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chuckw951's Avatar
 
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Tires again (speedo calibration)

I will be installing a set of Kumho 195/50 15s on Fuch 2.0 alloys this weekend.

I suspect my speedo will be way off. How can I get this callibrated?

Thanks.

Chuck
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Old 03-19-2002, 04:20 PM
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The speedometer is basically a magnet spinning inside a metal cup. As the magnet spins, the field created causes the metal cup to rotate against a spring. The needle is attached to the cup and shows the speed. According to the speedometer repair article here on the PP site the only real way to calibrate the speedo is to, change the spring tension, alter the magnetism of the magnet or change the gearing of the speedo drive gear. The article however doesn't suggest how to do any of those things. The implication is that you learn to live with it.

Perhaps gauge repair experts like North Hollywood Speedometer can help, though.

If you would like to know how far your speedo is off, download the Tire Size Calculator (also on this site.) Enter the dimensions for the stock tires (correct me everyone!) which is I believe 165/82/15 (though my Haynes manual also says 155 for the early cars) and then enter the dimensions for your new tires on the next line. Subtract the smaller number from the larger numeber and then divide by the larger number and you will know your error percentage. This should tell you how far off your speed will be and you can always keep that in the back of your mind as you bend your local speed laws
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Last edited by HMeeder; 03-19-2002 at 05:32 PM..
Old 03-19-2002, 05:24 PM
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Makes me think of my own speedo on the teener. I looked at the owners manual and studied the chart of RPM vs speed. all indications match what ever the tach indicates the speedooo matches. BUT there aint no way this car is that fast. Keeping up with traffic normally 75 MPH Speedo and tach both indicate I am doing like 95MPH..... Tires are 195/60-15
I will some day pace the teener against my beast which I know is 12 MPH slow has been for ten years.
Old 03-19-2002, 05:38 PM
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You can measure the error between your speedometer (i.e. indicated speed) and actual speed by using a handheld GPS unit. Make a chart with two columns (one for "indicated/speedo" and one for "actual/GPS") and take your car out for a spin. It helps to have an assistant to either drive or write down numbers. This will tell you the precise speedo error for your car and tires.
Old 03-19-2002, 07:33 PM
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Old-school rally types used to have a "gear box" that went between the speedo and the speedo drive cable. If you can find one of those, you can fiddle with the gear ratios until you get things right.

Just about all other solutions (other than extensive re-working of the speedo & odometer guts) will still leave the odometer reading wrong.

Easiest way to make the speedo match reality is to either calculate or measure the error, then remove the glass bezel and put stick-on labels over the face plate. Ugly, but it works...

--DD
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Old 03-20-2002, 10:33 AM
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The best idea is the use of a GPS unit as noted. I recently checked our 1974 914 and while the tires are larger (205) they are lower profile than the originals. I used the GEARS.EXE program I wrote for testing this (avail here on the P.P. site) and it said the speedo should be about 7 MPH fast and the GPS showed 10 MPH at 70 so I was pretty happy. If the tires are larger in dia then the speedo will be slower, but I bet it is not that much. Good luck.
Old 03-20-2002, 11:52 AM
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Here's another tire calculator, this one gives an estimated error for the speedo.

http://www.paspeedo.com/calculator.htm

Justin
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Old 03-20-2002, 12:20 PM
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All this is academic, of course. The only way any "calculator" is going to work is if the speedo was accurate to begin with. According to an article in Car and Driver last month, that means none of them . The most accurate speedometers were GM products. Some were as far off as 7%! (if memory serves, that was BMW which had the highest number of speedometer errors of all cars tested.)

The only thing that would be close would be the GPS method (or if you know a constable willing to shoot your car without writing a ticket) or use an optical testing wheel like the car mags do.

Me? I don't care that much. My steering wheel blocks a good portion of the speedo at hiway speeds anyway and I pay far more attention to what the Tach is doing. There's a reason it's bigger and right in front of your face, right?
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Old 03-20-2002, 02:29 PM
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well theres another method that i haven't seen mentioned...

you could always go down to your local dyno and drive and see the speed versus what your speedo says.

just an idea
Old 03-20-2002, 02:42 PM
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I was just thinking that the car I just bought probably has less mileage than indicated. I assume it's had smaller than stock tires for a long time.

My friend paced me in my Jetta and 70mph in the Jetta was 80mph in the 914.
Old 03-20-2002, 07:46 PM
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