Quote:
Originally Posted by M6-Coupe
what do you mean speedo reads high? you mean the actual speed is like 90 mph when speedo is reading 100 or something like this?? why???
|
Yes, every speedo reads high, for a couple of reasons
Difference in rolling circumference of tires (standard and optional tires might have a +/-5% difference in rolling circumference). When going from summer to winter tires, or from 19" to 20" tires (both available on the M5) you still want the speedo to show a higher speed than you are actually travelling in...)
A speedo is by law required to show a "fairly accurate speed reading, but not less than actual speed". This is to avoid a driver going faster than the speed limit, even though his speedo said he was going at the speed limit.
So, in order to allow for tolerances on rolling circumference (a similar size tire but from two different brands might also have a slight difference) it's important that the manufacturer calibrates the speedo to show more than real speed in any scenario while allowing for all optional tire sizes specified by the manufacturer.
And, perhaps most importantly for the driver, make sure that the speedo doesn't show less than real speed at any time. It would be unfortunate if the speedo shows 55MPH but true speed was 60MPH...
The ususal tolerance is 5-10% over true speed. Meaning that most speedos will show 94,5-99MPH when travelling at a true speed of 90MPH
