Accuracy of speed on GPS units

[+GrateBear]

Just wondering what your thoughts are on the accuracy of the speed read-out on GPS units. I have a Legend Cx, and as I was zipping across the endless state of Nebraska today, took a look to see what the speed was. It said 75.4 mph, while my speedo read 81 mph. The car has OEM wheels and tires. I also used the stopwatch function to check it, and that came out to 76.0 mph.

Has anyone else ever done a comparison?

Thanks.

[+Cpt.Blackbeard]

I've found mine to be what I consider extremly accurate, I've checked it in at least 6 different vehicles so far.

However temporary false readings are always possible so I'd check yours again on several different occasions and see if it's an anomaly or consistently reads higher. You can also check the total milage against the milemarkers and see how close it stays, my last pickup appeared to be dead on in one mile but by the tenth mile was off by 1/10th.

[+jpred1]

From all my test/playing, the gps is waayy closer than my vehicals speedometer.Some cars will read higher, some lower.

[+Rattlebars]

You didn't say what kind of car it was. It may have saved you from some speeding tickets along the way.

 

I have a Magellan Meridian Color.

 

I check it against my Honda GoldWing speedo and the Wing speedo is always 5mph faster. Most Honda's are, but I don't know about the cars. I should note that on my GoldWing I have a Sigma 800 digital bicycle speedometer http://www.rattlebars.com/mtz/valkyrie.html#SIGMA which is DEADLY accurate because I measure the circumference of the wheel and enter that figure in the computer. This is the same setup that consumer reports uses to measure fuel economy (the bike wheel clamped to the rear bumper). My GPS matches that exactly and it shows 5mph slower than the bike speedo too.

 

I've compared it with my quad and it's right on the money and I've also check it with my Avalanche and it's also right on the money.

 

[+fastforty]

I'd trust the speed that the GPSr is telling you WAY before I'd even think about trusting the peice of junk in the middle of your vehicle's instrument cluster. The best way to know for sure is with a tachimeter, often called a "chronograph" on a watch. You'll need an accurate mile marker- do not trust your odometer (in our Camry, the odometer is *dead on*, but the speedometer is 6% off). In absence of a watch with one of those funny looking dials around the bezel, just set your cruise at 60mph & it should take *exactly* 60 seconds to travel one mile.

[Dj Storm]

...

in our Camry, the odometer is *dead on*, but the speedometer is 6% off

...

 

That's what I also found out in all the cars I have traveled with (4 or 5 cars, different models, different manufacturing years). Sometimes the speedometer was off by 10% (55 km/h on the speedometer, 50 on the GPSr), usually was off by 6-7%, but the odometer on the car was the same with that on the GPS.

Rumor says the car manufacturers do this on purpose, for the drivers not to go way over the posted speed limit.

[+wookie-athos]

It's not unusual for car speedos to be off, and I definitely trust my Legend's speed indicator more than the car. There can be "blips" in the GPSr speed (especially as you go around corners and amongst tall buildings/canyons) but they average out well.

Even when your GPSr is reporting an accuracy in the range of 10m, the reported position doesn't jump around quickly. Over the timeframe that the position varies by, the speed calculations should be accurate.

 

For years I've set the car's cruise control using the GPSr as a guide, and it's been confirmed as accurate through radar traps (friendly folk driving the radar wanted to know the answer too!). Now when I see a police car I don't jab at the brake pedal: I just cruise through at the speed I've set (although I do glance at the GPSr again to make sure the cruise control hasn't drifted).

 

It's important to note that the amount by which the GPSr and speedo are off usually changes at different speeds (I've seen the speedo be off by 10% around 60 km/h, but be almost dead-on by the time you get to 120 km/h - and I've seen them the other way around also!).

[+briansnat]

All my units have been very close to what the car's speedometer shows.

[+ICHTHYS]

I have checked a couple different GPS by Lowrance and Garmin. I have checked them against the calibrated speedometer in my police car as well as the calibrated RADAR in the car. I have found the speed readings of the GPS to be very accurate.

[+Wadcutter]

I have checked a couple different GPS by Lowrance and Garmin. I have checked them against the calibrated speedometer in my police car as well as the calibrated RADAR in the car. I have found the speed readings of the GPS to be very accurate.

Done the same thing here. What I found tho was the GPS unit has a greater lag time than radar/lidar. That being when the speed is reduced quickly the GPS will take a bit longer to get the correct speed where radar/lidar is pretty much instanteous. I also found to get more erroneous readings with the GPS. I've never gotten a reading of 993 or anything close to that on a radar/lidar unit but when driving all day with any of my 3 GPS units it's almost a given that there will be a dramatic spike in the reading often in the 900 mph ranges.

[+trainlove]

Well, speed as figured out by GPS is not instantaneous speed, it's interpolated speed based on readings taken a second apart with both of those readings having +/-30 meters accuracy. So of course GPS speed is not totally reliable.

If you want your actual speed, install a radar gun under your car pointing at the ground at a shallow angle and multiply that speed by the cosine of the angle.

[+ICHTHYS]

I have checked a couple different GPS by Lowrance and Garmin. I have checked them against the calibrated speedometer in my police car as well as the calibrated RADAR in the car. I have found the speed readings of the GPS to be very accurate.

Done the same thing here. What I found tho was the GPS unit has a greater lag time than radar/lidar. That being when the speed is reduced quickly the GPS will take a bit longer to get the correct speed where radar/lidar is pretty much instanteous. I also found to get more erroneous readings with the GPS. I've never gotten a reading of 993 or anything close to that on a radar/lidar unit but when driving all day with any of my 3 GPS units it's almost a given that there will be a dramatic spike in the reading often in the 900 mph ranges.

 

Yes very true. RADAR is not off, but the GPS can give erroneous readings at times. But for the most part it gives an accurate account of speed at a steady pace.

[+J-Way]

I'll add my experience to the others', although it isn't worth as much as someone who checked their GPS vs. a calibrated police cruiser and RADAR. Actually, it doesn't matter who's more accurate in that case; the police radar is right

 

I drive rental cars a lot, usually several times a month for several hundred miles each trip. My Garmin GPS varies from the odometer by as much as 3-4MPH at interstate cruising speeds, and it's different in each vehicle. I haven't tried stop-watch calibration yet, though.

[+StarBrand]

Assuming a good lock on the sats - not much reason for it to be off if you are driving at a steady rate. As somebody mentioned - it is not good for rapidly changing speed. Should be very close to your actual.