Speedo innacuracies

[Jon]

Of course that is in the asumption that gps is correct. As they are based on a round ball for planet earth with no hills or valleys so not always accurate for a speedo , a measured mile and stop watch is far better .Or the distance markers on a motorway.

If you are doing a 50 mph speed run then almost by definition it's going to be on the flat with a Honda C90!...

i've also watched my car GPS speed against the cars speed and with the needle pinned to a fixed speed, say 60 mph, watched the GPS version of the speed as the car climbs or drops and it doesn't seem to vary at all so either the GPS chipsets are factoring the rate of change of height into the equation (and it IS part of the chipset's ability because some of the garmin etrex's from the '90's have altitude readouts)...Or the inclines are not enough to make much difference.

[krazy kev]

i have a modern car with digital speedo and all the other little add ons. i have noticed that if i set the speed limiter to 30mph and go past one of those flashing signs that remind you how fast you are going it tells me im doing 27mph. the problem is which one is accurate. the speedo or the flashing road sign i will split the difference. im doing 28.5mph

[Jon]

i have a modern car with digital speedo and all the other little add ons. i have noticed that if i set the speed limiter to 30mph and go past one of those flashing signs that remind you how fast you are going it tells me im doing 27mph. the problem is which one is accurate. the speedo or the flashing road sign i will split the difference. im doing 28.5mph

The signs are spot on. they use a radar beam and do not have any inaccuracies at all. I've noticed the signs always tally with GPS.

An indicated 30 at a true 27 is pretty much exactly the 10% error almost all modern vehicle speed indicators are set to.

[Pjam]

I've got several digital warnings for speed in London, I always check a comparison with my speedo. I think my Innova, with standard set up, is very close and the C70, although a little harder to read, seems not far out too!

One of my check points is very close to Davebikes gaff....... handy for Dave

[ll81]

My Innova over reads speed and distance. I put this down to down gearing by possibly smaller front sprocket to suit the larger build of the UK rider compared to the waif like Thais' where the bike is designed for. Wouldn't be cost effective to alter it all just to keep the speedo 100% correct. Just a theory.

Sat Nav speed is the calculated speed from the data stream it's using. If the speed was wrong, even by 1/2 an MPH then it wouldn't take long to say you were somewhere completely different to where you actually are. The satellites don't tell it physically where it is the map data stored in it does with reference to the difference in time it took for time signals from the satellites to reach it.

Even the effect of the earths gravity and velocity induced mass has to be accounted for as the time signal on the satellites drifts by around 38,000ns a day which equates to about 38,000 feet error per day! As you can imagine if it weren't corrected they wouldn't work very well. Topical geography is always taken into account, if it weren't then again by the time you'd reached the top of an incline it would say you were still somewhere else to where you actually are. Many sat navs (mine included) show elevation as it has to know this to work accurately, it can work this out again by the difference in the time signals.

[Jon]

My Innova over reads speed and distance. I put this down to down gearing by possibly smaller front sprocket to suit the larger build of the UK rider compared to the waif like Thais' where the bike is designed for. Wouldn't be cost effective to alter it all just to keep the speedo 100% correct. Just a theory.

Sat Nav speed is the calculated speed from the data stream it's using. If the speed was wrong, even by 1/2 an MPH then it wouldn't take long to say you were somewhere completely different to where you actually are. The satellites don't tell it physically where it is the map data stored in it does with reference to the difference in time it took for time signals from the satellites to reach it.

Even the effect of the earths gravity and velocity induced mass has to be accounted for as the time signal on the satellites drifts by around 38,000ns a day which equates to about 38,000 feet error per day! As you can imagine if it weren't corrected they wouldn't work very well. Topical geography is always taken into account, if it weren't then again by the time you'd reached the top of an incline it would say you were still somewhere else to where you actually are. Many sat navs (mine included) show elevation as it has to know this to work accurately, it can work this out again by the difference in the time signals.

My Innova's pretty much 10% out. That seems to be the universal standard with modern speedos. The innova's still geared for a theoretical 80 plus in top so it's well overgeared anyway. My W650 also has an electronic speedo which is 10% optimistic, and my current car's is too. In fact the only vehicles that weren't was my old C90, which after 27 years might have drifted back from being optimistic to being accurate, and a diesel Citroen Xantia which had a petrol car's gearbox sender unit driving the speedo and by coincidence was spot on.
When i was in the Police in 1981 , our Volvo GLT speedo's were all 10% optimistic compare to the calibrated jobbie fitted, and my 1976 ex Police beemer was also fitted with a calibrated speedo and that showed the original one was 10% fast.

I've also noticed that driving through those average speed camera traps on the motorway, most cars drive at a true 46 mph whereas i use my GPS and drive at a true 55 mph...

Interesting what you say about the GPS and altitude......It figures...