Can you believe what this chevy malibu computer says about the gas mileage I got from Miami to key west and back?

That's about right. My daughters 2018 Malibu gets the same or even a bit more, on similar trips.

Your mileage would be even better, if you didn't drive around with the front passenger door open.

Just sayin'.

NOT BELIEVABLE at ALL! US the OLD way of computing FUEL MILEAGE and do NOT rely on a CHEVY mileage computer! FILL THE TANKE< then drive 100 miles! TOP IT UP and WATCH the amount you need to ADD! DIVIDE by the MILEAGE driven and THEN see the MILEAGE! MANY GM cars NOW use GERMAN engines and GERMAN OPEN TURBO engines that are FUEL Efficient! I know I helped my nephew get a used CHEVY CRUZ with the TURBO OPEL engine and German ZF tranny in it!  IT DROVE like a TAXI CAB or a RENTAL CAR< which in REALIT Y it IS! NUMB steering and WOBBLE< too! He was getting about 38 MPG on JUNK tires! DAMN DEALERSHIP was a RIP off artist!)  The NEW Prius cars will most LIKLEY use the NEWEST QUANTUM GLASS technology and the BATTERIES will last ONE HUNDRED YEARS and take ONE MINUTE to recharge! you can drive from NEW YORK to PAL BEACH in TWO CHARGINGS with ONE stop in between! That is GREAT technology, and VERY VERY GREEN! 

If the car has a low drag coefficient, you could possibly average the same economy going 70 mph as well.

Get 40mpg with my Crosstrek.

Do not believe the fuel economy in the dash.  Use the old school method of topping off fuel tank. 

Loafing the highway doing a steady 55 mph?  Easy.

Speed is the killer, which is why they set the 55 limit back in the '70's.

Trying a trip on your Prius at a steady 55 mph.  I would expect well over 50mpg.