jimonfly

My current tires are getting pretty worn and I'll be looking at new ones pretty soon. I currently have 33" Toyo MTs and was wanting to move to 35s. I have a 2015 JKU Sport and looking at the build sheet I have 3.21 gears with the optional Anti-Spin rear differential. I've done some reading on here and other forums about both and the consensus is that 3.21 gears aren't suitable for 33s, much less 35s, and that the anti-spin diff is OK when new, but the clutches wear out after a while and is basically an open diff.

So first, my power and performance are really good with my current setup. I run the interstate at 70mph at under 2000 rpm, and have no problems with acceleration, hills, etc. Is it possible that the gears have already been changed, and how can I tell (I've only owned it about a year)? Will going to 35s be that much of a change?

Next, if I'm going to regear, I assume that would be a good time to install a TrueTrack diff in the front (based on what gears I go with). Should the rear be done as well? Or should I do just the rear and not worry about the front, or do neither? Which is more important?

This is my DD and I don't wheel, but go offroad quite a bit for work. I'm an ag appraiser and find myself in orange groves (really fine sand), muddy farm land and torn up logging roads.

rob_engineer

You can lift the rear wheels off the ground, then rotate the tires. Mark the driveshaft and see how many rotations the shaft makes with one rotation of the tires.

nthinuf

Yeah, counting driveshaft revolutions, or popping the covers and counting teeth. Or, somewhat less reliably, measuring the actual tire height and checking the rpm charts.
3.6L auto, 3.21's, 31" measured height (again, measured, not spec, not sidewall)
= 2022 rpm @ 70 mph

And remember that everyone has different opinions on what power they need. It will likely be completely different for someone living at altitude in the Rockies and someone living at sea level driving mostly flat roads. Or someone with a heavy armored up rig vs one that is mostly stock. Or... Base it on your opinions/needs/conditions, not necessarily what is right for other people.

The truetrac is a great traction device. If you have the spare cash, and feel that you need the extra traction, hard to go wrong with them. I went with selectables this time around, but truetracs are still my favorites.
But should you do them? Your call, not ours. Many people have the opinion that you shouldn't put a dime into a D30, and should just wait until you are in a position to upgrade it. But, if you are already doing gears, I'd say go for it. You aren't wheeling, so an extra 400 on top of the gears isn't such a steep investment. If money is tight and it is a choice of only doing front or rear, personally I would swap out the rear clutch-based limited for a truetrac and leave the front open.