Im in the middle of restoring a '91 MJ. It has the 4.0 with AX15 and no transfer case. Im planning on installing a Ford 8.8 for the limited slip and disk breaks but im unsure on which gearing I want. Im considering 4.10's or 3.73's. This Jeep will be a daily driver and has 31" tires (probably closer to 30" with the amount of wear they have) on it. Im wondering if this setup will suck for any amount of interstate driving. Am i overestimating the effect of this change. If it was a slush box and i was coming from 3.55's i wouldn't be so cautious but i believe i have 3.07's in there now. This is my first non-AW4 Jeep. I cant wait. I remember seeing a chart of gear/tire size/rpm somewhere...
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Gearing for the street and interstate
- Thread starter sharq
- Start date
seanyb505
NAXJA Forum User
- Location
- Simpsonville, SC
Here are a few calculators/ tire & gearing charts. http://www.jeepstrokers.com/forum/viewtopic.php?f=24&t=485&p=5300&hilit=+calculator#p5300
With 31's and the 3.07 you're running ~1700 RPM at 65 mph. Going to 3.73 gears will make the engine run ~2100 at the same mph. I wouldn't go higher than the 3.73's, as I think you will feel plenty of pep from those. Gradon is running 4.10 with a 26" or 27" tire, but he really uses it for autocross. I think he said 1st is useless lol.
With 31's and the 3.07 you're running ~1700 RPM at 65 mph. Going to 3.73 gears will make the engine run ~2100 at the same mph. I wouldn't go higher than the 3.73's, as I think you will feel plenty of pep from those. Gradon is running 4.10 with a 26" or 27" tire, but he really uses it for autocross. I think he said 1st is useless lol.
My ideal ratio is 3.73 with 27" tires. When I blew up the 1st Auburn LSD, I was on auto 3.55s, so when I got the replacement I tried out 4.10s(way too much). With 31", I would want 4.10s, but 3.73s would be fine as well. 4.10s won't be overkill and you'll like the extra pull.
NorCalChris
NAXJA Forum User
- Location
- Central Coast, Ca
Id go 4.10s. Gradons right, you will enjoy the extra pull.
- Location
- WA state
4.10s should do better in town, but still be decent on highway. 3.73 would be alright. Depending on what gears you get in the 8.8 could be either I would stick with that.
plaincoupe
NAXJA Forum User
- Location
- wilmington nc
I have 4.10s with 235-75-15 tires, i love them. at 80 mph on highway tachs about 3000rpms. the extra pull is very nice.
currently i'm running 31 with 3.07s and it sucks there is not a lot of pull and and 5th is only an overdrive. i down shift to 3rd to pass on the highway but second runs out to 60 something. i just got a set of axles with 3.55 from an auto so i'm swamping them in and building the others down the line to 4.10. i miss the peppiness. i slowly stepped up to this tire height so i can't remember how quick it was before. its a lot of feathering the clutch with 3.07s
