Corvette V8 diesel conversion - 40 mpg (US) avg.
 

http://ecomodder.com/imgs/diesel-vette.jpg

More info & engine pic: The Diesel Page - Readers' rigs

That is a pretty genius engine swap...talk about having your cake and eating it too!

Very cool...

I recently saw a diesel conversion of a 5th gen Honda Civic hatchback (the same age as my car), at least I assume it was a diesel converion. It had an official looking Diesel logo sticker in the location where the Civic CX/VX.DX normally is on the tailgate. I'm guessing they had swapped a Rabbit or Golf engine/transmission into the car.

that is pretty amazing mpg for a V-8, even if it is diesel.

They can do better with trans/gearing upgrades. ;)

Good point. I'm not sure what this means... did they upgrade?

Means it has a .70 4th gear overdrive. And they probably have a toggle switch or something on the dashboard to lock the torque converter. :)

700r4 is basically the older non electronic controlled version of what GM is using in current cars. I would guess it has 2.73:1 rear end ratio plus the .70 overdrive and probably 225-70/15 tires so it does pretty low rpms at highway speeds.

According to that guess it's still at around 1250rpm@55mph. Good for a 4L engine, but still kinda high for a 6.5L lump. With something like 6+L idling down the highway at 55mph is probably the best bet.

after reading that id be curious to see how the 6.2 diesel would do , or a powerstroke in a mustang