Just took a 630 mile roundtrip last weekend using my Garmin Streetpilot III GPS. I tried different speeds and here are the results...all tire sizes, sprockets, etc..,. on my bike are stock. I always thought I was going slow when I was on the freeway.

Speedo GPS
30mph 28mph
45mph 39mph
60mph 54mph
70mph 65mph
80mph 72mph
90mph 81mph
120mph 108mph

A little bit of a discrpency I would say. By the way, the GPS is very accurate as I have tested it with a police radar gun (adavantage of being a firefighter with friends) and have noticed only about a 1mph offset.

 

The GPS is MORE accurate than the radar gun. Most GPS units are accurate to within .1 mph.

 

I have always felt that I was not going as fast as my speedo said I was going, espeacialy in traffic on the freeway. How do we get them calibrated without spending a fortune at the dealership?

 

All speedo`s are set to over read they have to be set this way by law, normally up to 10% overread.

 

In the UK, on 04 model 1000rr's there was a recall on the speedo. Check with your main dealer about this.

 

Buy a Speedo Healer http://www.speedohealer.com/eng/intro.htm
Brad from Suprebike Supply should be able to hook you up.

 

At best, GPS is accurate to something more like 0.25 mph...and that's only under certain conditions. On average, the accuracy is probably more like 1 - 3 mph.

Recreational GPS receivers are only accurate for velocity if you are travelling in a straight line at a constant velocity for, say, several seconds. This is because all recreational GPS receivers don't actually measure velocity directly. They compute velocity by differencing position from second to second. This method has several deficiencies:
- Because its differencing positions to determine velocity, your velocity errors are very dependent on your position errors. Most recreational GPS receivers have a position error on the order of 15 meters or so. You can get this down to <5 meters with a real-time differential GPS unit or a WAAS-enabled unit.
- The sampling rate is usually at 1 Hz (once per second). A motorcycle's speed can change drastically within that 1 second.

Couple these two together and you will have a large error in velocity. You can see this when you ride. Notice how on a twisty road or a quick change in acceleration (either throttling or braking), there is a bit of latency (up to a few seconds) in the GPS and it takes some time for it to catch up to your speedo. During that time it can be off by as much as 5 - 10 mph.

The only way to get more accurate velocity data from a GPS receiver is to use a GPS reciever that measures carrier phase and/or delta ranges (doppler beat). I don't know of any recreational GPS receivers that do this. It would also help if the receiver has a higher sampling rate like, say, 10Hz or 20Hz as compared to 1 Hz.


Radar is much more accurate for velocity under those dynamic conditions. This is because it uses much more accurate phase ranging techniques (doppler beat). The physics is in its favor. Under any dynamics of the vehicle being targeted, it will be to about 1 mph.

[/geek]

anyway, my point is that GPS is not as accurate for velocity as you think it may be.

 

mang...abad is a dork...what? does he work at a GPS company or something...
oh yea...he does...

 

Radar certainly isn't what the cops would have you believe either -- it much prefers bigger stuff far away over the little stuff that's closer. It also has issues with twisties. I'll agree GPS is a pain if you want real time results -- the 1 sec delay bites when accelerating. The .1 mph is from my old GPS's manual -- this unit wouldn't show speed at all unless it had a good lock. Also a pain because going under bridges would kill it until it got enough signal again.

 

u can trip a radar buy moving it across a brick wall, defonatley not a sofisticated piece of machinery.