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Tyred of speedo error

Question : I read many of your articles regarding the issue of rim upgrades and I understand that we need to keep it below 3% of the upgrading rules. I found many software (tyre calculator) in the website for this purpose in ensuring we get the correct upgrade size. From these calculators, I understand that the original stock tyre of 195/65/15 will be perfectly fitted with 225/45/17 with 0% of diameter and speedometer difference. Whereas, if I fit it with 215/45/17, there will be a 1.5% difference in both diameter and speedometer reading. From what I understand, our speedometer will be reading 60mph but our actual travelling speed is 59mph if we have the above 1.5% difference. My question is, since 225/45/17 has a larger tyre width by 10mm compared to 215/45/17, why is the actual speed of 215 tyre slower than 225 since 225 have extra 10mm of rolling resistance.

Answer : The 225/45R17 tyre has an overall diameter of 634 mm and the 195/65R15 is 635 mm.

The 215/45R17 tyre has an overall diameter of 626 mm. The height of the tyre is calculated from the tyre width and 215/45 gives a height of 96.75mm or 193.5mm when added to the diameter.

Rolling resistance is a different issue. Agreed that wider tyres may have a higher rolling resistance but here we are talking about a theoretical speed. When you actually run the vehicle and measure the actual speeds, you will find that with identical engine set-up, etc. the top speed with wide tyres may be higher as generally speaking, lower profile tyres have less rolling resistance. This is not gospel truth because there are other factors such as vehicle aerodynamics that may change with the lower tyres and this may affect performance also. That is why Formula 1 teams spend so much time in the wind tunnel!


Tyred of speedo error

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