I think I have a big problem. I got a 700R4 trans installed several months ago. My tach use to jump around, until I replaced coil. Seems to be fine now. I have checked RPM vs Speed of mine vs websites that have me enter in tire size, trans ratio, and rear end. 1st, and 2nd gear for sure match dead on. Can't remember if 3rd does, but no reason it shouldn't. Most 4 speed transmissions are 1:0 ratio in 3rd. In 4th gear though, I was taching 2500@74mph with a 3.08 gear, 255/60/R15 *27.05inches*, and supposedly a .70 4th gear. Well, http://www.angelfire.com/fl/procrastination/rear.html shows I shoould be running at 94mph. I KNOW my speedometer is off by 3mph, I checked it with a GPS, even at high speeds... Well, that then shows I have a .90 4th gear. Tested two other rpms, with same result. can someone run their tiresize, gear, and trans ratio on this site, and let me know if it is accurate? I have tested it against other sites, and the one under tools, and the results are identical. I do know I have run at 3000 in 4th, and definently have was not going 112mph. I am going to run two wires in the car with me next week and check that tach vs another tach.... Do these numbers match what you are doing on that site??