2014 Wrangler Unlimited 35"tires, 6-speed manual, 3.21 gears
I spent a lot of time in forums reading the posts concerning tire size, gearing, and gas mileage. After reading all the posts stating that you can't run 35s with 3.21 gearing I almost cancelled my appointment to put the lift and tires on, I'm glad I didn't.
My average MPG before the lift averaged between 19 - 19.5 around town, mix of stop and go traffic and a little highway. These figures were calculated by hand over many full tanks of gas.
I had a 3.5" lift put on (that actually lifted my jeep 4") and 35" Maxxis Bighorn tires. I calibrated the new tire size with Procal. I've been running this setup for approximately 4 months and I track MPG manually on every tank of gas. With the 4" lift and 35s I've lost approximately 3 MPG on around town driving, where I previously averaged 19 - 19.5 I now average 16 - 16.5. If I have more highway than stop and go I'll see that number push past 17.
I recently drove from Tampa to Gatlinburg, TN through the Smoky Mountains. With strictly highway driving I averaged 18.5 - 19.5 mpg. I kept the cruise control right at 70 mph the majority of the trip. For those concerned that you can't run 35s with 3.21 gears I haven't found that to be the case. The major difference is that I don't use 6th gear unless your on a flat stretch of highway. I traveled through the Smoky Mountains and at no time did my Jeep start rolling backwards due to loss of power as some on these forums would suggest.
I am very happy with the decision to lift the jeep and put on the 35s. The slight loss of fuel mileage is not that bad. I went from refueling with 365 miles on a tank of gas to refueling with 330 miles on a tank of gas. I refuel when the gas light comes on and I have never been able to put more than 19 gallons in the tank, usually it only takes 18 - 18.5. Apparently there's another three gallons of fuel in the tank when the gas light comes on the the gauge shows empty.
While I don't have a problem with the 3.21 gears, if I could go back in time the one modification I would make when buying the jeep is I would have ordered a model with the 3.73 or 4.10 gear package just to keep 6th gear if nothing else. Hope this helps.