Speed Accuracy In Gps Receivers

[+dandrewk]

Greetings, first time poster here.

 

I have recently purchased a Legend C, primarily to use on my motorcycle. I have observed a discrepancy of around 5 mph in the speedometer on the bike vs. the speed readout on the GPSr.

 

At first, I just assumed the speedometer on the bike was off, but today I rode in two different cars and the speed was off by around 3-5 mph. Pretty consistently, and always the GPSr reading was lower than the speedometer. IOW, the GPSr showed 61 mph when the car's speedometer showed 65.

 

1. Is this a normal inaccuracy for GPSr's in general, or the Legend in particular? The positioning on the map is dead on, so perhaps the GPSr miscalculates the actual speed.

 

2. Or is it because car and motorcycle makers under-calibrate the speedometers to help keep speeds down. e.g. if you think you want to exceed the speed limit by 10 mph, you are in reality only exceeding it by 5 mph. Fewer tickets, fewer accidents, better gas mileage.

 

This is no biggie as I always have a speedometer available. Which is the accurate reading - the GPSr or the speedometer? It seems to me that the GPSr should be the accurate reading - far fewer variables than a speedometer. But I could be wrong.

 

Hope this makes sense. Thanks for any info.

 

Dave

[+fizzymagic]

1. Is this a normal inaccuracy for GPSr's in general, or the Legend in particular? The positioning on the map is dead on, so perhaps the GPSr miscalculates the actual speed.

 

2. Or is it because car and motorcycle makers under-calibrate the speedometers...

Your GPS speed will be quite accurate.

 

The laws are written so that speedometer errors are asymmetric. It is OK to have them read too fast, but not to have them read too slow. As a result, manufacturers tend to make them read just a little bit too fast.

 

The GPS speed is usually good to < 1 MPH.

[+Olar]

A GPSr will however lag behind about 1 second so if you are doing a rapid speed change you will get an inaccurate reading. A good example of this is when you come to a complete stop at a traffic light you will see the speed reading on the unit continue to drop to zero for that 1 second or so.

 

Cheers, Olar

[+Sputnik 57]

GPSr's don't use distance divided over time to give you speed. If they did, a 10 foot position error would make them too inaccurate. They use the doplar effect, and as a result, are accurate to with less than half a mile per hour.

 

Manufacturers overestimating your speed keeps them from getting blamed for your ticket, and has the added advantage of causing your warranty to run out more quickly. Class action lawsuit, anyone?

[+IV_Warrior]

1. Is this a normal inaccuracy for GPSr's in general, or the Legend in particular?  The positioning on the map is dead on, so perhaps the GPSr miscalculates the actual speed.

 

2. Or is it because car and motorcycle makers under-calibrate the speedometers...

Your GPS speed will be quite accurate.

 

The laws are written so that speedometer errors are asymmetric. It is OK to have them read too fast, but not to have them read too slow. As a result, manufacturers tend to make them read just a little bit too fast.

 

The GPS speed is usually good to < 1 MPH.

Actually, I think it's more a function of Odometer readings than Speedometer readings. The odometer has to read AT LEAST the actaul mileage, and can only be over by a certain amount.

 

Since in most (all?) vehicles the speedometer and odometer get their signal from the same source, the error spills over to the speedometer, as well.

[+Night Stalker]

Every vehicle I have driven has been off from 3 to 5 mph from my GPS except the 92 Honda Accord I am currently driving. It was right on.