Re-calibrating a speedometer...

[konrad]

Is it possible?

Mine seems to be running about 5 miles an hour faster than I'm actually going, based on what it says, and what the assorted "Your Speed" radar boards I've passed say.

Just wondering if there's any sort of adjustment that can be made?

[Tevel Jean-Jacques]

Hello,

Is that difference constant at any speed or only when driving at a fixed one? If at 65 m/h for example, look at tire wear or even air pressure, they both impact the total wheel diameter and therefore speed calculation.

At the end of the day 5 m/h diff is not that big ......

[joefrazer]

KF speedometers tend to read fast - as do most cars of that vintage. The issue is in the head unit and while they can be calibrated, the work to remove and reinstall them as well as the cost for calibration, usually means you just get used to doing some quick math to figure how fast you're really going.

[MarkH]

My Aero speedometer is off too, at higher speeds. I was heading for a speedo drive gear change in the transmission (easy to change) until I did more checking with a GPS & discovered it read correctly at lower speeds with the error increasing as road speed increased.

[konrad]

My Aero speedometer is off too, at higher speeds. I was heading for a speedo drive gear change in the transmission (easy to change) until I did more checking with a GPS & discovered it read correctly at lower speeds with the error increasing as road speed increased.

Seems to be the same with mine.  Passed one of those speed signs at about 30 and the speedo was spot on, but the other day passed one that said 40, though the speedo said 45.

I think Joe has it right...do the mental calc., but also use general traffic movement as a guide...at least for the upper limit. 

[Terry T]

a local speedo shop charged me a c-note to calibrate mine

[JoeKeys2010]

I have an app on my phone and my 49 runs almost 10 mph fast at highway (55-60) speeds

[kaiserfrazerlibrary]

Most car makers errored on the high side rather than read lower than actual measured mile road speed.  The variation on Pontiacs could be as high as 7mph at 65. 

My suggestion is to drive the car on I-94 back towards Milwaukee and have a passenger time the distance between a couple of the mile markers and work out the calculation.  To do a theoretical calculation as noted her earlier is the fact that part of the calibration matter is the number of gear teeth on the speedometer cable that meshes with the gear on the transmission.  You should be able to get numbers to work off of by the time you get to the Wisconsin Highway 73 exit and you can come back into town on U.S. Highway 18 if you want or take Dane County CC back in if you want to go on the "old highway" that runs from Madison to Waukesha where you hit Blue Mound Road to get into Milwaukee.

[vt2000driver]

THE REAL QUESTION IS THE DISTANCE TRAVELED.  IF IT IS CORRECT (AS COMPARED TO MILE MARKERS) THEN IT IS IN THE HEAD AND CHANGING THE GEARS WONT HELP AT ALL.