Your gearing is similar to mine. I turn 2,500 RPM at about 62 MPH. That is with an automatic transmission though. I know that the manual transmission for my otherwise the same 14" box truck has a much higher top end gear ratio. I have been told that these box trucks are geared for short haul around town stuff, not for down the freeway gas mileage. I get somewhere between 12 and 14 MPG, but that is with a diesel. I also have side skirts, but no fairing above the cab. The top front of the box is nicely rounded though.
I suspect that you and I will both need to do the same thing to get better mileage. Slow down. I try to keep it in the 60 to 62 MPH range.
You didn't say whether you have a manual transmission or an automatic. Being into race cars you may have more experience with changing the rear end gearing than I do. (Any experience would be more experience.) I can't promise how much it would buy you though. Others on this forum know more about that sort of thing than I do.
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