120/70 front tyres

This weekend I had new Diablo Stradas fitted and decided to go with a 120/70 front after riding my exes bike which has the same sizes as mine now has, only she has normal Diablos. I like the feel of it in corners but have noticed that the speedo now reads slower (and when I stoppie the front tyre just catches the nice shiny rad guard I just fitted to the SV1000 rad my 650 has...b**gger).

I'm no mathematician, not by any means, so how far out is it compared to what it was before?

Originally Posted by Lozzo

This weekend I had new Diablo Stradas fitted and decided to go with a 120/70 front after riding my exes bike which has the same sizes as mine now has, only she has normal Diablos. I like the feel of it in corners but have noticed that the speedo now reads slower (and when I stoppie the front tyre just catches the nice shiny rad guard I just fitted to the SV1000 rad my 650 has...b**gger).

I'm no mathematician, not by any means, so how far out is it compared to what it was before?

120mm x (70-60)% = 12mm difference in the height of the tyre

12mm / (120X0.7 + 17x25.4/2 ) = ~4% difference in radius/diameter/circumference

so your speed will read ~57mph where it would previously have read 60mph, for example