Grunt

Hi Everyone,
When I decided to re-gear my 98 xj on 32x11.5 Mickey Thompson ATZ P3's I had to decide between 4.10 gears and 4.56 gears. After much research I went with the 4.56 ratio. I now realize that for my usage i should have gone with a 4.10. I have to do long freeway trips to get to where I like to wheel and this thing screams on the highway. It's awesome on the trail, but I can't comfortably go above 65 on the freeway. When I push it to 70 it feels real wrong. At 70 my tach needle is on the last notch before 3,000 rpm or 2,800 rpm. The speedometer gear has been replaced with one with the proper tooth count.
My question is will going to a 33 inch tire make a significant difference? My AZT P3 tires measure an actual 31.5 inches in diameter. I'm thinking about going to a BFG AT KO2 in 33x10.5 which measures 32.5 inches in actual diameter thinking the narrower tire will give me fewer problems with the fronts hitting frame or control arms in tight turns than a wider tire. I used Outlaw Star's Gear calculator in the gear ratio sticky and came up with 31.5 @ 4.56 = effective gear ratio 4.053 and 32.5 @ 4.56 = 3.928 EGR. It looks like a pretty small difference. I can't determine if this will give me a decent reduction in rpm at highway speed and if so how much. I do NOT want 35s.
If anyone more knowledgable can tell me what kind of rpm difference the extra inch will get me I would greatly appreciate it. it's a pretty big investment in rubber but if it will help enough I will slap the 32s on my other xj. I've seen the charts that show rpms with a given tire size at certain ratio, but the numbers on those seem wildly different depending on which chart you look at.

Thanks, Dennis

4.3L XJ

Without going through some basic algebra, you just take the ratio of the small diameter divided by the larger one and it will tell you how much drop. So 32.5/32.79best guess of true diameter) X100 = 96.3% So yu would see a drop of about 3.5% of your RPMS

00t444e

This is the chart you need to use. http://www.grimmjeeper.com/gears.html
or if you want to do the math yourself it is a simple formula.
RPM= MPH x transmission gear ratio x axle gear ratio x 336 divided by tire diameter in inches.

Also have you checked your actual speed with GPS and do you have an auto or manual transmission? 32s with 4.56 gears wouldn't be a bad combo at all, I have 4.56 with 35s and I often have to keep it in 3rd so it doesn't keep downshifting out of overdrive on every hill.

robsjeep

you will see a bit less than 5 percent reduction in engine rpm going to33 inches from 31.5 inches in tire diameter. so maybe close to 100 rpm at freeway speed just for number estimated.

robsjeep

ps, it is a fine line too, i got 33 tires and 4.1 gear, she screens at 70 mph if there is any upgrade, as she will drop to third gear. But on the level, she hums along at a very sedate just over 2000 rpm on freeway in fourth, but she will down shift in a heart beat, then scream at 3000.

robsjeep

ps, it is a fine line too, i got 33 tires and 4.1 gear, she screens at 70 mph if there is any upgrade, as she will drop to third gear. But on the level, she hums along at a very sedate just over 2000 rpm on freeway in fourth, but she will down shift in a heart beat, then scream at 3000.

EvanM

I run 4.88 and 33s ive gotten used to running 2500-3000 rpm. Drivabiliy every where is improved though its not a rig that youl be running down the freeway at 80 in 65 mph and 3000rpm come quick. I think 4.56 would be virtually the same

If your not happy with 4.56 youl likely switch to 4.10 and wonder if that was even worth it. If I wasn't happy with 4.88 I'd go to 4.10 . Dropping a half gear ratio and expecting results wouldn't in my mind be a realistic goal. More like setting up for disappointment

Grunt

Thanks for the replies guys. I tried to include all of the pertinent info in the last post but forgot to mention that I have an auto trans. I haven't checked the actual speed with a gps. I'll google how to do that. My actual speed may even be lower the way everyone is zooming by me on the freeway. It doesn't sound like bumping up a tire size is gonna help much. Another gear change is out of the que$tion. I love the acceleration for driving in town ( jumps pretty good when you step on it ) and on the trail. It looks like I'll just drive a little slower on the highway.

4.3L XJ

If you are at 3,000 rpm, that is not bad. All the old school motors were geared to do 3K at 70 mph