Quote Originally Posted by StruanR View Post
Similarly adverse gear ratios/final drive changes will have the same effect.
This would depend on how the speedo is picking up its "input". For me, with a 166 engine and gearbox I will have to have the speedo sensor "watching" the bolts fly past on one of the CV joints, so only tyre size as any impact to accuracy. But changing tyre sizes has a surprisingly big impact. On one of my day cars I took the tyre width up one "step" but kept the profile the same, and all "slack" in the speedo reading vanished .. it's now spot on. Definitely something to keep an eye on if your looking to change tyre sizes.