It's not so much the engine, but the aerodynamics that kill MPG above 60-65mph. It's hard to make a big square box efficient when pushing air out of the way. Moving to a Buck Rogers looking shell, like Buckminster Fuller's Dymaxion car ( https://en.wikipedia.org/wiki/Dymaxion_car
), would help a lot. There is a really cool car made in Germany in the 30's that had the best wind tunnel score ever until efforts in the 90's, and it was basically shaped like a flattened raindrop. With an exceptional aerodynamic shell, even a little 100hp motor would drive a Transit size/weight vehicle at fast speeds with low fuel usage.