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Speedometer or navigational device?

When my tomtom reads 55mph my speedometer on my new Tacoma reads 60mph. Which is correct?

TomTom is likely to be far closer to correct. The car’s speedometer is intentionally reading high. All manufacturers I know are doing this. They do it to help you avoid speeding tickets and so you don’t blame them when you get one. Also so they don’t get hit with legal action if someone was speeding when they killed someone only to find the speedometer was a reading a little low.

You can tell for yourself. Go to the freeway and set the cruse control to 60 mph. Have your passenger use a watch to time how long it takes to travel five or ten miles using the mile markers on the side of the road (small while signs with black numbers. They are very accurate over longer distances.

Note1: You can also use your own odometer as a good measure of distance because they are not adjusted to read high or low.

Note2: If you change wheel or tyre size, that can make a difference in the readings of both your speedometer and odometer.

I disagree. I’m familiar with GPS devices, especially portables, and they are not extremely accurate. For position measurements at stand-still, they can be as much as 5 feet off. When moving at highway speed, they can be more that a several hundred. I do like the idea of checking your speedo calibration technique. That is the real test to see which is more accurate.

I would not trust either one to be very accurate. I would treat them both as estimates.

Of course the mile markers aren’t necessarily correct either. Some places have “speedometer check stations” which are basically 5 or so mile posts that are actually precisely placed, so you can do a time/distance check there which should be relatively accurate.

Like bustedknuckles was saying, the GPS has a certain amount of inaccuracy, usually around 5-20 feet depending on the unit. This makes the instant speed readings fairly inaccurate because the variations in readings can add or subtract several miles per hour. However, over a long trip these variations should even out and you can use the GPS to calibrate your speedometer by setting the trip odometers on the GPS and the speedometer and then comparing the numbers at the end of your trip. Because the speedometer will be off by a consistent percentage, you should be able to work out a rough idea for what speed you’re actually going when the needle is pointing at a given number.

Also, note that your GPS unit will record the variations in the readings as movement so if you’re doing a lot of stop-and-go or leave the unit on while you’re parked the GPS odometer will keep adding a few feet here and there and throw off your reading.

You have to check it with a stopwatch and mile markers if you want to know for sure. The navigation system can’t be a speed measuring device that you should trust. There are hills and curves that keep it from being accurate.

GPS will be most accurate on flat ground. If the road is in a hilly area, GPS won’t correct for this. It will only measure a change in position (latitude/longitude) across the earth’s surface and not the accompanying change in altitude; both factor into actual mileage.

Accuracy does vary. It depends on several factors. Generally they are less accurate if you are making turns.

Most if not all newer models have a higher sampling rate so those errors are minimized. The government has also turned off some of the intentional errors so you get a more accurate result than possible previously, unless you were military.

f the road is in a hilly area, GPS won’t correct for this.

Well some may and likely do. It depends on the design of the unit. Newer ones will be more likely to take altitude into account. Actually even my old cheap hand held GPS gave me readouts of altitude above sea level. However I don’t think it mixed that information into any distance calculation.

How does it take altitude into account? Triangulation only deals with distance across the earth, not anything in between. If you were at 120W/45N and then moved to 125W/45N GPS would only know that you moved 5 degrees west but nothing about what is in between. It has no time-of-flight capabilities so it can’t measure height.

GPS units output in a two dimensional coordinate system, but have to think in three dimensions and can figure out your altitude if it’s getting a signal from four or more satellites, though the degree of uncertainty is generally about half as much more than the horizontal uncertainty. And most outdoor-oriented GPS units do include vertical travel in distance traveled estimates and I don’t see why the car ones wouldn’t, and even if they don’t you can just figure out what the elevation change is for your trip and add it to the total distance traveled.

You can also use your own odometer as a good measure of distance because they are not adjusted >to read high or low.

I don’t think entirely this is true.

Hmm, yes, every car I’ve driven has had the odometer a direct function of the speedometer with both being off by the same ratio.

That also adds two other reasons why automakers like to err on the side of the speedo reading fast-- it makes it look like the car’s getting better mileage and it runs the warranty out faster!

Triangulation only deals with distance across the earth,

Triangulation only works on a flat surface. The earth is not a flat surface and the satellites are no where near the surface. They need to think in three dimension. They may or may not compute the output using that third dimension.