General Wheels ago ago!

[PNL]

Just to add a few extra things to that calculation on tyres lads/lasses your speedo reading with the 215/45/17 tyres is reading at 70mph actually 68.99 but with the 225/45/17 tyres the speedo reading is nearer the 70 mph at 69.98 and for rim sizes 215 tyre rim width from 7-8.5 and 225 rim width from 7.5 to 9

 

[PNL]

Well I would have thought so as well but I was browsing a tyre site and they had the rolling radius calculator there and that's what it said so you tell me do I need to go for another visit to Specsavers wellI just checked again and here it is.
215/45/17 Dia of 625.3mm and 1964.44mm Circumference

225/45/17 Dia of 634.3mm and 1992.71 Circumference
a difference of 28.27mm in Circumference

An update here for Stripey Stilo now you are running on 20" wheels/tyres so if for instance your tyre size is 225/30/20 your circumference is 2020.04 compared to the 225/45/17 of1992.71 so come down from up there Mr.4x4 stilo

 

[StiloBoy]

Actually on checking with accurate GPS, 70mph is 65mph! I've said for years that speedo's under read considerably.