Speedometers and GPS units

I’ve noticed that the speedometer in my car says I’m going faster than the speed reportedstopwatch by my GPS.  On reflection, this makes a lot of sense, and the GPS is almost certainly right.

First, one very good way to compute speed is to have a very good clock and to know more or less exactly where you are at time1 and at time2.  You then compute the distance from where you were at time1 to where you were at time2 and voilà! S = D / T where S is speed, D is distance and T is time.  To do all this you need the ability to make a few simple but reasonable precise measurements, including knowing the (more or less) exact time and your more or less exact location (Heisenberg be damned!) and the distance from one point to another.  All of this is what a GPS does for a living, and it is absurdly exact; able to detect your location within a radius measured in a small number of feet and able to detect the time to an absurdly accurate degree.

Your car, on the other hand measures your speed quite differently.  It measures the number of rotations of the tire over a set amount of time.  The clock involved is none-too-accurate and the computed speed can be thrown off by any number of mechanical factors (not least is using the wrong size tire).

Thus, I speculate, the GPS, by its nature, is more capable of finding the correct speed.

What is more, I think car engineers know this and they allow for it.  Their thinking must go something like this: “if the car shows a speed that is higher than its actual speed, then the worst that will happen is that you will not get that speeding ticket and if you crash the impact will be slightly less, but if the car shows a speed that is lower than it actually is, then you are more likely to get a speeding ticket and more likely to get hurt in an accident than might otherwise be expected, all of which is likely to come out in court in a very expensive law suit.”

Thus, I suspect, as a matter of good engineering, the automobile manufacturers make up for the inherent inaccuracy of their speedometers by tooling them up by a couple miles an hour  — better to be going slower than you think than faster.

And this fits with my experience; the speed measured by my car is not only consistently different than the speed measured by my GPS, but the car’s measured speed is typically 2-4 mph faster than that measured by the GPS.  This is across a couple dozen rented cars and both my family cars. 

Your mileage, speed and experience may differ

I note all of this because I think GPS navigation systems are the most magical devices that I use on a regular basis; including computers, dvr’s, cell phones, etc.  We are surrounded by the science fiction of a previous generation, but the GPS totally blows me away.

More on GPS soon.

Trivia question of the day: how far away are the satellites used with GPS? and for extra credit, who was that area of space named for?

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About Jesse Liberty

Jesse Liberty is an independent consultant and programmer with three decades of experience writing and delivering software projects. He is the author of 2 dozen books and multiple Pluralsight courses, and has been a Senior Technical Evangelist for Microsoft, a Distinguished Software Engineer for AT&T, a VP for Information Services for Citibank and a Software Architect for PBS. He is a Xamarin Certified Mobile Developer and a Xamarin MVP, Microsoft MVP and Telerik MVP.
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11 Responses to Speedometers and GPS units

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  2. Torben says:

    @Jesse …
    You are absolutely right about the car manufactures tinkering with the speedometers …
    When you watch the speedometer in a non-moving car. It shows 20 km/h here in Europe …
    (i’ll leave it to you to do a miles conversion).

  3. Peter Wone says:

    GPS satellites orbit at a variety of different altitudes. Some are geosynchronous, some are not. It’s all about coverage. There are roads outside of North America.

    As a curious aside, something that surprised me was that the United States invented almost no aspect of the automobile. Almost all of it came from Germany, some from Britain and everything else from assorted European countries. What the US gave the world was mass production — and GPS.

  4. TJ says:

    Any idea how much the inherent precision (or lack thereof) of some GPS units affects the reported speed? Most of the automobile GPS units I’ve worked with are accurate to around 20 – 30 feet. My cell phone GPS typically shows me within 50 feet or so of my actual position but is occasionally wrong by as much as 10 blocks. (On the other hand, the survey-grade GPS units my company operates are typically accurate to within an inch or so.)

  5. It’d be nice to think car manufacturers care about whether or not you get a ticket but more likely they just want the odometer to run faster and your warranty to expire quickly.

  6. CarlD says:

    They are in Medium Earth Orbit at an altitude of 20,200km. This belt is known as Medium Earth Orbit – not named after anyone.

  7. Tim says:

    I missed one possibility: when you’re doing the comparison are you following a straight line? If you’re changing lanes, or veering off a straight line and then back, your car/tires would be traveling a greater distance than would be apparent to your GPS, thus the GPS would report a slower speed.

  8. Tim says:

    To speculate, I can think of a few possibilities. The easiest is the one you already mentioned: tire size — I wonder if this could even vary in a single trip as the tires heat up after driving on the highway for several minutes. The other thing you said was that GPS is accurate down to a couple feet — could this be enough to throw it off? Next, since you’re not receiving GPS updates several times a second, I think the phone will do some interpolation of your position, which could throw things off unless you’re on a cruise controlled speed. Finally, maybe your speedometer just needs calibration 😉

  9. Pete Barber says:

    Intriguingly I’ve noticed the opposite on by cycle. The GPS app. on my iPhone consistently rates my average about 1MPH over my Cycle computer. I like my iPhone’s GPS:-) However, one trip it stated my max. speed was 48.5MPH. I think I would have remembered that!

  10. Gary Paul says:

    They are in geosynchronous orbit at 26,199 miles. It is known as the Clarke Belt after, Arthur C. Clarke.

  11. Corey says:

    Every time I use a GPS and I am given my route, I think back to my graph theory classes and shortest path algorithms. Every time. And, I wonder how the programming looks. I am fascinated that my phone can tell me where to go. I am walking around with a tri-corder that would shame those used in Star Trek.

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