In the following govt web site
they list the diesel
Grand Cherokee as receiving 17mpg City and 22mpg Highway
Mercedes ML320 18mpg City and 24mpg Highway
Mercedes GL320 (bigger and heavier) as 18mpg City and 24mpg Highway
The current Wrangler JK 4wd is listed:
15mpg City 19mpg Highway.
Asuming the Wrangler is about as heavy as the above vehicles...
The incremental jump for a Wrangler would be about a twenty percent fuel mileage increase to change over to a diesel of the the same size. It isn't as dramatic as I would have thought or was hoping for, but maybe the mileage goes up as the diesel breaks in.
It is also noteworthy that checking the fuel mileage of some cars and trucks (Honda Element 2wd for example), the automatics are sometimes giving much better fuel figures. Again, I would have thought otherwise! DC is currently investing in new automatics that might make their way into the JK someday. Someday.