rays0852

Interesting theory. Outside of tire diameter and air pressure which affects diameter.....nothing you just said makes sense.

In top gear the transmission and differentials have a direct relationship to your tires turning. None of the other stuff matters. Cut the HP of the Jeep in half and as long as it could hold the gear....the rpm would be the same as a fully powered Jeep.

Tooadvanced

X2 only truth to what he said is if windowd open or hardtop make it come out of 5 gear. That'll make it change rpms

raycn

I have a 2012 JK running 35 BFG KM2s. It came with 3.73s but I bought a set of 4.10s from a guy going to 4.88s for $75. Had them put in when I had the lockers installed so labor was a freebie. Been running for awhile and they're great. I live in the mountains so would notice if I lost my oomph! Off road pretty much every weekend and the combo seems to work well.

OutlawJK

I've been running 35 BFG's on a 12 with 4.10's for 18K miles, while it is fine, 4.56 will be in my future.
calculations show 35" tires with 4.56 gears is equal to the original rubicon tires running 4.10 gears.

rays0852

When using the calculator....use the actual mounted height of your tire on the Jeep. My 35" KM2s are actually exactly 33.5" mounted at 30 psi. That makes a big difference.

OutlawJK

I actually used the roll out measurment for both the factory and BFg'S, this gives you the actual size of your tire

flatattack

Duratrac 315/70 17's (measured on the Jeep 34")

rays0852

Not sure what a "roll out" measurement is. But the "actual size" of your tire is your tire mounted on the Jeep if you are doing math calculations.

So if were talking about using a gear ratio calculator.....your pinion and ring is an exact number ( not an estimate)....so is the final drive ratio in your transmission. If you're using a number that is not the exact tire height ( which affects the diameter of the tire ---- pi x 33.5 < pi x 35 or pi x 34 ). There are 5 things total....transfer case ratio and speed being the last 2.

Unless you are calculating 4 wheel low you should be using 1 for your transfer case ratio. And 70 should be your MPH.

My 35" BFG is 34.8 un mounted. 34" mounted....and then 33.5" mounted and on the Jeep. That 1/2 inch to inch makes a real difference in RPMs at 70 which is what the gear charts are based on. keep in mind the gear chart is assuming that a 35" tire is 35" on the Jeep......NEVER TRUE.

OutlawJK

Roll out is measuring how far the tire (mounted on the jeep in real world conditions) travels per revolution. Probably more accurate then measuring the height of a tire. this is the method (revolutions per mile)chrysler used a few years back to set the speedometers. and No i dont use a gear ratio calculator, just math. with my daughters help,

I do not remember the exact #'s (been too long ago and I'm too old) but I used .83 fifth gear ratio since mine is an automatic

Just for example:

RPM from factory 32" tires 4.10 gears 2328 @65 mph
with 35" BFG KM2's and 4.56 gears 2362 @65
with 4.88 gears 2527 @65
with 5.13 netted 2657 @65

So with my combination since I added the "35"" tires if I wanted the engine and transmission operating in the range closes to the factory, 4.56 gears would put me within 34 RPM's of what it rolled off the assembly line.

rays0852

35" KM2s will yield exactly 2500 RPms at 70 MPH with 4.10s. I know that because my buddy has that exact Jeep setup and I've been in it....I checked with my gear guy and he said 35KM2s with 4.56 would be 2709 RPMs at 70.

That's why I went with 4.10s. Plus I know a couple guys running 37"s with 2012 Autos and say its great.