I don't know how you verify a GPS, but the only approved way of determining speedo/odo accuracy by GM is with a GPS unit. So they think they are accurate.

 

Well if it's good enough for them, I guess we can trust it.

 

Mine is off by 2 to 3 mph also. Always slower than I think. Double checked with the DIC and it agrees. When I had it in for service last to get the rotors turned again, I mentioned this to the service manager and was told it was within spec. Just a Chevy manager said.

 

ChevySS, I have had the GPS in several vehicles and the HHR is the only one it is off in. Not a guarantee, but the odds are in favor.

Also, I tried resetting the DIC avg speed after I was moving on a long stretch of flat, straight road and sure enough, the DIC agreed with the GPS. So, that tells me that it is just the "needle'. Hopefully the mileage is correct. (thanks Jeff)

Chevy mgr, if you note the picture in my original post, you will see that the GPS reads 37, the speedo reads 40. If this is a mechanical error, I will live with it, I just thought there may be some easy programming that could correct it.

 

Sorry, nothing to program on an HHR cluster. They are plug and play.

 

 

You going to see this in just about any car you drive.

 

I've noticed my Speedo is off (according to my GPS) by a percentage... when speedo says 75, GPS says 70, 35/32 and so on, and this is most of my vehicles.

 

The Global Positioning System (GPS) satellites are synchronized primarily from the Cesium clock at the US Naval Observatory in Annapolis, MD. (There is certainly cross-checking involved with other atomic clocks around the world). The "second" is defined by a precise number of oscillations of a Cesium ion (I don't recall what that number is, but it's huge). The speed of light constant "c" is universally accepted and proven beyond any doubt for every imaginable human purpose of measurement.

The GPS system is constantly measuring how long it takes for a radio frequency signal, traveling at the speed of light, to be sent back to the satellite from a transponder (your car GPS unit). It is NOT measuring the distance an object has traveled. If you are at "Point A" on a highway, it might take 3-millionths of a second for the signal to get from the GPS to the nearest GPS satellite. Since you are moving, the GPS unit will be at "Point B" and the transit time of the next signal pulse might be 3.000022 - millionths of a second. The GPS knows, unerringly, exactly how far an rf signal has to travel to be detected. And that's all it knows. However - with that data, the GPS can easily calculate how far a transponder had to physically move in order to effect a change in the transit time of the signal. And, consequently, the velocity of the transponder based upon the known time interval of the output pulse frequency. 7th grade mathematics, really. The complicated stuff is how the GPS knows precisely where on the planet a transponder is to begin with.

The GPS system cannot be wrong if it is functional (with respect to the overall system - mobile units can of course have programming anomalies or glitches). It is the most accurate measurement system in existence.

Vehicle speedometers and odometers are virtually prehistoric in comparison to the GPS system. And yet, they can be considered accurate for nearly any practical purpose. Perhaps, in the future, the GPS system will supplant all methods of mechanical measurement in vehicles.

 

My yamaha road star motorcycle is off by 5 mph when I asked about that they said it was ok, yamaha has a 12% tolerance