I'm about to put 31's on my 99 jimmy (after I get done with my SL and BL) now I'm wondering about the gears. Mine are 3:42's I calculated I should regear to 3:73. What is everyone else running? I'm not worried about the speedo and odometer being off since blackbear will retune it for me. If you are running the 3:42 how does it handle the 31's? Mileage? Performance? Thanks to everyone who replies!

 

Ive got the RC 2.5" SL and BFG 31x10.5s my speedo is off 5mph at 60mph. I have stock tune and stock gears. I get 15mpg performance is still real good.

 

I'm running 3.73s on the rc2.5 & pa2 BL all on Yokohama geolander 31s & my speedo & odometer are off the speedo by 5mph. Other then that performance is the same as always. But mpg is near 11-13 depending on the day. But since the odometer is off I suspect it to be closer to 15-16 actually.

 

You should be fine with 3.42. no need to change em until you get up 33 inch tires. Spend the money on something else, fender flares, high flow exhaust, brush guard, lots of other fun stuff

 

yeah, 33's will kill ya on the 3.42.
but i have had other budget needs..maybe next year.
i would love to get the 3.73 because right now i am def slow moving.

 

31s and stock 3.42 gears work fine. 4.10s would be far better.

warthogdriver, 4.56 gears would be a perfect match for your 33s.

 

I found that with 31's and 3.42 gears in my truck that I lacked power cruising up hills on the highway (I was living in pretty hilly country at the time) which made it shift and hunt gears way more than I'd like. I swaped my stock axles out for 3.73 axles and for my truck it was way more driveable. Way better power starting out, and I rarely drop out of overdrive climbing hills on the highway.

I'm sure you'd be fine with 3.42's, especially if you're expecting to lose a bit of power. Not a lot, but noticeable. It's all a question of budget and how much work you're willing to do.

 

Thanks for the replys! I knew the performance and mileage would suffer so I've been busy trying to make it as powerful and some what efficent on the gas as possible. You can't have both lol. So it looks like a re gear will be in the near future after the 31's. Hopefully the local pick and pull won't charge me an arm and a leg for the axles.

 

Yeah, I have 31s with 3.42 gears and it can be sluggish at times. Also on a side note, it seems lately I've been mashing my break pedal to come to a stop (very annoying). But you will notice some sluggishness even with mods added. Aka cold air intake, less restrictive exhaust, throttle body spacer if that tickles your fancy. Some of that might be contributed to the weight of my blazer though. I've added several pounds since stock including complete skid plate set, trailer hitch and receiver, brush guard, aux lights, spare tire carrier and 31 inch tire. My gas mileage is complete crap.

Swapping gears is expensive though! Although that expense would be due to the fact that I would have to buy the gears then pay for an install because I'd be afraid I would mess it up.

 

It depends. If you're lucky and patient you can find decent deals. I got my complete ZR2 rear axle and front diff together for $200, and I did all the install work myself. After that my only cost was new seals for the diffs and a few quarts of gear oil. For me the increased driveability with the 3.73 gears was worth the money and work.

I will freely admit that even $200 can be a lot of money. It sure is for me at the moment.