Gearing and MPG's

stoutdog · **Posted:** Wed Apr 07, 2010 9:18 pm

geordi wrote:

kapalczynski wrote:

stoutdog wrote:

Thanks guys!

Geordi, let me see if I understand you correctly (sorry, this is going off into an area of vehicle mechanics that I am entirely unfamiliar with). If I were to gear to a higher number, I would be asking my vehicle to "do less per turn, but faster," which would give me no mpg gain - but, and I'm guessing here, faster acceleration?

On the opposite side, gearing to a lower number would increase mpg and slightly decrease acceleration and towing capacity. Correct? And, theoretically of course, you could gear the CRD all the way down to torque peak?

I have yet to tow with my Jeep and, when I do, I won't come close to 5k lbs. Just a small boat and trailer, at most, so that factor doesn't concern me too much....

You're right on man.

Yep, the trick to HP is that at higher RPMs (like when you are on the highway wanting passing power) you downshift which increases the RPM and reduces the torque-per-rev... But applies that torque more rapidly.
Reducing your gearing lowers the RPM at every point across the power band, and asks each pound of torque to be more fully utilized. The danger is that we have a wide demand range. You don't want to reduce the rpm too much, so that at low speeds you are below the torque peak and REALLY lugging the engine.

Woohoo, I'm glad I'm beginning to understand this.... Well, at least sort of understand it :dizzy:

So I gather that we run 3.73 gears stock, correct? And the next two levels down numerically are 3.55 and 3.21. So the question then is, what is the optimum gear to run to maximize mpg without sacrificing too much towing power and causing the engine to dangerously lug at low speeds? Is it 3.73? And, if not, would changing it to 3.55 or 3.21 ever be worth the investment given that I don't plan on lifting or putting on bigger/wider/more aggressive tires?

On a related side note, what do these numbers actually refer to? The gear ratio, as in 3.55:1, thereby multiplying your torque by 3.55?

tjkj2002 · **Posted:** Wed Apr 07, 2010 9:27 pm

stoutdog wrote:

Woohoo, I'm glad I'm beginning to understand this.... Well, at least sort of understand it :dizzy:

So I gather that we run 3.73 gears stock, correct? And the next two levels down numerically are 3.55 and 3.21. So the question then is, what is the optimum gear to run to maximize mpg without sacrificing too much towing power and causing the engine to dangerously lug at low speeds? Is it 3.73? And, if not, would changing it to 3.55 or 3.21 ever be worth the investment given that I don't plan on lifting or putting on bigger/wider/more aggressive tires?

On a related side note, what do these numbers actually refer to? The gear ratio, as in 3.55:1, thereby multiplying your torque by 3.55?

If you tow more then 3000lbs keep the 3.73's,you will see a drastic mpg decrease when towing then with the stock 3.73's.

3.55 gears is referring to the ratio of the pinion gear and ring gear,easy way is the pinion shaft will spin 3.55 times(driveshaft) while the ring gear will only spin once(axle).

Play around in the gear calculator in the link below,will give you final drive ratio's,crawl ratio's(for wheeling),and engine rpm's at certain speeds in a certain gear.You can do side by side comparisons also.

http://www.grimmjeeper.com/gears.html

kapalczynski · **Posted:** Wed Apr 07, 2010 9:37 pm

I'd stay stock unless you are going BIG on tires. Cheaper, less worries that someone installed them wrong and didn't space them right. I really don't see running anything bigger than 245/75-16 on our jeeps w/o trimming or bigger than a 3" lift and with those regearing isn't necessary.

stoutdog · **Posted:** Wed Apr 07, 2010 11:14 pm

Great, thanks for the final recommendations... It was worth asking, but I definitely won't be messing with the gears.

Gearing and MPG's

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