I have a unlimited x manual with 3.21s and I just got some new rims and tires, the 8179 pro comp and 285/70/17 Goodyear duratracs and man does it make it look a ton nicer then the sahara wheels I had on there before. Anyway I was wondering which would get me better mileage on the highway at about 70ish, 5th or 6th gear.

5th gear is about 2500 and 6th gear is about 2000. With goin 70 or so there is alot of wind and i don't have the instant mileage readout but im tossed up between which is better.

Any ideas or thoughts about this topic. I'll try and post pictures of it tomorrow.

 

I'd roll in 5th gear. If there is any difference in fuel mileage it will be nominal, and less downshifting.

 

Congrats on the new wheels and tires. The gear you choose totally depends on conditions. I have the 285/65R18s on stock Sahara rims and will use 6th if on a flat windless stretch (at 110 you make your own wind). I have 3.21s also.

Haven't gone on a long highway run though since I put on the Duratracs but ran 6th with the stock duelers all the time on highway. I'll probably need to shift more now.

 

If you're going the same speed in 5th at 2500 rpm as you are in 6th ay 2000 rpm I don't understand how you could be getting better mileage in 5th?

 

6th. Unless you are in a area with alot of hills.

 

I get better mileage in 6th...but I do have the 3:73 rear. If I have lots of hills I usually leave it in 5th so I am not constantly shifting gears.

 

6th gear is useless in my JK. 3:21 with stock sahara wheels and I can't even climb a bunny hill without slowing down. Getting 35's in a month so it will be even worse. Wish I got the tow package (better gears), I will eventually re-gear.

 

At 70mph if you hit something as simple as a 2 degree grade, such as an overpass, the 3.8 engine can't make enough power to cope with it at 2,000 rpms. The result is you either won't be able to maintain speed, need to downshift and will fully open the throttle blade removing negative pressure from the manifold and increase the amount of fuel needed.

At 2500 rpms you would have available more than a third more horsepower and nearly 10% additional torque. The fuel being consumed is going to be dtermined by the throttle blade position and the negative pressure in the manifold. So yes we have more strokes by the engine, but the power we need per stroke is lessened. The available power left over will cover you to deal with twice the grades, head and cross winds.

If your dealing with long highway flats and no wind running at 2,000 rpm at 70mph wouldn't be an issue. Add in some realities of the road, wind or perhaps needing to accelerate to pass someone and there is nothing there without a downshift. Also if you totally start bogging the motor down by feeding it more fuel when there isn't the headspace to cover it, that's where you start hammering down your efficiency.

 

I have 35's on mine with 4:10 gears and I never ever run 6th I find it gets better mileage in 5th and less shifting !!

 

It depends on your speed of course.

I frequently cruise the interstate at 80-85mph and it runs quite well with the 35" Toyos, heavy Hutchinsons, 6-speed, and the 4.10 gears. Sometimes I have to downshift to 5th but usually not as I'm at almost 2800 rpm. A downshift to 5th puts you at over 3300, and 4th is over 4K.
Mileage at 80mph isn't anything to brag about, but it is better in 6th than in 5th.