How to tell if speedometer is way off?

I know this isn't the best way but here is how I did it..

I have one of those Garmin GPS recievers. There is a mode that will display the current heading and speed. There is a little lag but if you are keeping your speed pretty steady it should be fairly accurate.

Mine sits on that little shelf to the left of the tach. Well as long as I don't accelerate too fast it does...

Anyway now I know why my speedo said 75 and all those mini-vans were still passing me.. Mine is off by close to 10 mph!

Scott
#1523

Mine does this too!! I've been ignoring it but my miles have been running up needlessly and now it's a very big irritant.

Anyone know of a fix?

Just get on a highway with mile markers. Run at 60mph and record the time between markers.

here is how to check. it requires an open road and a stopwatch. drive steady at 60 mph. start the stopwatch at 0 on the odometer and see where ther odometer is at after 1 min or a few minutes. exaple 5 mi at 60 mph should be 5 min. i belive it 1mph is = to 0.1 mi distance. its been a while but this a good indication.

Check out my posts in the ZF Trans section of this site, under MPH. I still have not corrected the damn thing, but some tips are listed here. In the mean time, I just keep racking up the miles, doin 100 on the guage while VW Bugs just pass me by.

Years ago, my wife & I ran T & D Rallys in our '72 Pantera, I ran a 26-1/2" dia rear tire & the speedo was very close; the odometer was dead-on. Smaller, and larger dia tires will give wrong readings. Either measure it or go to the Tire Rack's website, enter the size & brand of tire you have and see what the diameter is. There are a number of calibration schemes including an in-cable gearbox used by cops to calibrate their speedos. The Yellow pages list Speedo Calibration services (t'ain't cheap, though)

Being a former Georgia State Trooper and Accident Investigator, the proper and most accurate method to check your speed is..

Maintain a constant speed, any speed, and measure the seconds per 3 miles. Divide 10,800 by the number of seconds it took you. The answer is your speed. This will also work with a tachometer as well. The formula is taken from 60 seconds per mile or 3,600/60 where 3,600 is a constant and 60 is the seconds it took at 60 miles per hour. The formula 10,800/(seconds per 3 miles) is simply more accurate. 10,800 is simply the constant of 3,600 multiplied by 3. To fix the problem will require mechanical means such as changing your tire diameter or by reducing the speedometer cable ratio. Most guage or speedometer service centers can make up a reducer. You will need to take your speedometer so they can match up the threeds to the head and you'll also need to tell them the results of your speed test so they can come up with a reduction ratio for your setup.

[This message has been edited by Pantera1252 (edited 02-26-2002).]