Speedometer Accuracy

[+MeSteve]

Does anyone know how accurate the speedometer is on a GPSr? I dont mean compared to a vehicle speedometer, those arent very accurate themselves.

[+Learned Gerbil]

I suspect my GPS 315 is more accurate when travelling at a steady speed than the speedo on my similar vintage Ford Mondeo. However, the Mondeo speedo seems terribly innaccurate.

 

The main problem with GPS speedos is they work by comparing position over time. As the position can vary by several metres a second even when stationary, I would not trust a GPS speedo until travelling at a steady 10MPH plus. Even then, they cannot measure speed accuratly when accelerating or decellerating.

 

I was once getting very strange readings near Old Sarum where the GPS was saying I was several miles from my position, and then every second or two was saying I was several miles in the opposite direction. The track looked like someone had scribbled on a peice of paper. IIRC. the speedo said I was travelling at over 3000MPH when I was stationary.

Edited August 15, 2004 by Learned Gerbil

[moopgroop]

Can't speak for others but my 60cs correspoonds well to both our Caravan and Neon. Don't know how accurate those are but haven't got a speeding ticket yet!

[peter]

According to the Navstar GPS User Equipment manual

http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf

 

"GPS receivers typically calculate velocity by measuring the frequency shift (Doppler

shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent,

but 0.2 m/sec [0.45 mph] per axis (95%) is achievable for PPS [military] receivers. SPS [consumer] velocity accuracy is the same as PPS when SA is off."

 

I agree with the previous responses that accuracy is degraded if the speed is varying. The filtering done by the GPS unit results in a slight delay in response to changes in your speed.

[Neo_Geo]

My brother and I were going down the road just yesterday. He had his yellow eTrex, and I had my 60C. I just happened to notice the speed were displayed on both units, so I compared them... They were off from one another by a full 2 MPH!!!

 

My 60C was a full 1 MPH slower than the car's speedo. So which was most accurate? Mine, his or the car's? The world may never know.

 

ANOTHER RELATED THREAD

Edited August 15, 2004 by Neo_Geo

[+Renegade Knight]

Over time you get feel for your car's spedometer.

 

My corolla is Reads faster than I'm giong by about 10% My van is dead one and my Bronco used to read like my Corolla but after bigger tires it's actual speed is about 5% faster than indicted.

 

I've also checked speeds using radios when driving in a caravan. "Hey how fast are you going I'm reading 85mph..."

 

All in all I trust the GPS as more accurate than a cars speedometer in general.

[+EraSeek]

Well, here's the specs for my Garmin map76 which seems to be less than the 0.2 meter/sec listed above.

 

GPS accuracy:

 

Position: < 15 meters, 95% typical*

Velocity: 0.05 meter/sec steady state

WAAS accuracy:

 

Position: < 3 meters, 95% typical*

Velocity: 0.05 meter/sec steady state

[+jollybgood]

There's a school zone near my house where the police set up one of those automated radar detectors that flash your speed as you are approaching.

 

I've generally found my GPS is within +/- 1 mph whereas my van's speedometer is consisently showing me going 5 mph faster than I actually am.

 

I have an external attennae hooked up to my GPS which brings my accuracy down to 6 to 7 ft.

 

Not sure if that's a factor or not.

 

I recall a year or so ago there was a very heated debate on this subject on another website that got quit techincal (and personal).

 

In basically confused the hell out of me. All I know is based on the radar station my GPS is more accurate than my speedometer.

[+NightPilot]

Your car's speedometer is subject to many errors. Most manufacturers intentionally set their speedometers to read about 10% higher than actual to avoid liability problems. Added to that, your indicated speed depends on the air pressure in your tires (pressure difference produces different diameters) and the amount of wear on the tires (as the tread wears, the diameter decreases). Plus, the temperature makes a difference in tire pressure, thus tire diameter. Your speedometer only gives a rough estimate of your speed, but you should be able to get a feel for how it reads by comparing it with the GPS, which should, under most conditions, be very accurate. Different algorithms give different speeds, though. My Legend, connected to my Palm running Mapopolis, often gives a little different speed than Mapopolis indicates, and it seems that the Legend responds quicker, while Mapopolis just reads the speed now and then, perhaps every few seconds. It can make a difference. Years ago, I used to fly with an old LORAN unit installed, and that gave an average speed over the past 2 minutes. I can't say for sure how Garmin or Magellan does this, but I don't see a huge difference between them, and it's certainly updated more often than 2 minutes - probably closer to once per second.

 

Most police radar units round the speed down to the next integer speed - thus, 59.9 will usually read 59, but that's putting a very fine point on it. If the speed increases by .1mph, then you'll be clocked at 60.

[+EraSeek]

"Update rate: 1/second, continuous"

 

on my Garmin Map76

[gm100guy]

I have had my gps in many vehicles and watched the speed.

 

It seems to match the speedo in the cars most of the time. Except my car it show I am going 10 km slower then the car says. Thats ok it gives my a cushion when I pass radars.

 

But what do you do when the gps and the car match and you look at the vmg reading on the gps and it says you are going 10 km slower then what the speedo is showing. Could you capture this and show a judge that you were not speeding at that time.

 

[+NightPilot]

I don't think any judge is going to buy that. It's just your word that the GPS was reading your speed when you were caught, and that the image was captured before you slowed down. It ain't gonna fly, Bubba.