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If my odometer reads exactly correct mileage, does that mean that my speedometer is also reading the correct MPH? I want to determine how accurate my speedometer is. I know you can do this several ways inc. timing the car ove a measured mile while running 60MPH etc. but I think the easiest way may be to drive the car on an interstate with mileage markers for 10 miles or further and see how many miles the odometer reads. If you go 10 miles and the odometer says exactly 10 miles it would mean that the odometer is very accurate, but does that mean the speedometer will also read accurate MPH? What if you go 10 miles and the odometer says you went 11. That would indicate that the odometer reads 10% fast. Does it also mean that the speedometer is reading 10% fast? How closely is the accuracy of odometer correlated to the accuracy of the speedometer?

Thanks, Joel
 

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Your assumptions are 100% correct.


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1970 Chevelle SS Clone 502-580HP
1992 Trans Am Conv(4Sale)
1992 Taurus SHO
Mercer, PA
 

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With all of the years of use and wear, the speedo and odo may not match. I have a car with a good speedo ( I had to change the gears to make it so) and an odometer that is 20% too high. check them both with a watch and look at the odo , too.
 

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The longer your test trip, the more accurate your math will be. Have you ever seen a mobile radar trailer thing in your area, that clocks you and displays your speed?? They use them around here in areas where there are too many accidents caused by speed, or when they change the speed limit on a road. You also see them a lot when it's back to school time for the kids. They basiclly give you a warning without the cop. I have called to find out where and whenthey will have them set up, and test away on that speedometer. Just don't go 70 in a 25 zone to see if your speedometer is accurate! They don't think that's funny!
 

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There is a calibraton of the speedometer itself. You would change the gears to get the odometer right, and then if the speedometer is off, it would be adjusted by moving the cal lever (for lack of a better word). I think what it does is presets a certain amount of drag on the needle so that it takes more or less RPM's from the cable to move the needle a certain amount.

[This message has been edited by Gene McGill (edited 11-22-99).]
 

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You can check the speedo accuracy on the interstate too. Run a constant 60MPH (don't get run over
). Should take 60 seconds to go 1 mile. Same as odometer, the longer distance you check it, the more accurate the check will be.

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Rick (Abilene Texas)
72 El Camino
TPI350/700r4
 
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