Diesel will make your bike stink to anyone that's behind. It also may make your engine knock. It will also make it run leans since the viscosity is higher than gas, less will flow through the carburator jets. A fairing would help your mpg, if it doesn't have one already. Maybe a front sprocket with another tooth or riding with a big tail wind or pushing it up hills.
http://craigvetter.com/pages/470MPG/470MPG%20Main.html