Speedometer Slightly Off?

I went down a street with a radar-enhanced speed limit sign.
It reads "25" when my speedometer reads "26-27"ish.

So, is my speed always 1-2mph below, no matter how fast I go? Or will I actually be going about 51 when my speedometer reads 55? Maybe 59 when it reads 65?

Or is it more likely their radar needs to be calibrated? :D

I would think that if your sppedometer is digital, then if you're off by 1 MPH at 25 then you would be off by 2 MPH at 50, 3MPH at 75MPH, etc. We can expect the vehicle speed sensor to put out twice as many pulses per second at 50MPH compared tp 25MPH. If the digital circuitry sees 10 pulses per second and converts that to 25MPH, then 20 pulses per second should convert to 50MPH since digital circuits are very accurate. In other words I would expect a digital speedometer's percentage accuracy (not absolute accuracy) to be the same over its range. Now the speedometer could be innacurate due to tire size, for example, but at least it would be consistent. By consistent I mean it always converts a certain number of speed sensor pulses in to a certain MPH.

Now if you have an analog speedometer, then I would think that the accuracy could vary over different parts of the range due to inherent innacuracies of analog vs digital circuit design. So with an analog speedometer you cannot make the same assumptions as with a digital.

"So, is my speed always 1-2mph below, no matter how fast I go? Or will I actually be going about 51 when my speedometer reads 55? Maybe 59 when it reads 65?"

So if you have a digital speedometer, and if in fact you do have an error at 25MPH (assuming the radar is correct), then you are correct in assuming that the actual MPH error will increase as your speed increases.

From what I have read, it is more common for a factory speedometer to read high rather than low, so many of us are actually going slower than our speedometers read. The manufacturers make errors on the conservative side so we do not get so many speeding tickets. And if our speedometer reads high, then our odometers also read high (odometers get the same speed sensor pulses as the speedometer) and thus we have not driven as many miles as indicated by our odometer. And so our warranies run out sooner than they should and our resale value goes down. And our gas mileage calculations make our MPG look better than it really is since we did not really drive as many miles as our trip odometer said. What a bunch of bummers.

You did not say what type of speedometer or vehicle you had so I covered both cases.

Sorry; I haven't checked this thread in a while.

Yeah, I should have been more specific-- It's a 1992 LeSabre. I'm guessing that's analog? I thought about the tire-size, but everything looks normal. The idea of the manufacturer "erring on the side of caution" sure makes sense. (Besides, if the PD's radar says I can give it some more gas, who am I to argue? ;))

I would say your 4 to 8 % ish, off on the fast side.

I liked to watch my odometer when I'm on a interstate with mile markers, but lately I use a GPS..

Maybe you could test it by running it between mile markers on the highway. If it is going slower than it actually says, it should log slightly more than a mile between mile markers. Are the tires P205R15's? I believe that is the size they call for. I have been running P215R15's on my '97, so I am actually going a little faster than my speedometer says. I figure by about 2-3%. It logs just slightly less than a mile between mile markers.