peddy wrote:going from 255/40 to 255/35 will mean when the speedo says 70mph it's actually be 67.24mph, so if car manufactures put the speedo out by what is suggested to be around 3mph then the speed will be more accurate?
I checked on the following site: http://www.alloywheels.com/Tyre_Calculator
Your assumption for using the tool is that 70mph on the clock is 70mph on the road - which we know it isn't - so the original speed in your calculation needs to take that into account.
So original speed (70mph) minus error factor (5%) equals road speed (67mph) for a standard 255/40r18 tyre.
Fit a smaller (255/35r18) tyre and it's reduced by 2.5%, so you get a speedo showing 70mph when you're actually doing 65mph.
Fit a larger (265/40r18) tyre and it's increased by 1%, so you get a speedo showing 70mph when you're actually doing 68mph.
However, you also have to take into account that the tyre sizes are 'approximate' and the Michelin SuperSport is a wider/taller tyre than the claimed dimensions - I did have a thread on here where I'd researched the actual size vs the claimed sizes - so mine is about 2% extra, which makes my 70mph actually 69mph.
All approximately of course