2 hours ago, kenfowler3966 said:

?? Its the rolling radius that matters and that virtually is the same within a few mm at most. Otherwise the speedo error would be even worse... or just possibly spot on with true speed.

 

I drive on 17" low profile in the summer and 16" winters in the winter! According to my true speed app the speedo change is less than 1 mph, and the ground clearance is near identical.

True - if you drive 55 profile on 17” wheels and 60 profile on 16” wheels, otherwise the rolling radius (i.e. circumference)  is quite different. For example 205 x 55 R16 tyre circumference is 1985mm (+/-) while circumference of standard 225 x 55 R17 tyre circumference is 2133mm. That is a difference of 148mm = 7.5% difference, so whatever your speedo error is now it will be 7.5% more inaccurate with the 55 profile 16 tyres. Not a shattering difference but still worth knowing!

 

13 hours ago, Expatman said:

True - if you drive 55 profile on 17” wheels and 60 profile on 16” wheels, otherwise the rolling radius (i.e. circumference)  is quite different. For example 205 x 55 R16 tyre circumference is 1985mm (+/-) while circumference of standard 225 x 55 R17 tyre circumference is 2133mm. That is a difference of 148mm = 7.5% difference, so whatever your speedo error is now it will be 7.5% more inaccurate with the 55 profile 16 tyres. Not a shattering difference but still worth knowing!

 

Is the std Yet 17" tyre size not 225/50*17 ... making the circumference 2063mm

2 hours ago, TruckbusUK said:

Is the std Yet 17" tyre size not 225/50*17 ... making the circumference 2063mm

Hmmmm you could be right! I belong to the 16” wheel society so may have got the 17” profile wrong - in which case the difference is only 78mm  = 3.9% - so halving the “inaccuracy”. 

Not much to worry about just to take notice of.