actually depends on how efficiently the car runs and weight of the car and the driver mod.

Six speed has two overdrives, in my old Firehawk I could regulary get 26mpg with 6 speed and 4.11's on the highway. terrible if I drove it hard.
Car ran high 12's

I also had a low 12 second fox coupe regularly knock down 23 highway with 5 speed and 3.73 gears, drove it 47 miles each way to work every day.