How Accurate Is the MPH Speed on a GPS?

Your car's speedometer calculates your speed based on the revolutions per second of one of your tires -- usually one that's tied to your power train -- and it gives an instantaneous calculation of your speed at a given time. Factors that influence it are how much air is in your tire, your driving speed and road conditions. Auto safety regulations also mean that your car's speedometer will round the speed up as a safety measure. Your speedometer needs periodic recalibration for maximum accuracy, which most drivers never bother to do.

Your GPS is a positional speedometer. It will show your speed based on the average distance you've covered in the last few seconds, with adjustments for the Doppler shift from the range signals from the satellite constellation. GPS speeds will be based off of several readings over the last few seconds and are generally accurate under normal driving conditions. They don't vary by tire pressure, or the gear ratio on your car, for example. Where they can introduce errors is when the receiver can't get a good view of the sky. They can also under-report speeds when the car is climbing or descending on a steep road, since GPS altitude position fixes are less accurate than their horizontal position fixes.

GPS units will report driving speeds with errors down to about 0.1 to 0.5 MPH. GPS units have a maximum reported speed of 999 MPH -- a few models that are no longer on the market had a limit of 90 MPH for recorded speeds. Commerce Department regulations on self-guided missile technology keep GPS receivers from reporting speeds greater than 999 MPH, or altitudes in excess of 60,000 feet. As this is higher than Mount Everest, and nearly 20,000 feet higher than airline cruise altitudes, you won't see it on your typical commute.

You're safest using the highest speed shown -- either your GPS or your car's speedometer -- as your indicator of how fast you are going on the highway. You should always obey the speed limits appropriate to your driving area. If your GPS is one of the models that provides an estimated arrival time function, it's basing those estimates off of the speed it calculates and the map information it has in memory. Other uses of the GPS unit's speedometer include tracking whether or not a vehicle owned by a business was consistently speeding, or identifying a particularly slow route for delivery drivers. In the future, self-driving cars, like the ones being developed by Google, will make extensive use of GPS speedometer information.