I've had my 'Silver Bullet' for about 55,000 miles now. In '96 it got 4mpg. Has the hole Banks sit up and got 4mpg. Has 4 speed Allison 545 trany, got 4mpg. All the old thing could do, in 1st gear, to climb a mountain, but got 4mpg.
Put EFI on it and I get 5.9 to 6.9mpg. Run the dash air, same thing. Go 75 mph, same thing. Go 55mph, something. Pull the car, same thing. Would love to get 15mpg, but someone would have to fill it up three or four times when I wasn't looking for that to happen.
I've had three gas motor home and they all got from 3.5(that was a 413cid dodge) to 6.9mpg.
If I could get the wife to ride in the toad and keep it floored I'd still get 5.9-6.9mpg.
Before I put the EFI on I tried to find a way to remotely run the toad engine, but that's above my pay bracket.
Maybe GM will offer a package deal someday and we can slide a new Duramax/Allison under our doghouses and just plug it in