Initialism of miles per gallon.
"The new car achieved an impressive 40 mpg on the highway test drive."
In plain English: Miles per gallon is how many miles your car can travel using just one gallon of gas.
"My old car gets about 25 mpg on the highway, which is enough to make me happy after gas prices went up."
Alternative form of MPG
"The driver asked if his car could get 30 mpg on a long road trip, using the lower-case version as an alternative spelling for miles per gallon."