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post #3 of (permalink) Old 28th January 2008
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The answer is yes... and no.

On a straight, level road at a steady speed, GPS is a lot more accurate than the car speedo. I've heard of a standard sat-nav being proven to within 1mph when tested against proper calibration equipment. Uphill/downhill gradients will make GPS underread. A bendy road may also cause it to measure the straight line distance between two sample points rather than the path the car's actually following, also introducing some underread.

Car speedos should always be considered wrong unless they've recently been calibrated. There are two many variables in the rolling radius of the wheel, what with tread wear and tyre pressure (the latter even varies with tyre temperature.) Manufacturers will often give you options of different wheel sizes with slightly different rolling radii (for example, even though it has lower profile tyres, the rolling radius of the 17" Laguna wheels is slightly greater than for the 16") They don't change the speedo for specific wheels though - they just program the thing with a margin for error from the outset. The result? The speedo on a Laguna with 16" wheels overreads by slightly more than one with 17" wheels.

To get the best of both worlds, use GPS on a level straight road to find out what speed on the speedo corresponds to an actual speed on the road, then drive by the speedo bearing the GPS result in mind.
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Last edited by Horatio; 28th January 2008 at 08:32 AM.
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