speedometer/odometer accuracy question
 

Hello I was driving my 83SC yesterday to Daytona for the 24 hour race and I noticed that by useing the highway milemarkers it seems that my odometer is very accurate,but I have always felt that the speedo reads about 10 mph high,is it possible for the odometer to be accurate but the speedo is not?Also how do you go about figuring out if the speedo is accurate or not?I was thinking of following my wife in her 99 Civic and compare speeds via cellphones,any suggestions are appreciated.My tire sizes are 205/55-16 in front and 245/45-16 in the rear on 7/8 inch Fuchs.Thanks

you're speedo is probably off by 4-8 mph at 80. Almost all (all cars) speedos are off, usually reading a tad high. The best way to tell for sure would be to use a GPS or a radar/laser gun. You could probably use mile markers. Maybe test against 4 or 5 and use a stopwatch.

245 45 16 are a tad shorter than the stock 225 50 16, but only about 2%.

So if I was to use mile markers I'm thinking that if you were going 60 mph and if you used a stop watch you would cover 1 mile in 60 seconds,correct?The amount of seconds under/over 60 seconds would be the amount of error?

Don't trust Interstate mile markers. They're often pretty approximately located. Check it against 20 miles or so, not just one. The more mile markers you lose, the lower the percentage of error will be.

Stephan

Meant to say the more mile markers you _use_, not lose...

Stephan

http://forums.pelicanparts.com/porsche-911-technical-forum/116573-vdo-speedometer-calibration-how.html?highlight=speedo

http://forums.pelicanparts.com/porsche-911-technical-forum/83337-86-carrera-speedometer-calibration.html?highlight=speedo

http://forums.pelicanparts.com/porsche-911-technical-forum/97658-speedometer-error-need-some-help.html?highlight=speedo