Gps speedo

I’ve asked this before but got no answer.How accurate is the gps spped reading?On my V11 Sport the very nice Traffic police checked my speedo out on the M6 and we both agreed I had been travelling at 84 to 96 mph which means my speedo was accurate.At an indicated 105mph on my lil’Breva my gps shows 94mph.Does anybody know if this is the speed traveing bewtween 2 points or the proper speed on the road.I know some people prefer to rely on their gps speedo and if it is underreaing it could get you in trouble.Plus I would like to know what the bike is capable of.

GPS can be extremely accurate but the secret is to know when. GPS speed is calculated by looking at the difference of successive positional measurements and the time difference between the measurements. The absolute accuracy of the positioning is a function of how many satellites the system can “see”, atmospheric conditions and the deliberate inaccuracy built in to the civilian GPS system which can vary. That said, relative position versus the previously calculated position is reasonably accurate so a reasonable estimate of speed can be calculated. However, the update frequency is very low which means that GPS speed is only accurate after a few seconds settling time so only rely on it for steady speed measurement. Basically, if you hold a constant speed on your speeds or tacho and wait for the GPS speed to stabilise ie. reads the same each time it updates then you will have an accurate speed measure. Off the top of my head I can’t remember how accurate in terms of +/- mph but certainly way better than a typical speedo.

I use my Garmin Montana for walking (OS maps ) and over the same real distance, the measured GPS distamce can vary quite a bit. I’ve always wondered if this is down to where the satellites are at that time, my position and even the vertical distance.

Based on this variance, I would question it’s accuracy for speed measurements on the bike unless I’m in a dead straight line at a constant speed with no incline. Basically, the same conditions that make you likely to be seen and caught by the old bill!

JonSquarepants2014-07-02 09:20:00

The accuracy of a GPS based system is largely a factor of the sampling times. Yes, a GPS does measure your speed between two points, but if this is updated every second or less, it is going to give you a pretty accurate speed, and respond fairly quickly to changes. It’s conceivable if you made several very rapid changes in direction you could spoof the system, but as we all know, Guzzi’s don’t do that sort of thing ;)A good satellite fix, with half a dozen or more SV’s in view should give great accuracy. Despite what some think, the old system of ‘selective availability’ whereby the US Govt degraded accuracy has been turned off, with most domestic systems able to return street number type precision. That is not to say however, that the system can’t be turned off without notice, or degraded again for whatever strategic reason. A ‘dual’ system that utilises the GLONAS SV’s would provide redundancy.Just my two shekels.Phil

I concur.The digital speedo in my Citroen is within 1mph of the GPS, but the analogue one in my Saab is significantly fast, as is my B750.If relying on a GPS, don’t expect as much leniency if PC Plod has a chat. ie, if you are doing an indicated 80 on your speedo, you will probably be only a whisker over 70, 80 on the GPS is 80.