Reviving this old thread...my '91 80 Series is waaaay off and I'm about to do a rally that includes TSD legs. We bombed this part last year, trying to find a way to improve the speedo.
The number for Speedometer Service doesn't seem to go there anymore, the link on Slee Offroad is dead and Yellow Box seems more geared towards motorcycles.
Anything out there nowadays?
I've got the same issue with my '72 KE20 Corolla rally car, and just run my Garmin with the trip data selected. I do the same for my 80 since I went to larger tires. Accurate speed & mileage without yellow boxes or custom gear units. Bonus: it's also got Bluetooth, so now the 80's got hands-free calling capability.