Gears & tire size & RPM

JL Rockies · **Posted:** Sat Apr 24, 2010 8:27 pm

I currently have a 32" tire and 4.10 gears on my KJsaurus. At 65MPH, I am sitting at about 1800 RPM with the setup I have now. If I installed 30" tires on my whip what would be the impact on RPM? Online calculators indicate my current setup should be 2800 RPM and with the smaller tires, 2900 RPM (results rounded). I am pretty sure I will get increased RPM but not sure what it might be.

Anyone rollin' 30's and big-boy gears?

JeepinJarhead03 · **Posted:** Sat Apr 24, 2010 8:48 pm

don't the CRD's have a 5-45 with a different final transmission ratio than KJ's with the 45?

There's at least a 100-150 RPM difference in the KJ's with 45's in comparison to KJ's with 42's

ATXKJ · **Posted:** Sat Apr 24, 2010 9:07 pm

bugnout lists 245/75/16 & 4.10's
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=6&t=32357&p=334309&hilit=+gears#p334309

I don't remember anyone running 4.10 & stock diameter tires.

JL Rockies · **Posted:** Sat Apr 24, 2010 11:37 pm

I have the opportunity to pick up some black MOAB wheels with brand-new MT/Rs (old style) for next to nothing. Although the tires are smaller than what is currently on there (free Discoverer STT) I feel they will perform better for my May MOAB trip. It's a 3.5 hr drive to MOAB and I just want to be sure I can hang.

When I used the calculator, I plugged in 4.1 for gear ratio..... maybe it was user error. :dizzy:

JeepinJarhead03 · **Posted:** Sun Apr 25, 2010 12:02 am

http://www.4lo.com/calc/gearratio.htm

that calc should be pretty accurate, there's another one but i forgot the addy for it but it would tell you the mph/rpm's for speed in 4lo after you put in the gear ratios and what not

JL Rockies · **Posted:** Sun Apr 25, 2010 12:54 am

I'm not sure that applies and if it does, I'm not sure what numbers it's looking for.

JeepinJarhead03 · **Posted:** Sun Apr 25, 2010 2:17 am

32" tire + 4.10 rear end + 1:1 transfer case + 0.67 (5-45 5th gear) at 65mph says 1875 RPM

30" tire same other particulars says 2000 RPM

with a 45RFE with a 0.75 final gear with a 32" looks like 2100 RPM and 2240 RPM for a 30"

4th on the 45 is 0.75 and 5th on the 5-45 is 0.67

not sure what was wrong with the calc you used, i have ran across a few that come up with some absurd results lol

so it sounds like about a 150 RPM increase for the 2" smaller tire

InCommando · **Posted:** Sun Apr 25, 2010 8:59 am

I thought true enthusiasts only ran 235/85/16's?

http://www.grimmjeeper.com/gears.html <-- I like this one

linewarbr · **Posted:** Sun Apr 25, 2010 9:57 am

JL Rockies wrote:

I have the opportunity to pick up some black MOAB wheels with brand-new MT/Rs (old style) for next to nothing. Although the tires are smaller than what is currently on there (free Discoverer STT) I feel they will perform better for my May MOAB trip. It's a 3.5 hr drive to MOAB and I just want to be sure I can hang.

When I used the calculator, I plugged in 4.1 for gear ratio..... maybe it was user error. :dizzy:

That sounds like it would look tight.

JL Rockies · **Posted:** Sun Apr 25, 2010 12:34 pm

A new set of 235/85R16 MT/R is going to cost me $700 trough one of our vendors, these will cost me $100 with wheels. I got a free set of these Discoverer STT but they performed like they were made from ground Chinese peasants rather than any kind of rubber at LJS 2010. Even though they were free, I believe I over-paid.

The new set of wheels will buy me time and performance in the short-term and allow me to have an on-road set and off-road set of wheel/tires when I get the other set of MT/Rs...... a true enthusiast would understand this.

I might powder coat my other set yellow just to make durangotang jealous.

Thanks for the help dudes.

Gears & tire size & RPM

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