Had something happen during a recent trip that I thought offered some useful insight into a couple things: What gear to use when tooling down the highway, and what gearing to select for your differentials.

What happened is I noticed that my catalytic converter temperatures were running hotter than I've ever seen them before (the values I'm presenting are based on Torque Pro, which I have running on a Nexus 7 in my Jeep). They were in the high-1700s, where normally they are in the mid-1600s, though I've seen them get into the low 1700s briefly on occasion. The second thing I noticed is that my instantaneous gas mileage had dropped 2 MPG. This was about 500 miles into the trip, and I was holding my speed at 65 MPH for the bulk of the Interstate travel. I glanced at the Jeep's MPG gauge, and it had also dropped significantly--from the mid-15s I'd been getting to the low-13s. Looking back at the Nexus 7 I also noticed that the coolant temperature was running about 215, which is 10-20 degrees above normal. Again, I'd never seen it that high before. Oil pressure (I have an OP gauge installed) looked normal. Then I figured out what was wrong. I had the stereo too high! Yes, I had it high enjoying some music and had not noticed (from exhaust noise) that I was running along in fourth gear--over 4K RPMs instead of well under 3K RPMs. LOL. Shifted, and everything began to quickly correct itself.

Anyway, funny story, but the more I thought about it, the more I figured there was a lesson here: I run 4.88 gears on 35s. Had I put 5.38 gears on 35s, I suspect that would have essentially duplicated my mistake of running in fourth gear, except that's what I'd see in sixth gear with the 5.38s. So, thought I'd just share it on the forum for it's educational value. Of course, gearing and tire size selection (and even specific tire selection, since they weigh different amounts) will all go into affecting your Jeep's ultimate performance, so I would caution against drawing any generalities from my experience. But, still, I think there's one firm statement I can make: Select differential gearing for highway usage, and not for off-road usage. The only exception is if you don't use your Jeep on the highway much--primarily trail rigs.

 

Good story Mark. I have to say, I have done the same thing but it wasn't a big deal because I still have 3.21s. Planning on regearing to the 4.88s in the near future. I will keep this in mind!

 

Your highway terrain will factor in as well. If you are somewhere like florida where it is flat and you are at sea level you don't need to run the lower gearing to keep highway speeds whereas if you live in the mountains with 7% grades the lower gearing will benifit you more.

 

Excellent point, though I always like to point out that if people dream of traveling with their Jeep, don't build just for their home terrain.