My Mom's last car, a RWD 1982 Cadillac Coupe deVille with the 1st aluminum Caddy 4200 V8, had an "Instant Gas Mileage" display on the dash/center console (along with a fuel gauge and a speedometer/odometer). Read "0" when stopped, hit 40-50 mpg when coasting.
Probably the 1st really & truly computer-controlled GM vehcle. It was an optional engine, used in the Coupe and the Seville for one year as opposed to the 47X?+ cast iron V8. Same cooling system transmission, drive train, etc.
In 1983 they turned it sideways and it ( + the 4.5 & 4.9 varients) was the engine in all the FWD Caddys until the NorthStar. (Maybe there was a 4-cylinder GM compact "clone" in there somewhere?)
Got 22-25 mpg on regular (Unleaded!) around town, up to 30 on the highway @ 75+ with the cruise control.
Couldn't get out of its own way off the line.
My Brother still has it in Florida (no hills) and had all the suspension, body & paint work done, looks like a new car, still under 100K miles, great gas mileage, still SLOW!
Cheers!