Speed accuracy?

I've compared the speed of my GPS to my three vehicles, which are a 1993 Yamaha Virago, a 1996 Nissan pickup, and a 1998 BMW 528i. The speedometers of my motorcycle and my truck show exactly the same speed that my GPS shows. The BMW's speedometer shows that I'm driving five MPH faster than what the GPS shows.

I still don't know which of the speeds are correct, but I was surprised that my motorcycle's speedometer showed the same speed of my GPS.

i have a 2006 Avalanche and last week compared the Nuvi 660 against the speedometer. 500 mile trip always within 1/2 - 1 mph agreement. Speedo slight ly higher if any difference.

It may mount to the engine but it will be sensing the output of the gearbox. If you know what's coming out of the gearbox, the final drive ratio and the diameter of the wheel you can determine road speed. There is a connection to the wheel, via the chain and sprockets (or drive shaft and axle).

You're missing the point. It does not have to be connected directly to the wheels in order to measure the wheel's rotation. In this case, it's connected to the transmission and driven by one of the internal gears, which is eventually connected to the drive wheel. No matter how you look at it, speedometers are measuring something which eventually results in calculating how fast the drive wheel(s) spin and by knowing the circumference of the wheel, can calculate the speed. There might actually be some somewhere that use an actual motion sensor, but I could not imagine why they would use something so elaborate and costly instead of the existing technology which just simply measures wheel or driveshaft or transmission revolutions.

If you want to prove it to yourself, just peel out hard sometime on gravel or ice and watch your speedometer jump up. You're not really going 30 mph, were you?

Ok, I'm done debating this, but you are both missing the point.

The original comment was that the speed measurement was related to the the circumference of the TIRE. Clearly it isn't.

I was picking up on what you said earlier, I think others were too:

Consider johnc's point about peeling away on ice. The speedo will indicate you are moving. Put your bike on it's stand, put it in gear, the rear wheels turn and the speedo will indicate but you're not going anywhere. Knowing the circumference of the driven wheel(s) is key to measuring speed accurately.

My speedometer matches the GPS as close as I can read the speedometer in my 4Runner. As stated earlier all speedometers are calibrated to the tire circumference whether the pickoff is at the wheel or in the transmission. It will be affected slightly due to tire pressure (if pressure is low you will be going slower than the speedometer reading). I notice this particularly on my bicycle when I travel a known distance which changes when the tire pressure is low.

Not to get too technical, but speed is distance divided by time. So, if you want an accurate speed number you have to measure the distance and the time very accurately.

All other measurements are indirect (you're measuring something that's "related" to what you want to know). This goes for speedo's, GPSr's, RADAR, Laser, etc. The problem with indirect measurements is they introduce other variables and, as a result, more errors. True, I have confidence in GPSr's, but always remember that it's an indirect measurement and thus subject to atmospheric conditions, accurate satellite position info, multipath errors, etc. just to name a few.

For what it's worth, I tested my Mio C320 (SiRFstar III) vs. my Garmin GPS-18 (NMEA/WAAS/EGNOS). I found really good speed ageement between the two with the GPS-18 reading perhaps 0.5 MPH higher. This could have been a round off issue since the C320 reports only whole number MPH. Both agreed with the speedometer in my Catera as close as I could read the analog dial. My Volvo's speedometer read 2-3 MPH higher than the GPSs or about 5%.

Also interesting, elevation or altitude agreement between the two was usually within about 5 ft. This was an interesting test, especially the dueling spoken directions Add a couple more GPSs and you'd have a great youtube video!

Jim

I guess we cannot convince you on this. As an automotive engineer I can tell you with absoluteness that the tire (or tires) rolling radius is indeed the method that all vehicles use to compute the vehicle speed. They may measure the wheel speed, driveshaft speed, transmission speed, or other rotating member that is in the driveline but ultimately it comes down to a calculation using the rolling radius of 1 or more wheels. No vehicle in production today uses GPS, radar or other non-contact means of speed measurement. So no matter how hard you try to dismiss the tire circumference element (or rolling radius which is what engineers use) it remains the method for speed and distance measurement.

If you go back to the beginning, the debate was on accuracy of the GPS or the SPEEDO. and the GPS is in fact more accurate. That is it.
You can not ask a rubber tire to always be new, so the diameter will be different so will be the accuracy of the speedo.

My speedo, gps, and the interstae speed signs all say the same thing for my truck.

... It's still measuring the rotation of a part of the drive train that is directly connected to the wheel.

The same is true of cars. Used to be the speedometer cable came from a point at the rear of the transmission where the speedometer gear was located. When you changed the size of the wheels, or changed the gear ratio in the differential, you had to change that gear too or the speedometer reading would be off.