Speedometer accuracy



I recently purchased a fairly high end portable GPS device for the car. One of its features is a display of vehicle speed. At highway speeds it often reads about 4-5mph slower than my car’s speedometer. Any idea which is more accurate? My car’s speedometer is analog and the GPS is digital.


make sure your tires are at proper pressure. every instrument has an error plus/minus some value…your differences seem to be within that range of about 5% error…check using highway mileage markers and your GPS is only as good as the number of satellites being referenced.


Your GPS is correct. It’s real Science.
My dad was a traveling salesman starting in 1950 and he showed me that his speedometers over-registered by four or five mph at sixty mph. So I?ve been checking every vehicle I?ve owned starting with a 46 Dodge panel through my current 94 Civic and 94 Ford E-150. A total of about twenty-five vehicles. They all over-registered. I thing the 64 VW beetle was the least at 3 ? and the E-150 the most at 4 5/8 mph.
I use the mile markers on the side of the highways. I think I know how carefully they are surveyed. In California, they mark every culvert and bridge with a mile marker, measured from the county line, IN TENTHS OF A MILE!
So when I blow down the Interstate registering 80 in a 70 zone I?m really going 74, only 4 over, and I?ve never gotten a speeding ticket.


What kind of car is this?
I have a Honda Civic & am debating slightly larger tires to rectify this same situation. The tires that came on the car (bought used) are el cheapo & noisy but are stock size. Yes, they are properly inflated too.
As previously mentioned, this is within the “slop factor” so it’s only a minor annoyance. It might save a few people from tickets though.


The GPS is not exact (there are several reasons) but it is almost certainly more accurate than the the speedometer.

The speedometer is intentionally set to read about 3-5% high. That is to error in the direction of saving you a ticket. In addition it is subject to tyre wear and someone putting different size tyres on it while the GPS is not.

Some speedometers are easy to calibrate but most will cost you more than you would likely want to pay and then would become inaccurate for the reasons listed above.


GPS units are not as accurate as you might think… Get on the highway and reset your trip odometer at a mile maker and note it’s reading at the next mile marker…
.9 miles = 10% error, .8 miles = 80% and so on…


It’s probably a combination of the two. Most car speedos I’ve seen (assuming the correct tires and pressure of course) are usually only about 2 MPH off at the most and that’s at highway speeds of around 60.


We had an 84 Mazda that read 64 when the car was doing 60. After numerous phone calls up through the Mazda chain, I finally got to Mazda’s USA technical reps. They told me Mazda’s specs were +/- 7%. And since 4mph/60mph was less than 7% off, it was well within spec.

My observation in the 23 years since then is that many manufactures have at least a +/- 6% tolerance on their speedometers.

Follow the advice of the replies on this board and use the highway mile markers to calibrate both your GPS and your car’s speedometer.



Just to cloud the issue a bit more… I’ve used my gps in my neighbor’s Jetta. It agrees with that speedometer.
I expect a little error in all things man-made, and that surprised me.


Clarification on my reply:

My observation in the 23 years since then is that many manufactures have at least a +/- 6% tolerance on their speedometers.

I meant to say +/- 6% maximum allowable tolerance in their specs. In practice, most cars are off by less than the maximum tolerance.


Each device (GPS, speedometer, radar gun) are NOT 100% accurate. They are built to a certain tolorance.