Member - Troll 81 (QLD) replied:

Laura

The GPS will be correct. Have you fitted larger tyres? if so your Speedo reading will be out and you need to get it adjusted

Truckster (Vic) replied:

quite common these days for speedo's to be out by "Acceptable levels" according to the dealer, which doesnt help when you get fined.

I drive by GPS now, dont even look at the speedo.

Take your car back to dealer and complain, but they will only test it to say "Its within acceptable levels", in otherwords - go away.

Grungle replied:

Yes that would be correct. Vehicle speedo will always be out by around 5% for standard tyres to 10% or higher for larger tyres.

Standard cars will have an actual speed lower than indicated (ie 95km/hr actual, 100km/hr indicated). Modified cars (bigger tyres)will most likely be opposite.

We always use the GPS as a trip computer due to accuracy but I have also match the diff ratios to my tyres so am out by less than 1km/hr @ 100km/hr on the speedo / ODO.

The only thing I can see that isn't quite right would be the GPS would show a shorter distance travelled than the ODO unless you had 35" tyres on.

Regards
David

Gerhardp1 replied:

The GPS will also measure when you go backwards. Did you go backwards for 400km? :)

I'd tend to trust the GPS - you can also tell if you tend to be flying past the bulk of the traffic, pointing to a speedo and odo reading less than you are actually doing.

There is also a possibility that the odo is out of step with the speedo, this would probably not be common since most use the same pulse for both. You would suspect this if your speedo and the GPS speed are reading the same.

pprass replied:

We have found over the past 4 years using a GPS that they are deadly accurate in measuring your speed. However regarding distance travelled - especially long distances there seems to be a problem there. This issue has been discussed at length on GPS forums such as Magellan with no clear understanding as to why it happens. You would think that a deadly accurate speed reading would equate to a deadly accurate distance measurement - but we figure wrong!

Member - Mike DID replied:

When the GPS loses satellite lock you suddenly see your track jump off several kms in some random direction, and then jump back - that gets added to your distance travelled.

Member - Doug T (FNQ) replied:

Laura
Trust your GPS. When I was doing Pilot/Escort work I always charged the Klm rate by the GPS Reading , If i had used the Odometer I would have been losing dollars.

.

Member - Alan H (Narangba QLD replied:

GPS is only accurate when travelling in a straight line.
It is showing the speed of displacement of the receiver therefore if you take a large curve, the receiver is being displaced in a straight line (short cutting the corner) and the speed shown will be slower than around the circumference of the bend.

Odometer readings are recording distance. The GPS does this by again measuring the displacement of the reciever at fixed intervals and again will be be in effect a series of very short straight lines. The GPS distance is therefore less than the rolling distance done by the tyres.

The GPS is also not pinpoint accurate (say about 3-5 metres) and uses averaging to give your location. On a long trip this small distance error caused by the averaging can all add up to a large amount if used for distance measuring.

Member - Captain (WA) replied:

Hi Laura,

I have had my GPS clock up over 100kms overnight while parked in my garage!!!

As others have said above, when the GPS loses "lock" on the satalites your position can end up anywhere. My garage windows allow satelite signal most of the time but occasionally it loses lock depending on the satelite location.

Out of cursoity I downloaded my tracklog from my GPS from an overnight "trip" and found while I was mostly stationary, I could suddenly end up metres to kilometres away from home, but only for an instant.

So, trust your km/hr reading for speed but overall the GPS odometer may not be that accurate.

Cheers

Captain

Member - Duncs replied:

Kiwi,

You are comparing the distances measured by the various devices.

Some others are comparing distance travelled to actual distance. I am wondering where the actual distance measurement comes from.

I was speaking to a cartographer a few years back who was busy re-measureing distances on his maps using a GPS. He told me that many distances were proving to be considerably different to those shown on the old maps. He claimed that a small SW Queensland town was in fact about 200km away from it's location as marked on the maps.

If we are measuring distances based on traditional maps and mesurement methods then it could be that the GPS is spot on and the 'actual distance' as we know it is wrong.

Duncs

jeepthing replied:

As far as I'm concerned I'll back a gps over a speedodometer any day. There is a variable error built into the satelite which can change from day to day or hour to hour. I understand this is controlled by the US military.

This is why Laura has experienced the variations of 300 and 100m. Although I've never experienced those sort of errors. In my boat's gps the error can be virtually eliminated by comparing the charted position at a given location with the gps position and entering the error in the gps.

My understanding is that a vehicle type gps has been calibrated prior to sale, particularly the more upmarket ones.

I don't agree that it measures distance in a straight line when travelling on roads. I do a fair bit of travelling in the outback and use my gps in conjunction with my rasta maps and have always found that if the map indicates that a certain road/point of interest is x k from my starting point the gps is spot on in distance and position when the target is reached.

Same applies when I'm fishing I can go back to that same small piece of reef any time.

Member - Bucky (VIC) replied:

I used our GPS to check uot our speedo, and its spot on.

Just remember, Australian Design Rules, 10% speedo variation, is considered normal ( I could be wrong, maybe less now )
Most vehicles that are out, are reading up 10% higher than the actaul speed, so its a built in "no speeding fine technology" (hehehehe)

Also consider that when the road distances are measured by some of the most intelligent council/main road people on the planet !.

So that has to be built into everything as well..
Trust your GPS for the above reasons, and drive your carefully.

Cheers Mate
Bucky

Member - Coyote (QLD) replied:

GPS's also do a small amount of 'rounding' when recording a track, eg if you are driving along a track that winds a lot, you will find that the actual recorded track / distance measured gets shortened by the GPS and it takes redingevery so often (you can adjust the frequency/ number of points it records to make a track) but it is still a collection of short straight lines put together to give you your track.. the further between the 'points' the less accurate the actual recording.. over the distance you travelled say 2000 and something k's if it was a route the wpound around a fair but, the GPS will have rounded off a lot of the corners giving you a shorter distance travelled.

Gone Bush (WA) replied:

Some of the more expensive, and therefore more accurate, GPSs have a flux capacitor in them which prevents errors.

True as I sit here.

Member - Mick O (VIC) replied:

If it all boils down to a speedo inaccuracy, hang the GPS and why not get your friendly neighbourhood traffic plod to put his hair dryer or laser thingo over your car at 100kph? What about the "Speed check" radars on major motor ways in Vic and NSW?

Mick