I have 2 2 Door JK's with 3.8L 1 Manual and the other auto, similar weight, lift and tire weight. Everything is constant and they are traveling at the same rpm at relatively similar speeds +/- 2mph.
My question is which one will get better gas mileage if they were running at say 75mph given that that the 6th gear in the JK is .84 and in final in the auto the 4th gear is .69.

I have a friend who is running the exact same set up I will be running in an auto 2 door with the same tires, lift and weight 5.13 and he's getting ~16-17 going 75mph at around 2700 rpm.
I've driven his JK and really like its performance, of course you are on 37's but it didnt have to down shift ever.

Thanks I know that they don't make gears for them to match up exactly at the same rpm at the same speed for either just wanted to make sure my logic was sound.

 

I'm not sure if your post is confusing or I had too many beers. Your trying to decide which to re gear?

Modern auto's get better mpg's. Although idk about the 42rle. Haha.

Do you have 37's? Does your buddy have 37's?

 

There is always going to be power loss in an automatic even with a lockup. It will be less efficient and mileage will suffer at some value.

Thing is that auto accepts a Rubicrawler where the 6-speed doesn't so advantage (if you want to get one) automatic.

 

I usually see 3.8 automatics using numerically higher gears than a similarly equipped manual. This is usually due to a lack of power complaint, so If you use the same gearing and are downshifting more often or revving higher between shifts with the auto, I would expect lower fuel economy. I drove a stock 3.8 automatic and a stock 3.8 6 speed, the manual was noticeably more powerful, I found myself on the throttle more often in the auto for equivalent performance.

"they are traveling at the same rpm at relatively similar speeds +/- 2mph." I think this situation would be close to even... until you hit a slight incline...

 

If you look at the JK gear charts you will see that 5.13 in a standard is equivalent to 5.38 in an auto. Very common ratios for a 3.8L JK on 35 or 37s.