What is more accurate? GPS or radar?

[Supercharged111]

You know what I do for work, stuff we use is accurate in inches. I laughed when they told me that when I first started, then they showed me.

The newer stuff is also tough to jam. Pretty wild when you figure how insanely weak the signal is.

 

[rfpowerdude]

Here is an open source project to make centimeter accuracy available for us mere mortals.

RTK Rover

Centimeter-accurate stand-alone RTK receiver available in two practical form factors, leveraging a robust decentralized geospatial network.

www.crowdsupply.com

 

[Bonnewagon]

I contacted some enginerd cousins of mine. The one that works on radar agreed the courtsey radar to be the most accurate for real world speedo testing. He said at best the GPS will be off by at least 1 or 2 mph, and you MUST maintain speed during testing or it will be wrong. And even he suggested the mile marker method because he is so old school. But then another cousin says that in PA for a small fee the State Police will put your car on the rollers and give it the same speedo test that their cruisers use. His totally stock car was off by 4.5mph at 70mph. That is a speed camera ticket in the mail.

 

[Rt Jam]

I would take GPS as the best method just based on the fact it's ground position vs time. The only chance for inaccuracy here is the variance in YOUR speed during the test of the refresh rate of your equipment. Which is zero concern if you are going at a steady speed.
Radar is a signal hitting a moving object and calculating the speed.
Measuring on rollers is theoretical since we do not drive on rollers, we drive on flat ground. So maybe there is a correction factor involved there.

Apps with smart phones will get it closer than you will ever need for a speedometer.

 

[Supercharged111]

One thing worth mentioning on the radar is the fact that if you aren't driving head on for it, your calculated speed will never be perfect. Think of it as a triangle: the angle from you to the gun is the hypotenuse of a triangle whereas your true bearing is the second longest side of it.

 

[Hurricane77]

Go to straight, flat stretch of Interstate. Set the cruise at 60mph. Take your phone, turn on the stop watch.
Time yourself driving between the mile posts.
Time greater than 1:00min your speed-o is reading to fast
Time less than 1:00, you speed-o is reading too low

This. Except use the GPS speed.

GPS will be bad at giving you an instantaneous speed, but if you're going at a constant speed, it'll give you a good accuracy.

Roadside radar will have some inherent errors, about +/- 1 mph or so. And as someone else mentioned, it has to be corrected for the offset to the side. This error will vary based on how far you are from the radar itself. Error will get worse as you get closer.

Frankly, either will be good enough to calibrate your speedometer within a reasonable margin of error. You may get your speedometer calibrated 'exactly' on a particular day at a particular speed. But as you vary from the speed, you'll likely see it shift from being exact. Go out on a different day and your tire pressure is a bit different because ambient temp is hotter or colder than they day you calibrated? It'll be off a bit again.

Don't over think it.

 

[69hurstolds]

I don't know what GM is using for their speedometers nowadays, but my digital speedo readout in the 2020 truck matches spot on with the Garmin GPS dash cam that shows MPH at 55 speed limits and below. The calculation differences amplify themselves I guess when you're at 80 because sometimes it shows 81 or 80 on the GPS cam while my heads up display shows 80 on cruise. This may be rounding errors or whatever, but I was pretty impressed considering it's just a dash cam.

 

[Bonnewagon]

You know the sad part about all this is that I am driving while looking up at the trees searching for cameras. I have been bitten a few times and I am determined to not get bit again. When I spot one I now slow down to way below the posted limit [25mph here] just to be safe. It sucks but a $50 ticket sucks more. I don't think the 10mph grace is working in every camera. Yes, you read that right, NYC DOT claims you don't get a ticket unless you are 10mph over the limit. BULL PUCKEY!!!