My $0.02 worth based on using GPS since 1993 for work.

A GPS is updating its position a couple of times a second and thus changing its estimate of your speed. Most newer GPS's keep a running average of your speed  in the display rather than your actual computed speed. Some older models (and newer, high precision models) show instantaneous speed. Inst. speed will jump around more and gives people the misconception of inaccuracy.

If you are running a constant velocity your reading will be fairly accurate.

The speedo on the GS seems to be pretty precise. The displayed speed for a given RPM on the tach is very repeatable. Therefore if you can borrow a GPS or hit the cop up for a one time calibration, you can rely on your speedo.

I think I am slipping into my accuracy vs. precison lecture, better quit now. :lol:

Hmm, I may have missed it, but I'm surprised no one has said anything about bicycle speedometers. I put one on my bike and it's pretty dead on accurate. You just secure a magnet to the wheel and the little device picks up how many rev/sec and caculates speed. Since it's all electrical and digital there's no mechanics to mess w/ the results.

There are better instructions/pics on Joris' website (http://www.bikepower.net/gs500e-power/modifications/appearance-related/speedometer/speedometerpage1.htm). I can post pics of my set up if any one is curious, it's nothing fancy. I got the bike speedo off eBay for <$15 and a better magnet at Radio Shack for ~$1. Anyway, it's a cheap fix if it bugs you knowing your speedo is off, but mine is a consistent 10%ish like the rest of you guys.

Now to find a way to reconstruct the whole dash and get rid of that worthless speedo....

trey

Be careful what you wish for with cops.  I asked one to use his "gun" on me...next thing I knew I'm starring down the barrel of a 45 cal. Sig Sauer in one hand and a 50,000 volt tazer in the other :guns: .  He thought it was hilarious...me...not so much.  He's a funny guy...I guess it was the 50 Krispy Kreme boxes I stacked on his porch that made him angry at me...  :dunno:

I have a buddy that is planning on putting  a Mountain bike speedo on his bike.  It works well although I don't think that you will get very accurate results at high speeds.  

I use a GPS all the time for checking my speeds.  They are very accurate if the speed is held.  Also I believe that most GPS's will not account for change in elevations in your speeds.  This will only matter on big hills.

Quote from: jakethesnakeI don't think that you will get very accurate results at high speeds.

Just out of curiosity.. why do you think that? Mine displays up to at least 105mph, haven't tried for more... But I think it's accurate. The little magnet passes the sensor... all it does is send a signal to the computer. I'm sure it can keep up just fine. The little electrons are moving faster than my wheel is spinning.

Anywho. Just wondering if you knew that it was inaccurate or that's just a guess..

trey

I have had problems with my mountain bike and it staying accurate.  The sensor is just two think plates that when the magnate passes they touch and allows a current.  On my mountain bike at higher speeds on ruff roads this connection chatters sends false signals.  On the newer models this is less of a problem but I would expect at high speeds that the same thing can occur.

Quote from: jakethesnakeThe sensor is just two think plates that when the magnate passes they touch and allows a current.....on ruff roads this connection chatters sends false signals.

The sensor sounds like a common reed switch. This is not a reed as in a woodwind instrument or a plant that grows in wet soil.  It is simply two metal contacts, sealed in a glass or plastic envelope, that are magnetically sensitive. The contacts either open or close depending on the design. Many older burglar alarms use this type switch. A better magnetic switch that is solid state, and completely immune to vibration, is a hall effect switch. The only problem a hall effect has is the need for a power source.

Most simple speedometers for lower cost exercise equipment use a reed switch. They are fairly reliable, but as evidenced here, are sensitive to vibration and shock. There is no cure for this and I suspect that at high speeds on a bicycle the last thing you are worried about is a little error.