Speedometer vs GPS speed readout


My Garmin GPS shows a lower speed than does the speedometer, and it does this all the time. Is the GPS the most accurate?

Probably, car speedometer’s are usually set to read anywhere from the true speed to a little higher than the true speed. They never seem to read less than the true speed. The diameter of your tires is the biggest factor in the accuracy of your speedometer. The diameter will vary with inflation pressure and tread wear. As the thread on the tire wears down (due to use), the speedometer will read higher and higher than true speed. With brand new fully inflated tires, it may be about right.

If you have factory recommend tires on your vehicle (which would give you a correct speedometer reading) I highly doubt your GPS is more accurate

With tire wear my guess is your talking 10ths or 100ths of a mph difference

If you have factory recommend tires on your vehicle (which would give you a correct speedometer reading) I highly doubt your GPS is more accurate

Every make and model car sold in America that I know of come intentionally adjusted to over estimate the speed of the car. They generally are very accurate when it comes to the mileage, but not the speed. The reason is simple. They don’t want to end up paying speeding tickets or getting involved in lawsuits for accidents if the normal manufacturing variations would show a speed slower than it really is.

Modern GPS units today are very accurate. Some years back they were intentionally inaccurate and that inaccuracy varied. The military did not want the bad guys (well the guys they think are bad) using GPS information. Today there are too many satellites from other countries to bother dumbing down out consumer GPS units.

If you want a second opinion, measure a route on the interstate and carefully measure the distance using a GPS, the odometer and the while mile markers on the free way (small white signs with numbers and one decimal.) The GPS and the white number will be equal with the odometer off.

Copy that. Thanks for the info.

They are both toys and they are both useful sometimes. Roads are not straight either. That should explain any differences.

My car does the opposite of yours, reads 60 when the GPS is reading 61. This is contrary to what some posted here. I’d go with the GPS too as being more accurate and often use it as a speedometer because it is more at eye level at the top of the dash.

Probably the GPS is the most accurate. Many cars have a speedometer inaccuracy of 1-2 MPH. If you’re driving on any kind of a significant grade, I think it can fool the GPS though.

I would have to say the GPS is the most accurate, but it’s not perfect. I haven’t seen an accurate speedometer on a car yet.

When GPS was approved for civilian use there was an intentional degradation of the signal, Selective Availability (SA), limiting position accuracy to around ~150 - 200 ft. SA was turned off in May 2000 and accuracy improved to ~50 ft.

Some GPS receivers use WAAS (Wide Area Augmentation System), this improves accuracy to less than 10 ft.

Garmin and Trimble have simple GPS tutorials



Ed B.

We had 2 K-cars in our family 25 years ago and their speedometers read 3mph slower than the actual speed with the original tires,

This is normal. Wife’s 2010 Camry speedo is 2 mph over the gps. My 2010 Prius is 1 over.

The GPS should be accurate. Most car speedo readouts are close but not as accurate as GPS. My '04 T’bird reads 4 to 5 mph higher than actual on factory wheels and tires. My '03 Civic is spot on accurate which is normal for Honda.

I would think that your speedometer would be accurate (so to speak) in the short term and would reflect quick changes in velocity, but unless it was calibrated might be consistenly off by a certain percentage. A GPS would be more accurate in the long run (again, so to speak) for a situation where the speed of the car is held constant, but, again, that would be an average speed.

GPS would be less accurate. It is at best case a 2 meter level of accuracy. That is 6 ft ± in one direction, 12 ft overall. If you have ever had a higher precision gps you will notice the arrow when at a stop is going all different directions. If you look at points recorded it will look like a shotgun blast at a target, ie a whole number of points around the bulls eye. GPS is good even with waas, but I would rely on my speedometer.

It depends. How much hill driving are you doing? GPS measures distance traversed across the earth, but does not include the ups and downs of hill driving. It will be accurate if you drive on a flat path. Most of my driving with the GPS is on flat land, and I find that the GPS and speedometer provide the same speed reading.

I beg to differ, GPS determines position in 3 dimensions, otherwise it wouldn’t be much use for the aviation community. The Wide Area Augmentation System (WAAS) was developed to improve the accuracy of GPS to the point of being able to land aircraft in instrument only conditions. When landing aircraft, the vertical position is more important than the horizontal position for reasons that are obvious.

However an aircraft GPS receiver is more accurate (and more expensive) than a car receiver. I wish my task lead was here, he could explain this a whole lot better than I can.

Ed B.

WAAS and LAAS are expensive systems, and I wonder if they are available fro automotive applications. And triangulation from 3 satellites might provide the capability to provide location in 3D space, but again, I wonder about the expense to implement the system for cars.

Anything is possible with the right amount of money.

But, if your using a $200 Tom-Tom or Garmin, I’d put bank against the speedo to be more accurate. Unless your not using the original-sized tires.