Best Gears For 31" Tires???

[Gary Briggs]

Thanks hellbent, did an rpm test today on the ride to work. @ 70mph I'm running 2050 rpm and if I set the cruise control I get 21-22 mpg at 65. I see people post rpm's of 2500-2800 at 70 with very little milage loss and I'm very skeptical. The math doesn't add up to me, how can you run 35-40% higher rpm's and not lose milage?

My engine is basically stock with the exception of the 703 injectors and some minor intake/exhaust mods and I have no problems with acceleration or freeway milage.

However my around town milage isn't very good with the 3.07 gears.

The 8.8 already has 3.73, I will have to re-gear the front either way. I'm gonna try the 3.73 first and if I don't like it, I can them swap the rear when I do the front.

It's a win-win either way.

 

[markaboo929]

FYI...3.90's exist.I may want to go that route...235/75 15's with aw4 auto 2wd 3.90's or 3.73's.My 4.6 likes to start making power around 2100 and up.
http://eastcoastgearsupply.com/i-11785810-chrysler-8-25-ring-pinion-3-90.html

 

[Marcelo73]

I got my 3.73 8.8 with serviceable disc brakes for 90$, so beggars can't be choosers.

I don't have any problem with flat land drivability on the 3.07 with a manual and the 31's. Like I said the problem is pulling a long grade.

It seems like 3.07 to 4.10/4.56 is a huge jump, I don't have any intentions of ever running bigger tires than the current 31's.

Does anyone with a MANUAL TRANSMISSION have any input?

I have MT with 3.07 and 31" tires. I use it as a DD and for weekends. Is my first 4x4 so I can't compare with anything. For me is good enough. I just do mud and sand, no rocks around here. I installed a 5.5RE lift kit with drop brackets. In my plans is go to 33" but first need to change the D35 with an 8.8. After reading a lot, I will go with 4.56.
MPG? Here the gallon costs almost 7.6 dollars. The advantage? All distances are short .

Just my .02.

 

[escout]

UPDATE:
Well, 8,000 miles later and I still stand behind the choice of 4.10's for those tires (265/70 r16). If this was a dedicated trail rig, I would have chosen deeper gears, but considering this is my daily driver, it works well.

During the the R&P install I also put in a set of Detroit Trutrac's Front and Rear. Highly recommend this setup for the daily driver in similar builds. They are transparent on the street under normal conditions, although they basically don't work at all if you lift a wheel off the ground. Either way, in 2wd it handles well in the snow and drifts like a boss.

The vast majority of the time, based on my driving habits and driving cycle, I get right around 17 mpg. If I'm nice to it and do hwy the majority of a tank (65 mph), I'll get 19 mpg. I got 20 mpg on one tank, but that was all hwy.

That's really all I have to add. This change in gears, among many other mods, has made this Jeep very enjoyable to daily around town and I plan to keep it chugging around for quite awhile

 

[Rod Knee]

Thanks for the update. I've got detroit TT front/locker rear but I sometimes wish I had gone TT's front and rear for the reasons you state plus I see mostly moderate, scenic wheeling in my future.

 

[Just Differentials]

In an effort to improve fuel economy, late model overdrive transmissions are geared very high which means RPM’s are very low on the highway. When even a slightly taller tire is installed the RPM’s are reduced below acceptable levels that the engine was designed to run. This causes constant shifting out of overdrive, sluggish acceleration, poor economy and rapid wear on the transmission and other components. Typically it is recommended to select a ratio slightly lower (higher numerically) than what the math will tell you. This will help accommodate increased rolling resistance, wind drag, and added weight of aftermarket accessories and gear.

We have a gear, RPM, and tire size calculator HERE

 

[Tony_SS]

Most people neglect to correct the shift points by changing out their speedo gear. If they don't do that, yes, it will have poor manners.

 

[Rod Knee]

Most people neglect to correct the shift points by changing out their speedo gear. If they don't do that, yes, it will have poor manners.

I always thought that the sole reason for changing the speedo gear after a change in gearing and/or tire size was for recalibration of the speedometer gauge on the dashboard, and that the transmission was blind to any such recalibration. When I changed to larger tires and then subsequently to lower gears I did not notice any particular issues with when gear changes occurred. Not that any such changes did not occur, just that I did not notice them. What am I missing?

 

[Tony_SS]

I can't speak for all years, but from what I read OD will only kick in after 40 mph.. so if your speedo is off, OD might kick in early and start lugging, etc. My 4.10's on my 31's behaved horrible.. never downshifting at the proper time, etc until I changed the speedo gear. I don't think the trans is blind, at least not on an ODBII setup.

 

[need-a-bow]

Im having this dilemma too. I used some 32 and my gas started to suck, then sold those and got some 31s and it got even worse for some reason. Im back to stock 235/75 15s, but im not sure why my gas mileage sucked so much with such a minor tire change. I went from 14-16 mpg to 10 mpg. its a 98 sport AW4 8.25 rear with 3.55 i believe. Would the speedo cause the tranny to shift too late and drop my MPG? Driving habits were leas aggressive with larger tires.

 

[Talyn]

How were you calculating your mileage if you didn't have the correct speedo gear?

 

[need-a-bow]

Basically did the math of 31.0/28.9*mileage. That was the size of the 2 tires i ran. With 32x11.5 i was running around 12-13MPG which was acceptable for an aggresive MT but the 31x10.5 set was ATs in better shape and wasted a lot more gas. Jeep also felt more sluggish for some reason.