Speedometer Accuracy

As you already found, your short wide tires are too small in diameter. From the tire sizes, you must either own a GT5 or GT5-S, or a converted car to those specs. You need a tire that is 26-1/2 to 27" tall for your speedo and odometer to work properly. Trouble is, there are NO new tires of any diameter that fit your wheels, either front or rear, with V or Z speed ratings. So your best bet is to go to a speedometer repair shop, or one that advertizes speedo calibrations and have them build a small adapter gearbox that sits between the ZF right-angle adapter and the cable. This is done all the time for police cars to calibrate their speedometers as a legal requirement to 'clock' cars for speed enforcement. Just be VERY sure that when the adapter-gearbox is attached, they don't pull the 8mm bolt near the right-angle adapter. Incautious removal of that bolt will cause the speedo gear to drop into your transmission guts, with disasterous results! Rebuilt ZFs are currently around $5500!