My new truck with the 10 speed tranny gets about 22 MPG highway not towing and about 14-16 mpg with the FW in tow. But as others said, it really doesn't matter. Even if MPG in a DP was half what it is in a truck, how many miles would you have to drive at the cost of gas for it to matter more than a few hundred bucks over the course of a lot of time? The answer is that doing the math isn't worth the time. People will spend $100k or more on a setup and fuss about a few mpg. In the end the money spent isn't really as much as you might think.