How accurate is your speedometer

[VetteOwner]

yea they can be off by a certian percentage, most cases they just program the buffer to compensate, as long as you know your speed is off by XX ammount at X speed your fine. just remember with the manual cable spedos that the odometer is prolly the most accurate while the spedometer can be off.

[GasSavers_bobski]

Originally Posted by dkjones96

Over 100 miles of road the car got ahead by 2 tenths of a mile. So it is only .2% fast which I didn't really think any cars did except police speedos.

Keep in mind that the speedo and odometer are separate devices, and can have different degrees of calibration. I did a similar check-the-mile-markers test in my CRX and found my odometer to be accurate within 0.3-0.4%, but according to GPS readings, the speedo reads 6-7% fast. This seems to be a common artifact in Honda speedos, which has led some people speculate that they are intentionally calibrated to read fast. Why? We're in the US... If the speedo read slow, somebody would surely sue when they got a speeding ticket.

[GasSavers_RoadWarrior]

Cable drive speedos aren't driven directly but commonly have a "magnetic torque converter" type arrangement where magnets are spun to bias an iron plate or other magnets against a spring. Since magnets lose strength over time, or ferromagnetic materials may become magnetised under the influence of magnets, this means that a speedo in an older car may gradually read slower. When it was new it might have been a few percent high, but by the time it's 10 or 15 years old it might be reading closer to "dead on" or reading slower. It is however possible that the odometer could have a direct driven gear reduction drive off the speedo cable.

Also Chrysler cars of the 80s and 90s for example have about 10 possible final drive ratios, although only 4 were commonly used, and varying tire sizes, but only 3 speedometer drive pinions, 19, 20 and 21 teeth. The factory would put in the closest match for the final drive and tire combo, but as you might guess, this meant that speedo error could vary by model and options across the range.

Also one's measure of odometer accuracy may be affected by the number of curves in the road one is traveling. This is because it measures the speed of the differential, which will be turning as fast as the fastest wheel, so it always measures the "outside" of a curve rather than the line down the middle. So on a series of 10 hairpin bends, you might show 1/10 mile greater than actual distance. However one should realize that the position error on a GPS reading of a route like that will make the comparison questionable anyway. (i.e. you can be 10x the position error off, or greater if it doesn't update very fast)

[GasSavers_bobski]

Originally Posted by RoadWarrior

Also one's measure of odometer accuracy may be affected by the number of curves in the road one is traveling. This is because it measures the speed of the differential, which will be turning as fast as the fastest wheel, so it always measures the "outside" of a curve rather than the line down the middle.

Wrong. The differential spins at the average speed of the two wheels. If you jack up a car (autos in park, manuals in gear with the engine off) and spin one of the drive wheels, the other side will spin in the opposite direction. The differential is being held stationary by the transmission, so the movement of the two wheels needs to average out to zero.
A more dangerous example is to put the car in drive (still jacked up) at a low speed and stop one drive wheel. Don't try this if your car has a limited slip differential - the wheel won't stop. The other side will pick up speed and spin at double it's original speed. Again, this is because the differential spins at the average speed of the two wheels.

[kamesama980]

compared to my GPS, my speedo's off 10% and the odometers off 14%. my guess is the odometers more precise (mechanical) and the speedo magnetic spinny part (technical term) is 20 years old and tired. Reason it's so far is I've got a different differential and tires than the speedo gear in the transmission is set up for. (3.9 instead of 4.1 and 205/60r16 instead of 195/65r15)