While we were swapping the R2.8 in our TJ, Cummins was doing a similar swap in a bone-stock TJ. Cummins pulled the 4.0L out and dropped in the R2.8 to do data acquisition. Its diesel TJ was third-party tested to verify fuel economy and realized mileage numbers of 30 mpg on the highway and 25 mpg in the city! A stock 1997 TJ with a 4.0L is rated at 16-18 mpg highway, 13-15 city. A 2.5L 4-cylinder TJ isn’t much better with 18-19 highway, 15-17 city. (Data from fueleconomy.gov.) Of course, mileage with big 37-inch tires will be much different than stock. (Stay tuned for our long distance mileage testing of Tube Sock).