My speedometer is off and has been reading low since I've owned it... It has the factory 2:76 rear ratio... it reads about 5 mph off between 35-55 mph. Then once your traveling 60-80 mph its starts to vary around 7-8 mph off. Example : When the speedo says 60 your only traveling about 54-55 mph... It has the correct 25 tooth pinion in the tail shaft... do i need a 23 or 24? adapter is stamped 21-25. If anything I should actually be traveling faster than advertised since the tires are taller than factory on the car (255-70-15)... Where can I buy a 23 or 24 tooth?
If I'm understanding you correctly, your speedometer reads faster than the speed you are actually travelling. This would indicate that you need a BIGGER tooth count speedometer gear, not a smaller one. This will slow down the speedometer for any given ground speed.
It also sounds like you currently have a 25 tooth gear installed in your car (you didn't give us info on what year, make, model. It sometimes makes a difference). From that I roughly calculated that you need to change the speedometer pinion gear to a 27 or 28 tooth gear to get fairly close to accurate.
The most accurate and easiest way I know to determine which gear to install is to follow this procedure. This requires a GPS unit of some kind, handheld, laptop computer, automotive dash mounted unit, it doesn't matter as long as you have GPS speed data available.
1. Drive the car at steady speeds of 30, 40, 50, & 60 mph, 70 if you can do it safely, as indicated on your SPEEDOMETER.
2. Have your co-pilot (passenger) operate the GPS and write down the GPS INDICATED speed that correlates to your SPEEDOMETER speeds in step #1 above.
3. Go back home to your shop/garage.
4. Calculate your speedometer error with the data you just wrote down. If as in your case, you have a speedo reading of 60 mph while your GPS indicates 55 mph, you have an error of about 9% fast. Calculate this error for ALL of the speeds that you took data, then figure an average error %. They will all be pretty close to the same %.
5. From this calculated error %, you can now determine which speedometer pinion gear tooth count you need to correct your error in the speedometer. If you currently have a 25 tooth gear (as in your case), multiply 25 times 1.09 (+ 9%) to get 27.25 (number of teeth required to correct the error). This works both ways, for speedometers too fast, or too slow. If your speedo is too slow, then you need LESS teeth to speed it up.
SPEEDO too fast = MORE pinion gear teeth
SPEEDO too slow = LESS pinion gear teeth
6. Remove original pinion gear from transmission, and install correct calculated pinion gear.
Following this method eliminates all the guesswork and trial & error work. One time and its done and its correct. Also, here is the factory Pinion Gear selector chart for your info.
