Originally Posted by BYuen
The later model TJs do have more emission controls than earlier models, but I'm not sure how greatly it affects mpg.
I think you're probably right.
The later models have a redesigned intake manifold to make up for power losses caused by the controls on the exhaust. The redesigned intake manifold lets more air into the engine (and consequently burns more fuel) raising power back up to where it should be.
I'm guessing since the engine is burning more fuel to produce the same power, that's why your mpg sucks so bad in later years. Sorry guys. But thanks for not killing the planet!
It's a popular swap for older style Jeeps (TJs, XJs, and Grands)... I've read gains of anywhere from 10 to 25 hp.
Old style left, new style right:
My '98 (3.07's on 31's, never use 5th gear) gets 15-16 mpg on a 50/50 mix of city/highway, though I have a depleted uranium right foot.