My speedometer is off by about 18 percent. It reads significantly low. At 65 mph, my gauge reads 55 mph. (Verified by driving next to a brand new 07 Camry and then also a police radar machine) At lower speeds it is less, at 35 it reads about 29. How can I fix this?

And consequently, how does this effect my ODOMETER reading? Is this why my gas mileage calculations have been so crappy, because I'm actually going 18 percent more miles than I think I am on a tank of gas? Does this also mean my car has a lot more than 177,000 miles, depending on how long its been doing this?

Important information:

This is a 91 with ELECTRONIC speedometer.

I have the original rear end with a 2.73 ratio. Yes, I removed the cover and counted the teeth myself. Stock gear.

Tire size is 245/50/16 which I believe to be the stock size for a 91 RS with the optional 16x8 wheels. Correct me if I'm wrong. Either way the difference in height from a 235/50 or even a 225/50 tire should not make this much of a difference.

 

not sure of the size, but it will affect your odometer, since your speedo is slow than it racks up less miles.....if it were fast it would rack up more, so in easy terms lets say you drive 1 mile, you odometer says .9 mile replace your speed sensor, looks like you did your homework...according to my Stadard Catalog of Camaros, it says a 1991 should have 235/55/R-16, doubt that affect it that much, as i got 255/60/15's on the back end... 215/65/15's were stock, and mine says 70MPH at 74MPH so mine is 4 MPH slow at that speed, not enough to worry about and thats ball park, it may be closer, need to get a cop friend to zap me at 70MPH and see what he gets, this info was based on someone following me, not side by side, but when im going 55 on a 55 MPH road i seem to pass alot off people, but than again everyhere is a turtle, and moves at the speed of smell and i even have to pass people in a big rig here

 

are the gears in the sending unit correct? the odometer would spin slower and the same rate as the speedo. I had the same issue for a while when i swapped in 3.73's, my speedo was way fast and it was that way for awhile, i figured i have over 500 miles on the car that are not really there .