the rim size has nothing to do with the speedo... its the tires outside diameter, as long as you keep a close sized diameter tire it wont throw it off much at all[/quote]
Yea, so if you have 16" rims you need shorter sidewalls.
Take the Trans Am and WS6 for example. Trans ams have 245/50/zr16 and WS6 has 275/45/zr17. in a sense one of the cars speedo will be off but by like 1/20th of a mph on the highway or so, its not a visible difference since, like ^ said, the OD of the tires remain basically the same.
So any car with 24'* on it, would need a new speedo gear and cable.