On my highway there's a "speed meter" on the side to tell you how fast you're driving (so to scare you to slow down I guess.) I took my 2007 Camry, make sure no other cars are around, and set cruise at 75 mph, and the speed meter picked me up as only going 69 mph. I circled back, repeat at 70 mph cruise and the speed meter picked me up at 65 mph.
Took my 2000 Maxima, same thing, 75 is picked up as 70, and 70 picked up as 65.
Then took my 1990 Corolla, wah la, 75 is 75, 70 is 70.