Help with my brick
 

I started driving my 97 Jeep Wrangler last week and I would very much like to maximize the efficiency since it is terrible on fuel. The Jeep has a 4.0L inline 6, 5 speed manual, 4.10 gears in the axles and 35x12.50x15 tires. Not exactly eco-modder material but it is just so much more fun to drive than my 93 Accord with 208,000 miles on it! Anyway - I installed a vacuum gauge in it over the weekend and it sits at 15 at idle and driving down the highway I can't seem to get it up past 10 - normally it is 5-8. I track mpg religiously and have for years - and the Jeep netted 13.98 mpg on the last tank. Dismal at best compared to the 29 I got with my Accord. Any suggestions on improving mileage with this brick?