Quote:
Originally Posted by M6-Coupe
well yes and no 
"pi" is always "pi" but what you are referring to is the actual diameter which is changed based on those flat spot you explained and cause different circumferences. My point is the percentage! those "flat spot" are not more than 2-3 mm and this amount for a over 600mm diameter is far less than 5% you mentioned. we are not talking about worn tires or low pressure tires which have a lot diameter and circumference deviation and we are not talking about rainy day and uneven roads which affect the calculation and you know those effects |
I think you misunderstood what I was trying to say
Of course a "flat spot" doesn't account for 5% difference. My point was that ETRO use a variable pi factor depending on tire dimensions (as it logically should since the relative size of the "flat spot" will vary between large and small tires etc). This means that you can't just use pi=3,14 or pi=3,05 (as an example) for all tires...
We are still not talking about a difference of 5% in the above calculations, but I have seen examples where a generic online tire calculator says the tire is just over 5% larger than stock tire, but where the ETRO tables list the same tire size as being 4,8% larger than the stock tire. This is due to the fact that ETRO uses a tire specific "pi" factor, whereas most online tire calculators just use the same generic factor for pi (in fact some use 3,14) for ALL tire dimensions.
But these are the exceptions to the norm
The point is that the speedo has a tolerance and that it needs to allow for aftermarket tires with as much as +5% rolling circumference and still not show less than actual speed.