If you want to make a good investment I would buy a late model Honda Civic. That should double you gas mileage and at 140 miles round trip add up in fuel savings.

 

The only reason to cite MPG increase for a tonneau is to sell the wife on the idea you need one.

Surely doesnt hurt ,but I would call it negligible, I have found going lighter on the peddle is a much larger increase.

Now, security, and keeping stuff out of the weather is the real plus IMO.

 

I got a hard tonneau because I wanted something to keep stuff in the bed dry and secure, and also wanted it to look nice and clean. Increased MPG wasn't even considered.

 

I have seen mixed reports on mileage with a cover. A guy I work with has an Colorado, he was aware of the mythbusters and made the argument to me that he would not see an increase but much to his surprise, and he actually admits it, he has seen an 2 mpg improvement. I was surprised to see that much. He drives a lot of highway.

 

The impact of a cover on MPG is pure aerodynamics, so bed and cab configuration is just as important as anything else. For example, a regular cab with an 8' bed will have different results than a 5.5' Screw. My 86' F250 Reg cab 8' bed saw a 2-3 MPG gain on the highway with the tailgate down. My '97 Scab 6.5' bed improved about 1 MPG on the highway with a tonneau. The Mythbusters didn't test a cover, they did a tailgate up/down test on a Screw 5.5' (IIRC). I realize that my "field test data" is pretty limited, but it makes aerodynamic sense that the long bed, short cab configurations would see the greatest benefit from a cover or dropping the gate.

 

Yeah, mileage is better with the cover ...
... but my reason for the cover is security / weather protection.

I normally drive with a light and very steady foot. Even crusie control mileage can be beat with the right foot technique, anticipating grades, etc.