Quote:
Originally Posted by Boss330
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  |
Thanks for great explanation.
Now, I agree with the fact that speedo may read a bit high but not that much!and I am not talking about people who change the rims and tires by their own with no attention to ratios and so. I'm talking about BMW wheels installed or offered by factory. Now let's do the some calculations.
You referred to circumference. M5/M6 have standard and optional wheels.
The standard one is 19" and tires are 265/40/19 for front and 295/35/19 for rear. The diameters are 694.5mm front and 689 mm for rear
The optional wheels (343M) are coming with 265/35/20 for front and 295/30/20 for rear. now the diameters are 693mm for front and 685mm for rear. as you can see the difference in circumference between these two sets is 0.5% and not 5%!
You mentioned the calibration for manufacturer of tires and I think Michelin, Good year, Dunlop or Bridgestone, that mostly provide BMW tires are high rated in quality and accuracy.
You also explained about law which makes sense to me but how much? 5%?
this does not make sense to me. overall having a number like 1-2% may be real but 5-10% is too much! OP captured 169 mph over 155 speed limiter. what I am trying to say is that definitely the actual speed limit is not what is claimed by factory and can be something about 160-165 mph.
Anyways I'm engineer and good in math and not lawyer to know the law