Speed reporting off by 4 miles an hour?

[rpavich]

Hi,
I own an XL 325SE and I'm liking it...just getting used to it having never owned any GPS before....however...I noticed that the speed that it's reporting is off by 4 or 5 miles an hour...
I know that doesn't sound like much but when you're supposed to be doing 25 in a residential and it's saying that you are doing 20...and my speedo says 25...I'm wondering who to believe?

Is there a way to calibrate this?

 

[dhn]

Your gps is accurate. Your speedometer is the one that is inaccurate.

 

[rpavich]

thanks for the reply...are you sure? (don't take that the wrong way...I'm just incredulous that my speedo would be that off)

How can you be sure?

 

[dhn]

Yes, I'm sure. There are threads somewhere that elaborate on the issue. Speedometers by law may show a speed up to 10% greater than the real speed. Also, things like the incorrect tire size for your vehicle affect your apparent speed.

Here is just one thread on the issue.

And this one is a VERY long thread on the topic.

 

[a.b.normal]

I would think that a GPS would be accurate if the vehicle is traveling in a straight line with no obstruction to Satellite view(horizon),but if the GPS unit is constantly changing Satellites there is no way it can maintain consist signal for speed reference.I've noticed my view(direction) of travel will(can) change when I'm stationary and the Unit contacts new/different Sats.Also I don't believe a GPS handles elevation changes accurately i.e. going up or down a mountain and one of the satellites is almost directly behind you.So unless you are traveling straight and maintaining the same elevation ,I would not rely on the GPS unit for speed.

 

[1XL-on-XR650L]

I'm going to pitch in here because I too was surprised by the difference between the GPSr readings and the speedo in my car(s). Having gone to great pains to calibrate my digital speedometer on my motorcycle (had to replace the OEM one due to a nice endo), I was able to compare my Tomtom to it (also mounted to the motorcycle) and found that they were so close it was not worth worrying about (within .01 miles over 20 miles). Also, having access to 3 different GPSr systems, all running different software and using different chipsets (Tomtom ONE XL, Garmin Etrex Vista HCx, and a Garmin Forerunner 305), I have found that they agree on speed and mileage to well within 2% regardless of conditions (hiking, biking, motor vehicle, etc..). The speedometers in both of my cars are off by more than 3 MPH at 60 MPH, and neither of them have oversize (or undersized) tires.
If your GPS says you are going at a certain speed, believe it!

 

[Arno]

Less than manufacturer recommended tire pressure will already show higher than actual speed.

Use your speedometer rather than TomTom to reduce the incident of speeding tickets as it gives you a bit more leeway.


When we switched to metric, the excess speed at which we may collect three demerit points on our license is 16 km/h which, by the sheerest of coincidences is just about 10 mph, dead nuts.

On the Freeway that crosses Toronto the flow of traffic moves at about 120 km/h according to my speedo and, except for actually passing a police cruiser we feel pretty much OK even if one drive at the same speed in either the collector or express lanes in which ever we ware not.
If he drives slower in the other set of lanes and there is nothing ahead of him it just may be astute not to pass him.

 

[rpavich]

Yes, I'm sure. There are threads somewhere that elaborate on the issue. Speedometers by law may show a speed up to 10% greater than the real speed. Also, things like the incorrect tire size for your vehicle affect your apparent speed.

Here is just one thread on the issue.

And this one is a VERY long thread on the topic.

Lol...thanks for the links...I only had enough guts to read a few pages...but the message that came through was...trust the TT....not the speedo.

thanks very much.

PS: I used the TT for the first time in the real world yesterday...I went to a neighboring down with a lot of little side streets and the TT took me to the address' doorstep!

And one side benefit that I hadn't thought of...I enjoyed the trip more because I didn't have to think about where I was going...I could enjoy the drive and not stare at each passing street sign...

Nice.

 

[mrpogo]

I have known for a while the speedo is off in my truck by about 5miles. I figgured it out by passing one of those check your speed radar stations on the side of the road. My Tom Tom tells me the same thing so I believe its pretty accurate