Jeeps have been produced for the military and for civilians since the 1940s, and there have been a large variety of Jeep models produced. Jeep gas mileage has changed with the times as well, not that much and some vehicles got worse, but change has happened. Jeep gas mileage does vary model to model, and if you want a Jeep there are models that have decent gas mileage. For example, the 2008 Compass 2WD gets 23 miles per gallon in the city and 28 miles per gallon on the highway, which is not that bad for an SUV.
Diesel models also get good gas mileage, so you can get a powerful SUV and get better gas mileage than you would with a regular model. The Jeep Grand Cherokee 2WD diesel model gets 18 miles per gallon in the city and 23 miles per gallon on the highway while the regular Grand Cherokee 2WD gets 13 miles per gallon in the city and 19 miles per gallon on the highway. The diesel model will emit more pollution, but it is an improvement in jeep gas mileage and power.
You can also get better gas mileage with two-wheel drive Jeeps than four-wheel drive Jeeps. The difference is usually just 2 to 4 miles per gallon, but if you do not need a four-wheel drive Jeep getting a two-wheel drive Jeep can benefit you somewhat.
Before you decide which jeep model you want you should research the gas mileage for each model because there is a good variety of models with different gas mileage to choose from.