Have recently purchased a new Zumo. On both the bike (Wee Strom) and the car, (Nissan Patrol) the Zumo shows 95/96 kph while the speedo on both vehicles is 100 kph. I guess that both speedos are under but what confuses the situation is that when i am sitting on the legal limit of 100 kph most are cars on the motorway seem to be travelling at the same 100 kph limit.

Could we all be wrong or possible that the Zumo is reading incorrect.

Anyone with the similar situation?

I transfer my sat nav between 6 vehicles - cars and bikes, from brand new to 30+ yrs old - and they're all the same; sat nav reads a few mph slower. The manufacturers do it to save you from yourself.

Is that why it is? I always thought they wanted people to think their vehicles were faster than they really are...
Stephan

Zumo vs Speedo > my Zumo was within 1mph of my speedo, that's 1mph slower..

Agree with you on the car Gary, but not the bike. I've yet to own any bike that is less than 10% optimistic - my bet is you're weestrom is only doing an actual 90 against an indicated 100 (unless you have changed the gearing on your DL60).

Worst offender was a Bandit I owned a few years - 14% optimistic! - what a shocker.

P.S. If you want to be doubley sure about the Zumo then install a bicycle computer on your bike - I guarantee it will tally to under 1% providing you accurately measure the distance of a full wheel rotation.

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Garry from Oz - powered by Burgman

Not only do the manufacturers make their vehicles appear faster, they also make them appear to be more fuel efficient. In NZ we have a few straight roads with odometer calibration markers. Her again my speedos usually indicate 5-10% more distance than the markers.

My GPS indicates 5-9 km/h less than my bike's speedo.

The compliance rule in Germany used to be that a speedo must not indicate more than 5 km/h too much below 50 km/h and no more than 5% too much above that. Indicating too little speed is not allowed. Looks like if I had bought my bike in Germany new I would have a case for getting a new speedo under warranty - or a new speedo drive.

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Cheers,
Peter.

Europe to NZ 2006-10

How about the police radar, that would have to be set spot on, you would think. Now thinking about some dubious fines.
The GPS reading and the speedo reading does vary proportionly as they are only a couple of k's out at 60kph.

I thought it was illegal (in the UK so therefore Europe too?) for a speedo to over read? I've never previously had a vehicle that didn't say I was going faster than the gps did.
Considering how much better the gps receivers currently are compared to only a few years ago with regard to speed of hook up even in less than ideal conditions, and the fact that the speed is just a simple calculation (it would be like checking a calculator to see if when you typed 2 * 2 it really was 4, how would it occasionally produce 4.167253 as the answer?), assuming you are receiving a signal for it to work from and aren't at the north pole the GPS would have to be right? It stops giving a speed when you go through tunnels etc, so it doesn't guess, but it does interpolate between fixes giving a continuously updated average based on a lot of readings and calcs. Driving through a forest may give a few hiccoughs but they'd be obvious prior to either back to normal or a no signal scenario. I can't imagine how gps could be contiuously a wee bit out for driving purposes.

Weirdly, although I have oversize tyres and a speedo not corrected to suit, the zumo matches the odometer better than one percent, but the speedo is over at slow speeds and under at high speeds (speedo and odometer driven from the same drive).

Slightly O/T, a while back there was an earhtworks construction contract in Cornwall. GPS was being used to control the machinery to create whatever it was to be built. At the time you would position a highly accurate surveying receiver in a safe stable position reasonably close to where you were working and leave it taking measurements contiuously, while you used a connected "slave" unit that also received GPS signals, but used the info from the main unit to acheive a better fix. You could then have this slave unit tracking a bulldozer for instance, which would have inside it surveying software with a CAD model of the job. The driver would decide where to drive, but the depth that the blade would add or remove was determined by the software, which I suppose you could tweak to improve fuel efficiency to an optimum depth per pass for that machine on that soil etc etc.
Anyhow, There appeared to be errors appearing, presumably someone had checked what the GPS was doing compared to a known height datum/benchmark, and found height differences of 100mm plus. This could mean a big cost to someone, that depth on a few miles of roadway is a big volume to pay for! I'm sure the person operating the GPS control network was getting quite concerned that he'd made a cock up somewhere, but couldn't find anything wrong in what he'd done.
After a bit of investigating, they discovered that the GPS readings were going slowly up and down throughout every day. Weird.
It turned out that this movement was mirroring the tide coming in and out. Cornwall being a long thin promentary and surrounded by the shallow sea on the continental shelf, when the tide came in the extra water on top of the shelf pushed it and therefore Cornwall down by more than 100mm twice a day! Until you had something remote and seperate in the shape of a satellite or two to measure from no-one was aware this was happening!
Sorry if that was just tooooo boring!!

I have extensively studied the correlation between speedo/odos on bikes (and cars) vs. GPSes and bicycle computers for over a decade.

My experience is as follows:

1. Odos are generally very close to actual distance travelled - typically 1% optimistic, with a worst case of 3.5% optimistic.
2. Speedos are on average 10% optimistic - worst case 14% (Bandit 1200) - best case 8% (Suzuki GSX1400).

The question then has to be asked, if the odo is accurate to 1% why is the speedo 10% out if both devices receive the same input feed/signal?

There is no other conclusion to be reached other than that this is a deliberate ploy by manufacturers - i.e it's as simple as the numbers on the speedo dial being painted in the wrong place.

Why do manufacturers do this (and it is all manufacturers, not just the Japanese). I can see only two possible reasons - to boost the rider's ego by making you think your bike is faster than it really is; or as a safety measure to get riders to ride slower than they otherwise would.

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Garry from Oz - powered by Burgman

Thanks for the ongoing technical explanations.

Farqhuar... you were correct about the bike reading, the Wee is showing 100 kph when the Zumo 550 showed around 92.

Went out in the wife's car today, a Mercedes 180c and guess what... the GPS and the speedo reading were spot on at speeds 60 kph, 100 and 110.

That indicates that motors of lesser quality, eg my Nissan Patrol and the Wee (sorry wee) are just not calibrated correctly. Would have been interesting to have tested my recently sold Ducati as they have more upmarket gauges.

regards
Gary