GPS Accuracy For Determining Speed

[+GrnXnham]

How does GPS accuracy compare to speedometers and radar guns for determining the speed of your moving vehicle? I would guess that it would be more accurate than most speedometers but less accurate than a radar gun?

 

I would also think that GPS would be more accurate the faster you go.

[KD6QZX]

I have had the opportunity to use a Radar Speed Gun along side of a Garmin GPS V and a Quest. Both Garmin units read the same speed as the gun while driving straight. I used them on the surface streets and the freeway. Speeds were accurate both at low speeds 15-35 as well as freeway speeds of 50-75MPH. I did find some error when turning on city streets, but this is more due to doppler shift causing an error in the Radar. Hope this helps.

[+StarBrand]

The problem with speed readings from a GPS is that it isn't continous or instantaneous. It simply compares where you are now to where you were at the last reading and computes what your average speed was to get you there. Now I do admit that a modern GPS does all of that in under 1 second. That makes it pretty good but not perfect. Radar is doing that same calculation but many times per second.That is why courts do not tend to give much weight to GPS data over radar data.

[Source_GPS]

The problem with speed readings from a GPS is that it isn't continous or instantaneous. It simply compares where you are now to where you were at the last reading and computes what your average speed was to get you there. Now I do admit that a modern GPS does all of that in under 1 second. That makes it pretty good but not perfect. Radar is doing that same calculation but many times per second.That is why courts do not tend to give much weight to GPS data over radar data.

 

I agree - However, with a steady foot, my GPS and some patience I was able to recalibrate a speedometer when I changed the tire size on a car I used to own. I needed a fast way to do it, popped the dash open and took the needle off the speedometer. Coasted at 5MPH put the needle on then checked it at speeds up to 60. Worked perfect. This did take several attempts as you need a steady foot and flat terrain.

[SiliconFiend]

The problem with speed readings from a GPS is that it isn't continous or instantaneous. It simply compares where you are now to where you were at the last reading and computes what your average speed was to get you there. Now I do admit that a modern GPS does all of that in under 1 second. That makes it pretty good but not perfect. Radar is doing that same calculation but many times per second.That is why courts do not tend to give much weight to GPS data over radar data.

This is incorrect. A GPS receiver calculates speed using the Doppler shift of the GPS signals, similar to the method used for the radar gun.

[SiliconFiend]

I agree - However, with a steady foot, my GPS and some patience I was able to recalibrate a speedometer when I changed the tire size on a car I used to own. I needed a fast way to do it, popped the dash open and took the needle off the speedometer. Coasted at 5MPH put the needle on then checked it at speeds up to 60. Worked perfect. This did take several attempts as you need a steady foot and flat terrain.

I'm surprised that worked. Changing the tire size will change the ratio, so it will be more incorrect as you increase speed. It's not just an offset (which is what you did by removing and reinstalling the needle). What you probably did was to remove the offset that manufacturers put in to make the speedometers read slightly high by default, so instead it was dead-on.

[+GPSlug]

The problem with speed readings from a GPS is that it isn't continous or instantaneous. It simply compares where you are now to where you were at the last reading and computes what your average speed was to get you there. Now I do admit that a modern GPS does all of that in under 1 second. That makes it pretty good but not perfect. Radar is doing that same calculation but many times per second.That is why courts do not tend to give much weight to GPS data over radar data.

This is incorrect. A GPS receiver calculates speed using the Doppler shift of the GPS signals, similar to the method used for the radar gun.

It's done both ways, but Doppler is certainly much more common these days.

[+Renegade Knight]

How does GPS accuracy compare to speedometers and radar guns for determining the speed of your moving vehicle? I would guess that it would be more accurate than most speedometers but less accurate than a radar gun? ...

 

Your thinking is about right. You can use a GPS to determine if a radar gun is mis-calibrated, but overall the Radar Gun will be more accurate.

[+Renegade Knight]

I agree - However, with a steady foot, my GPS and some patience I was able to recalibrate a speedometer when I changed the tire size on a car I used to own. I needed a fast way to do it, popped the dash open and took the needle off the speedometer. Coasted at 5MPH put the needle on then checked it at speeds up to 60. Worked perfect. This did take several attempts as you need a steady foot and flat terrain.

I'm surprised that worked. Changing the tire size will change the ratio, so it will be more incorrect as you increase speed. It's not just an offset (which is what you did by removing and reinstalling the needle). What you probably did was to remove the offset that manufacturers put in to make the speedometers read slightly high by default, so instead it was dead-on.

 

It didn't work perfectly but it probably did work close enough. If you move the needle to be accurate at 60mph it will be "close enough" at highway speeds. The catch is a bigger tire size is a set amount off. Call it 10%. At 10 mph you read 9mph on your speedo at 60mph you read 54mph. If you adjust the speedo using this method so at 60 it reads 60 you are perfect at 60 (sort of like degrees F and Degrees C are the same 40 below). At 10 mph though you are now 6mph off instead of 1mph. At highway speeds you are now much less likley to get a ticket. Town speeds may not matter so much.

 

What used to be 10% off is now 10% off plus a set MPH reading off at all speeds that just happens to be correct at 60mph. Overall I think it's a brilliant simple fix.