JerryB,

Mate, you're missing a link or two...

First you say: "I've gotten up to 31 mph on the hiway [sic]." and then "Your mistaken - that was 31 mph @ 75 mpg." Getting 75 miles per gallon are we? Mmm... boy these new diesels are VERY impressive aren't they? But I still think you should try to go a little faster than 31 miles per hour...

I think what you're trying so hard to say is that you have be able to get 31 M.P.G. (Miles Per Gallon, not Miles Per Hour dumbo)
when travelling an average of 75 M.P.H (Miles Per Hour, not Miles Per Gallon dumbo).

Put it all together (may take a while) and you'll realise that it is you, not I, who is mistaken...

David Rayment.