Speedometer underreading
 

The speedo (and odometer) on my '88 SE is seriously undereading - I've checked it several times against one of those portable radar units that read out your speed as you drive by. It's about 25% under. For example, 40mph on the speedo is 50mph on the radar. I checked with a Mazda dealer and there are only 2 speedo drive gears for the trans - 20 and 21 teeth - not enough to account for the difference. Rear wheels and tires are the correct size. Are there several differential ratios for these cars? The book says 4.10 on this car. Is there something lower (from the factory, not aftermarket - the car is dead stock) that might be in there? Any ideas? Has anyone had their speedometer head act up?
Thanks in advance.

Maybe they are calibrated wrong. Is your car bone stock or modded? thats including wheels

IF someone changed teh rearend to the other version w/o changing the drive gear it can throw it off by 10-12mph, Ive done it before. Even though your car is a 5sp, someone may have gotten a 3.9 out of an automatic and installed it to replace a faulty rear, or for better fuel mileage but didnt know to ge the gear. The 4.1 gear is whiteish/gray/black(if worn out) and the 3.9 gear is yellow/orgnageish. ID see which one you hav, then switch it out.