I posted some info here not long ago on the speedo. It's in the forum, run a search there is a lot of info about the speedo available, common problem with the MKIII. Uses 3 cables, any one of them might be the problem, or it could be electrical. As I have not yet had a problem with mine I only read it briefly and reposted it here somewhere. Google it and you will find tons of info from other sites as well. Check this out maybe this will help
http://www.supras.com/06/techcenter/display.php?QID=29
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1987 N/A 7MGE Left Hand Drive MK3 Red 5 Speed and Black Auto Supras
My Next project is the Batmobile!