I do know on the Insight, the speedometer will read out a positive number for speed in reverse. This is a digital readout. I've had it up to 12 miles per hour going backwards down a backroad.

But I don't know how it would affect the fuel calculation. Reverse is geared so low in most vehicles, I would guess that overall you would give up some MPG's.

Things that make you go "hmmm?"

Regards,