i did this, bigger tires, bigger tires. If you are talking highway mileage, there is no magic pill. Bigger tires you can drop a gear or regear. Flat out bigger tires mean more mass to rotate. Regearing will mean you can use 5th or 6th gear more often but gas mileage is a function of resistance to rotate.
Having done 35s on a TJR..... gas mleage will suck regardless of the regearing. I drove an '87 YJ with the 258 and a carb on 33s with the dreaded 3.07s. I never used 5th gear on the highway but it got 25mpg on the highway in 4th gear.
After doing 35s, I quickly went back to 33x10.50s, loved them. On a daily driver 33s are fabulous tires. Then I went a different route, I almost wanted an SE in the mountains and forests where I live but a 4.0L is a nice engine. I wanted a skinnier TJ. I bought 7.50R16s, kinda old school, I had these on my 1977 F250 on split rims new from Ford. Much lighter tires, I went from a best of 17mpg with 33x10.50R15s to 22mpg. I'll never go back. That skinny 7.50 is a fabulous winter tire slicing thru slush and standing water.
IF fuel economy and range count..... I added 95 miles to my range between fills. I know, few of us care but for me an extra 95miles between gas stations open exploring without carrying a jerry can. All done by picking a skinny 31" tall tire.
ps, go here
http://www.grimmjeeper.com/gears.html
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I ran 33s on my TJR for years, averaging 25K miles a year, thru 3 sets of 33s. I run lots of highway miles towing 2K pounds in the mountains, my TJR came with 4.10s, I still wish I had 3.73s. I now run 31s and really wish I had 3.73s.
Loaded for work tomorrow, looking at 200 miles over 2 mountain passes towing a trailer.
Honestly, the TJR is the best vehicle I have ever owned.
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