The speedometer in our GT500 has always been off. New 3.00 gears a few years ago and I can't figure out the proper driven gear to use. I think I've changed every color of the driven gear and still no luck. I see today that the speedometer reads about 10 MPH below zero when not moving, the needle doesn't rest on the zero. Am I losing 10 MPH from the get go? This is an automatic car. Is there any way to set the needle to zero? I've tried the tire measurement calculations and nothing works.
Thanks to all.
I wouldn't waste my time trying to DIY it. I would send it off to a instrument repair like Phidon Restorations or Instrument Specialties. Have them go through it. They should be able to give you information on what gear needs to be on the cable or in the transmission from factory. They most likely have speedo gear alternative information for different tire size.
It does sound like your speedometer needs professional recalibration.
Often times something as simple as changing your tire size will effect your speedometer readings. If you suspect your speedometer is incorrect, perform this simple test: Time yourself on the highway. It should take you exactly 60 seconds to travel one mile at 60 mph.
If you speedometer reads incorrectly, you can change easily change the driven gear. From under the car, remove the cable from from the side of the transmission. Increasing the number of teeth on the gear will slow your speedometer reading. Decreasing the number of teeth on the gear will make your speedometer read faster.
http://www.thecoralsnake.com/speedos.html
If the needle rests below "0" at rest it sounds like speedo head problem , fix that first before playing with gears.