I have a '97 4-door 328i, 5-speed. Some things that might make difference to my question are: a) BBS RK 17x8.5, b) Pirelli P Zero Nero M+S 225/45/ZR17, c)Brembo Sport slotted rotors. So, my question is this: I have the cruise control pegged on 70, and I check the speed on the onboard computer and it says 67. I peg the cruise control on 80, and the computer says 77.4. Which one is right?