That may be true with the software but all commercial hardware will inherently calculate the height above ground. Velocity measured by a GPS is inherently 3 dimensions, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Of course the requirements of (US) Defense department regulations prohibit standard consumer GPS receivers from functioning above 60,000 feet and 999mph (simultaneously). Receivers that can do that are classified as munitions. I have heard (but not sure) that most commercial GPS receivers seem to set hard limits at EITHER 999mph or 60,000 feet.
Garmin's specifications quote 0.1mph accuracy but due to signal degradation problems (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP), perhaps 0.5mph accuracy in typical automobile applications would be what you will see.
Not really sure. This could be a good research project....
I know that since putting on my michelan sport PS2's my speedo is accurate where the Falken 452's were not.
All that changed wheel size was the rears from 255 to 245.
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