In the USA we have no small car diesel engines available (VW 1.6 being the only "small" car diesel), making it more difficult to build our own diesel motorcycles with realistic freeways capabilities (i.e: 75mph+). I think I'm pretty close to achieving the speed as well as the desirable fuel economy. It's a work in progress, like so many of the other ones on your most excellent website.
Daihatsu/Briggs DM950D (26.5hp, non-turbo), new from Tulsa engine warehouse.
1976 Moto-Guzzi Convert chassis, tranny and rear end, stretched 5 inches.
Stock performance = about 30 to 35 mpg (US gallons) at 70 mph, 120mph top.
With Diahatsu = 46 mpg, 62 mph max (redline), better economy at 55mph.
With Vetter streamlined nose = 64 mpg, 65 mph
The goal is 100 mpg. I'm modifying a spare tranny for a 25% overdrive, which should give about 75mph top.
I also have a 5 speed tranny that will eliminate the 20% loss through the torque convertor, if I can figure a clutch to fit.
A streamlined tail will also help. As one of the Vetter Challenge competitors said " the first 100 mpg is easy".
I started building my diesel motorcycle in 2005, but only got it on the road in 2013. I've tried to document the steps as the project has progressed over the years, and will be pleased to share them here. One goal has been to make the finished result look "stock". I'm quite pleased with the result.