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GPS receivers can measure your cars actual velocity. Whereas your speedometer really only measures the angular velocity of your wheels (the number of rotations your wheels make in a unit of time). When calibrating the speed dial, the car manufacturers make certain assumptions of the effective radius
of your tires to deduce your speed (v = omega * r). They also add some padding to this estimate so theyre not liable if youre caught speeding when your speedometer says youre legal, but I digress.
It would be very easy to do this calculation in reverse. Get your velocity from the GPS and then use the speedometer to deduce the radius or your tires. With no extra effort, you will be able to: (1) Calibrate your speed gage, (2) get an indication of whether your tires are inflated correctly both a safety and a gas-mileage issue, and (3) detect a flat tire.
If you can make this system accurate enough, you might even be able to detect tire wear, and turn on a warning light when your tires should be replaced. Though to reach that accuracy you might need to account for the variable tire temperature and car weight as well.
Remote Tire Presure Indicator
One of several approaches. [csea, May 31 2009]
||//rite ware// sp. tire wear (or tyre wear.)
||It's possible that roadbed conditions might have a larger effect on v(gps)/v(odo) than tire inflation or wear. Several other parameters might have second- or third-order effects: road curvature, barometric pressure, wheel alignment, etc.
||Given that some cars now have remote tire pressure measurement [link], this could be factored in.
||sp. error corrected - thanks
(serves me right for writing at 2am)