my bike is -1 +2 geared and speedo healed i had her to 142mph at about 10500rmps, she would have went faster but that was close to her limit

 

I've worked on speedometers, GPS, and even traffic radar in the past. A GPS isn't really inaccurate, but the issue comes from its sampling rate, screen refresh rate, and Position Errors. Holding a steady 60, for instance, then checking the GPS will display 60, no problem. But if you blip up to 65 and then quickly slow back to 60, the GPS may never register it depending on the above factors. That's why, unless you get a GPS with high sampling rates, high screen refresh, and have a good, strong signal they aren't very good as standalone speedometers.

 

I know I keep posting this image, but the discussion keeps coming up.



Speedo read something like 174 (I didn't exactly take a long, thoughtful look)

Oh yeah, I was in full tuck as far as I could be but my camera mount was installed on the tank so I couldn't quite get all the way under the wind. I was wearing a mesh jacket, a backpack and jeans. I would say it's got another 2-3mph in it just from wearing leathers and removing the camera mount. This is stock height with stock front turn signals and mirrors, stock pipe, stock air filter.

*edit: yes, I was wearing my full-face motorcycle helmet too :P

 

Never intended to imply that a GPS is a great speedo. You are correct in that the sampling rate is usually a smidge behind actual. However, that does not mean the GPS is not accurate. If it has no signal (the Garmins at least) then it will not register anything. If there is a signal then it is accurate to 1/10 of a knot which is equal to .115077945 mph. This is more accurate than radar or the laser.

When using a GPS to verify top speed or to set a speedo healer you have to account for the small lag by holding the speed steady for at least 1 second. This gives most units a chance to refresh twice.

If you're trying to gauge acceleration or 1/4 mile times then a GPS is practically useless ... you'll get a general idea but the numbers don't move near as quick as a CBR will

 

Hit 172mph on the speedo indicated down the front straight at Road America...-1/+2 520 chain conversion.
I figure about 150-152 maybe........cant remember engine rpm though

 

Anyone ever try timing mile markers? Find a straight, flat stretch of highway, take your bike up to speed, and time how long in between the markers. It's not exact, but nothing is, and you don't need a GPS or radar gun to do it. All you need is a steady throttle hand and a stop watch.

 

Love you long time dude but there are a couple issues:

1. Mile markers indicates public roads. I can not encourage that
2. If you truly "at speed" then taking a split second to hit a timer is extra risk.

I have used mile markers at 60mph to verify odometer and speedo but for high speed stuff I only ever used GPS. But the mile markers are pretty good for checking 60mph -- takes exactly 1 minute

 

Timing would be accurate. The errors would be two fold--how accurate the measured course is and how accurately you can start and stop the timer. At 150 MPH, that's 220 feet per second. A half second error at each end--easy to imagine at that speed--would yield 440 feet over the measured mile or about 8 percent. So at 150, 12 MPH plus or minus. A lot.



GPS is very accurate. Even street navigators are using once-per-second positioning, I think, so the errors at 150 MPH would be trivial. In aviation, we obviously see speeds much greater than this at 5 per second updates. Accuracy is down in the tenths.

Above, someone posted a GPS-derived speed of 157. I'm surprised at that, but I'll accept it as accurate.

--Paul

 

Not recommending the stop watch over GPS. It's just something that most people could do. As for the timing, the more consecutive miles you can time, the more accurate it's going to be. If I were to do it, I would do several 5-6 mile runs and average them. But I have GPS, so it doesn't matter for me anyway.

 

True dat You get 5 or more mile markers to play with and you can fairly accurately dial in your speedo error. The only problem being you have to keep a constant speed ... which is sometimes difficult due to cagers