[911Rob]

Driving my '74 in MPH use to bug the crap outa me too; I'd see a cop and wouldn't know in a split second if my speed was OK or not with all the differing speed zones we have in my area.

I looked into an overlay and couldn't find one.
There are a number of speedo shops that do the swap and its easy to do too.
You can likely search on the 911 Tech forum and find info on who/what/where?
FYI,
Cheers