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Re: speedometer vs computer 6% difference Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 6 Aug 2001 08:42:15
In Reply to: speedometer vs computer 6% difference, adam  [Profile/Gallery]
, Sun, 5 Aug 2001 16:07:26
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All measuring devices have some accuracy - a tolerance from one unit to another. This is usually specified as a % error around a point. Since speeding isn't a good thing, speedometers are usually designed to read high. So instead of a tolerance of +/- 2%, a speedometer would be (for example) - 0%, +4%. So it would never read low, but could be as much as 4% high.
Yes, the speedo and the trip computer use the same data, but in different ways. For the speedo, pulses from the transmission are converted into a voltage, and that voltage moves a needle. All that electronics has some error in it, including how you read it (parallax error). The computer is going to count pulses over time. So chances are the trip computer will be more accurate.
But actually that doesn't mean that's your actual speed. The speedo/trip computer measures only how fast the driveshaft axle is turning. That would be fine if you know EXACTLY how big the tire is, but the speedo and the trip computer don't. They are based on the stock tire size - you can get a speed variation with other tire sizes. Also, each tire has a different size (that tolerance thing again) from the factory - not a lot, but some. Tire pressure and tire temperature make a big difference - Higher pressure and hotter tires are bigger. There is enough difference due to pressure that there are tire pressure measuring systems based on wheel speed differences.
The only true test of a speedometer is to find a measured distance, like a measured mile, and travel it at a set speed while timing it. I'll bet both the speedo and the trip computer are a little off.
What's more, you're going to do that at one speed (say 60 mph) - but to be truly accurate, you need to do it at a variety of speeds - 30, 45, 75, up to whatever speed you drive at. Systems aren't linear - the accuracy at one speed may be different than at another.
Sound like a pain? Yes. My advice - live with the speedo, and don't sweat it. Otherwise, find a measured mile and check the accuracy of your speedometer at the prevailing speed limit - usually 65 mph. You don't need to know the speed withing 2% unless you're doing rallys, in which case you do a complete calibration of your speed sensing system. All that speedometer does for you is let you know when you become cop bait. So if your speedo says 65 when the car is actually going 62, you know that you can pass that speed trap at 68 (in a 65 zone) and be relatively safe.
Good luck!
posted by 140.157....
, Sun, 5 Aug 2001 16:07:26, Mon, 6 Aug 2001 08:42:15 <-- Viewing This Message
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