@Gordon Nichols posted:Halda, makers of trip pilot and speed pilot rally instruments, also produced The adapters to adjust for tire sizes, but I messed with that stuff over 45 years years ago so nothing I know would be current, for sure.
Might be a lot easier to get your gauge calibrated to your chosen tire size or just get a new, GPS driven speedo.
I wish I'd have bought a set of those before they became unobtainium. About the only thing I'd consider adding to my dash.