For those of you with both manual transmission Rubicons and X model JK's, what RPM's are your engines turning in 6th gear at 80mph, 70mph, 65mph, and 55mph? Also will these vehicles hold up under sustained highway speeds? I am assuming that the R's will be somewhere on the order of 2500-3000 at hwy speed, so is this engine designed to be run at these higher R's for long periods of time or will it fail prematurely?

 

If I remember right I'm at 2500-2600rpm @ 70 mph. That's in a 4dr Rubicon with stock 32" tires, 6-speed, 4:10 gears. I'd say 3000-3100@80mph. The higher revs won't hurt the engine. the 3.8L is a higher revvng engine anyhow that makes it's power in the mid range and upper rpms. I keep my speeds at 70-75 mph on the interstate and that's fast enough for me in a Jeep.

 

2008 X 6 speed 4.88 gears and lockers 12.50 X 35 goodyears 2500rpms 65mph

75mph is close to 3100rpm and I don't run at that speed for long periods of time as fuel mileage goes south.

 

Yep. I drove to PA @ 70-75 mph over the mountains and got 18mpg. Anything over 75 it gets bad fast. I noticed that. I think I was getting 15mpg on the instaneous mileage reading at 80mph.

 

Suggest keeping it under 75 and enjoy the ride.