Speedometer Correction

Ron Scarboro

GT40s Supporter
Supporter
All,
I have a Smiths mechanical speedometer. It is off a mile. From indicated you need to multiply by 73%. 100mph indicated is actually 73mph, 60mph indicated is actually 44mph, etc. The odometer is off by the same %.

Has anyone had luck with the ratio adapters? Is there another solution that doesn’t involve tearing into the gearbox or replacing the speedometer with a GPS or electronic unit?

305ci SBF w/ZF gearbox, mechanical speedometer that looks like 1184 rev/mi


I’m not interested in a GPS, or electronic speedometer (trying to keep an original look).

Thanks as always,


Ron
 

Ron Scarboro

GT40s Supporter
Supporter
It is 4.22. My understanding is that the revs per mile of the speedometers are 1000 (not the 1184 I referenced). I don’t think the rear-end affects the speedo on a ZF box, but could be wrong.

Doing the math on the Avon’s (748 rev/mi), I think I get 1024. I could be off on my GPS unit, so 73%-75% would be about right.
 
Top