Well,
I had been unimpressed with my speedo accuracy from day 1. Didn't really wanna pay much to fix, and sending it into Hollywood Speedo every time I changed tire size was even less appealing. Found these chaps Down Unda,
Official home page of the Yellow Box Speedo Recalibrator. Ordered one up and patiently waited until they strapped it onto a seagull and it got here(couple a weeks). Install was easier than a stereo, and calibration was very straight forward. The only downside is they speak Australian which is different than 'murican! Just kidding!
I made my own harness so I wouldn't have had to cut my wires, easy peazy. Got it in and fired up corrected the already gps verified speedo error 11%. Went for a drive, crap no speedo, the sender uses 2 wires both brn/red, pulled over swapped them and bingo speedo! Verified with gps, spot on! 3.1416 banana job!
P.S. As seen 4 wires +, -, sensor in, out to speedo. Basic stiff!