences 2" will throw codes and potentially disable the ABS.

You also cheat yourself with the PCM, it will make shift decisions that you didn't want/expect. ;D
 
Maybe I am not getting the calculator right, I am putting in 31" for the original tires, 35" for the new tires and a 4:10 axle ratio. It is not bringing up the new gear ratio. What should I be entering for gear ratio, I assume that 4:10 is not correct.
 
Greymet,

I went to the calc, enterened in 31" tire, 35" tire and 4.10 gears... and it said new gear ratio of... 4.629032258064516

regards,

Grey
 
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