My wifes 67 fastbacks speedo has always been off, I would say 10-12 miles per hour slow. Pulled the driven gear off the cable tonight and it has 17 teeth. C4, so assuming 8 drive teeth, 2.79 rear, 215/65r15 tires. I've tried 4 or 5 different gear calculators and they all say it should be a 17 tooth driven gear. I had an 18 tooth laying around and that made it even slower. Looks like the only smaller gear I can go to is a 16 tooth. Wondering now if the drive gear is wrong, maybe a 7 tooth. Anyone have a tried and true gear calculator?