As mentioned in the referenced thread, I work with a guy who has an 98 TJ 4 banger, all stock, and he claims he can't get better than 14 mpg on his daily commute.  I have ridden with him to work and home a couple times, and he's by no means a hot rod with it, so not sure why he can't get any better than 14.  I have an 04 TJ with the 6L and I average 17.6 mpg on the same commute.  Mine is all stock except for the 30X9.5-15 tires.  They are Nexen with more of a street tread (similar look to Michelin LTX).  My bud runs the stock 235/70-15 Goodyear Wranglers.  Not sure what pressure he runs in his, but I keep mine aired up to 40 psi.  The ride is stiff, but it helps gas mileage.  Both our Jeeps are 5 speed.  Another buddy has an 04 Sahara with an automatic and he claims he can't get better than 14 mpg either, but he's running 31X10.5-15's on his with stock suspension.  All three engines are bone stock.  Personally, I think driving style has as big an effect on gas mileage as lifts and big tires.  Taking it easy off the line and shifting at optimal times helps a lot.