If you were traveling up a steep grade, it would need to compensate for that since it is measure the distance between point A and B and and using time to determine mph. In a big level curve, it would know the radius for sure I guess. I have one but have never looked for altitude on it. Can it tell me how far my house floor is from the salt water level in the creek by me?

 

After a brief search of, "How does a GPS calculate speed?"

"Using GPS, a device is able to calculate a lot of information about a moving object. Using even basic time and location data, a GPS unit can quickly calculate the relative speed of the object, based on how much distance it covered in a given time.

GPS devices are positional speedometers, based on how far the device has moved since the last measurement. The algorithm also uses the doppler shift in the pseudo range signals from the satellites. It should also be noted that the speed reading is normalized, and is not an instant speed.

Speeds are updated at short intervals to maintain accuracy at all times. It uses frequent calculations to determine the vehicle’s speed. For example, using a standard movement per time calculation, if you have covered 80 feet in one second, the GPS device works out and converts that to MPH, which in this case is 55MPH.

GPS speed calculations are more accurate than a vehicle’s speedometer since it is not affected by the same inaccuracies, including the vehicle’s wheel size or drive ratios. It is dependent however on GPS satellite signal quality but with the use of moving average calculations any errors are minimal.

Vehicle speedometers also require calibration to maintain perfect accuracy as general wear and tear, changing wheel sizes, and the manufacturers own ‘erring on the side of caution’ and setting the speedometer to read higher than the actual speed, all contribute to inaccuracies.

Generally most manufacturers claim a 0.1MPH tolerance with speed readings, however will go as far to say a 0.5MPH inaccuracy is possible, to make allowances for any satellite signal variations beyond their control."

However there is inaccuracy

Where they can introduce errors is when the receiver can't get a good view of the sky. They can also under-report speeds when the car is climbing or descending on a steep road, since GPS altitude position fixes are less accurate than their horizontal position fixes.

 

As some else stated using a stand alone GPS to check the Speedometer. I checked mine with my Zumo 665 and found the speedo to on the money. I do have 36,000 miles on the rear tires (because of the chrome rims on my '12 I had them swapped out to the '14). When I replace the rear tires I'll recheck to see if it's still correct.

 

WOW .... so much info ... now I got a headache

It seems the only way to get an answer to my question is to take my zumo 660 with me and check it against the other two ... maybe do a measured mile test on the speedo ... but I think in the mean time, I'll just go with the speedo and use the field on the gps for something else ... what's the worst that could happen ... if the gps in the infotainment system is getting it from the speedo and the speedo is Hi, I'll get there a few minutes later, or They are both Lo, I'll get a ticket ... as I routinely go 3-5 mph under the limit ... not likely

 

I doubt very much that HD arrived at speed using the mechanical pickup used for the speedo to register speed on the GPS, one is calculated the other is an input from the bike.

 

Go Here: http://www.csgnetwork.com/speedocalibcalc.html

 

If you have a Power Vision tuner you have the availability to change your ECM to match your GPS speed under the quick tune feature of the P/V. Works like a charm.

 

I would first want to know which one is the most accurate ... right now they read the same (within a half of a mile) so I guess it doesn't matter

 

Lets see multi billion dollar system that is used by most aircraft that has many lives at stake and a digital instrument that is said to be accurate to 2% or more. I know what I think is the more accurate system. Most car/bike GPS are accurate to within 6 ft or so I have read.

 

I have been riding for almost 50 years. Several of my choppers didn't have a speedo and frankly I didn't care how fast I was going. Even if there were a 10% error, at 50, you may be going 45 or 55.... I'm usually admiring the scenery, not pondering how fast I'm really going. Does it really matter??