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?
Buy or borrow a GPS and find out for sure. Or, you could do it the old fashioned way. Set your cruise for a 10-mile run on fairly flat road (so your speed doesn't vary much) and time it for a known distance. The longer you can go at a set speed, the better, but 10 mile increments make the math easier. Use road mile markers, not your odometer, which might be equally inaccurate. Distance traveled divided by hours used = mph.
Your on-board computer would most likely be more correct. The speedometer is designed to read faster than actuality, usually between 3-5 mph.
How does the onboard computer calculate speed? If I remember correctly speedometers work via a sensor on your differential. How does your onboard computer calculate vehicle speed?
By getting its reading from the sensor? The analog speedometer, as I said, is designed to show a faster speed, where a digital readout can't be fooled by live data. But, what I just posted is what I find to be logical, and cannot verify without someone +1'ing that.
+1 to Botond. THe computer readout is normally the more accurate of the two because the analog display is intentionally designed to read high to compensate for variability in tire sizes/inflation/etc. because if they read even 0.1mph low in some countries the manufacturer could be fined.
