sorry for the stupid question. its probably been asked hundreds of times, but i searched and found nothing. so what is the difference between them. better mpg, speed? thanks.

 

SONIC nailed it.
In layman's terms, think of your mountain bike gears and imagine your legs as the engine and drivetrain. The lower the gear you are in, the less stress is put on and the easier they turn. Just like the bike, the lower your gears, the quicker you are off the line and the easier you will climb obstacles off road. Going with larger tires necessitates lowering gears to keep the ratio close to stock for engine performance.

 

alright thanks guys, that helped.

 

Actual formula to explain all of this would look something like this:
Engine rpm= E
Transmission ratio = R
Drive shaft rpm = S
Diff ratio = D
Tire rpm = T
Tire circumference = C
Vehicle MPH = M


E / R = S
S / D = T
(T x C) / 88 = M ( you divide by 88 to go from fpm to mph)
Or
(((E / R) / D) x C) / 88 = mph



Ex
2500 rpm 6th gear 4.11gears and 32" tire would be
2500 / .84 / 4.11 x 8.37' / 88 = 68.8mph

If you change to 37's and 5.13's it would be
2500 / .84 / 5.13 x 9.6' / 88 = 63 mph

Now if you kept the mph the same and change the tires and diff gears it would be this:
65mph x .84 x 4.11 / 8.37 x 88 = 2359 rpm for 32's and 4.11

65 x .84 x 5.13 / 9.6 x 88 = 2567rpm for 37's and 5.13's

 

Another way to look at it:

V6
6 speed in 5th (1:1)
Rear axle = 2:1
You get 6 power strokes per tire revolution.

Change the rear axle to 4:1, you get 12 power strokes per tire revolution, which feels like a bigger engine. You would go up the hill slower, but you could haul a larger load up that hill.

 

This is by far the best way to describe what's happening. Just keep in mind the engines gonna be turning faster (higher rpm) to achieve the same speed your stock gears would have....it's just gonna be easier for it to do so.
Now consider this...I was running my 33" tires with 3.21 gearing for the last six months...I just re-geared to 4.10 and though I'm spinning my engine faster to move the tires...I have improved my MPG...I'm making more effective use of the engines power, not just wasting gas trying to get the bigger tires to turn...you can go too far though if I had put say 5.13's in I would have maintained the poor MPG but my performance would have been pretty impressive....it's all about balance.
Have a look at this chart and see what you think(keep in mind the numbers are based on a 1:1 ratio at 65mph, not accounting for OD):


stock my '07X came with 3.21's and 29" tires....it performed alright, got great mileage, just not alot of get up and go...you can see in the chart where that fits in...
when I went to 33" tires with same 3.21 gear ratio, my performance was BAD...I made it work..but there's a reason I re-geared...also, trying to turn those tires with those gears ate up alot of gas and equated to poor MPG.
Now I have 4.10's with the 33's and personally I believe this is an absolutely perfect ratio...I couldn't be happier with my Jeeps performance and mileage...

 

Thats awesome! I love it!

 

BlackNorthernJK

I'm getting my 33's in a week. I have a X Unlimited, Automatic with 3:73 gears. So according to the chart I'm pretty much on the border of poor/good performance and good/best gas milage. Is that correct?

Thanks.

 

If you go from 3.73s to say 5.13s, it doesn't add ANY power. This is a common misconception. Think of it as adding a long breaker bar to a socket to break loose a stubborn bolt. Deeper gears just give you more leverage WITH the same power.

 

You used the R variable twice!!!!!!

Thats a great formula, hope you dont mind but I am going to copy it down and hold onto it.