Went to a RR day @ John Nobles on Saturday with a few FRST.
After the RR was finished a couple of cars had a speedo calibration test, very interesting!! The car with 17" wheels (2" bigger then standard) had a speedo that was spot on.
Apparently the speedos on standard manufactured cars are always down from the realtime figures.
I wouldnt believe it myself but I saw it first hand.
Anyone have any other experience of this??
Doc

 

Rolling diameter should stay about constant when you change tyre size. EG, I moved from 205/45R17s to 215/40R17s, so there's not a huge difference in rolling radius.

A larger diameter, which can result, will lead to increased speed per speedo reading. As an example, Rum's sti with 18s was reading absolutely spot on for the speedo - the Origin Blue i confirmed it to the dot...

Nick.

 

i didnt think it may a huge difference as when you increase the diameter there is normally a reduction in profile ie. I went from 205x50x16 to 215x35x18. Yes the wheel is 2 inches wider in diameter but the side wall has been reduced from 50 to 35. Its not exactly the same and using this tyre calculator it means my speedo will read 0.6% too fast but isnt a huge amount.

http://www.miata.net/garage/tirecalc.html

 

So as the speedo over reads, perhaps fitting larger diameter wheels/tyres will make it over read less??

Matt

 

I happened to have this handy - sad, or what

Tyre diameters in millimetres:

215/40x17 = 604
205/55x15 = 607 OEM MY97<
205/50x16 = 612 OEM MY98-00
225/35x18 = 615
205/45x17 = 616
215/45x17 = 626 OEM MY01 WRX
225/45x17 = 634 OEM STi7
225/40x18 = 637 OEM UK300

Most swops don't involve a change in rolling diameter of more than 2 or 3%.