The Speedometer or the GPS?

While driving back and forth to the Yellow Barn Music Festival rehearsals last week, I noticed something strange about my speed.  Since it was an almost 3 hour trip, 95% of which was on the highway, I had set the cruise control to somewhere just over the speed limit and was not really thinking about whether or not the speedometer was correct.  After a while, I noticed that a lot of cars were passing me, which was fine, but it seemed to be more cars than usual.  I glanced down at my GPS, which gives a readout of my speed, and saw that it was reading significantly lower than my car’s speedometer.

So if my car is telling me that I am going 72, and my GPS is telling me that I am going 63, which is correct?  I tried speeding up until my GPS said 72, but then my speedometer hit 80 and I got very nervous.  I do not want to be driving 80 down the highway, not because it is not safe, because it totally is, but because I do not want a giant speeding ticket.  But it was a really long drive and I really didn’t want to make it any longer by driving under the speed limit either.  What to do?

Well, I decided that, since I was driving and couldn’t really do any online research on the subject, I would check the speedometer app on my phone.  I opened it up, and it also read 72 mph.  Now there were two votes for 72 and one vote for 80, and yet somehow my bias was still to trust my car, or at least to trust the higher number just to be on the safe side.  After all, cars have been around a lot longer than GPS devices.  A few years ago this wouldn’t even have been a debate!

Finally, I decided to employ some good old fashioned math to find the answer.  I waited until I hit a mile marker and then looked at my watch.  When I hit the next mile marker I looked at the time again.  50 seconds.  It took me 50 second to drive one mile, which means that it would take me 50 minutes to drive 60 miles, which means I was driving at exactly…72  miles per hour!  The GPS was precisely correct.

So even though I was still nervous, I drove the rest of the way with my speedometer telling me I was going 80, mathematically confident that I was indeed driving at 72 mph.  But why was my car so far off the mark?  Well, when I got home I looked up everything I could find about such matters, and it turns out that cars are generally not at all accurate when it comes to speed.

Your car measures your speed based on how fast the wheels are spinning, but the wheels can have fairy different diameters depending on the tires.  Many people have tires that are not the exact size recommended for the car, and even when they are the same size, loss of air pressure and normal wearing away of the tire over time will cause the tire to become slightly smaller.  Even tiny differences in tire size can affect the speedometer reading fairly significantly.

A few millimeters of difference in tire diameter make a few centimeters of difference in the circumference, and if your circumference is off by 5 or 6 cm, and an average tire is maybe 180 cm, well that’s over 3%.  And the law requires car speedometers to be accurate within 4%, so if your car is legally off by 4% and your tires are under-inflated or worn down, your speedometer could easily be off by 7%, and 7% of 72 mph is just over 5 mph!  And don’t even talk to me if you have differently sized tires on your car.  Your speedometer could be 10 mph wrong and you might have no idea!

So when it comes to your speed on the highway, you may have no idea how fast you are actually going.  But the police do.  And so does your GPS.  Just thought you might like to know that.

Posted in Car, Driving, GPS, Math, Speedometer.


  1. I have wondered the same thing. Our GPS routinely says we’re going about 3 miles slower than the speedometer. I always trusted the GPS over the speedometer (since that meant I got to go faster); glad to know that I have a logical basis for doing so now!

    • Also, I was wondering if this could have implications for the mileage on cars. It seems that mileage would rack up faster than the number of miles actually used. So if you have a 50,000 mile warranty, could you have a legal case if your car broke down at 50,100 miles?

  2. Yes, the speedometers fitted in cars are not always correct. That is why, in speed tests conducted by car companies to test the top speed of their cars, they always fit a calibrated speedometer that measures the exact speed of the car on top of the hood to get precise numbers. That’s why, if you’re on a highway, it would be better to just consult your GPS instead of looking at your speedometer if you really want to know how fast you’re going.

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