GPS v Speedo reading

I'm not sure what you are worried about Wato.....Do 110km/h using your speedo and you'll never get a fine (assuming the appropriate speed zone).

When doing 110km/h on your speedo, you may only be doing 108km/h or thereabouts in reality, so you are on the safe side. IIRC Australian Design Rules stipulate that it should be in this range, so it seems to be all good.

Hope this helps,

Andrew

If you want proof Wato - drop you tyre pressures right down and do 2 runs
eg 40 psi then 20psi.

The gps reading of 110 will now occur when your speedo reads 120 whatever , instead of 113 and the camera won't flash.

If the camera does flash , please send any bills to me addressed as
Santa Clause
1 ICY lane
North Pole.

Your GPS is the accurate reading and yes it is common for your vehicles speedo to overead it's speed, not a bad thing imo. The camera typically knocks off 3 kms. If a camera books you for 115km's hour you were most likely doing close to 118kms. Use your GPS for an accurate speed Wato. Nice and easy.

I know that my vehicle is overeading by around 4-5KM's hour at 100KM's so I also keep this in mind if my GPS is not on.

The principle for calculating both is the same so they should essentially be the same, speed camera versus GPS. Your Speedo reading is another matter altogether, and depending on tyre size etc may over-read or under-read actual speed as recorded by the GPS or speed camera.

Cheers

Your speedo should never under-read (I believe it is illegal for it to under-read) - so reading 113kph when your actual road speed is 110kph is good, and fairly typical.

It is legal in Australia for car speedos to read up to 10% fast.

They are not allowed to read slow.

EG my wifes car reads 107kph at 100kph

mine reads 103 at 100.

Asked dealers about it and they said

A they cant fix it and

B they dont have to cos its legal.

To fix it would take a set of tyres 40mm larger which in itself is illegal in QLD

Wato35
Does your state have 5km markers along the main roads?
If it does, find a flat straight stretch that you maintain a steady 100kph.
It takes 3 mins to travel 5kms @ 100kph
If you find the speedo needs to be on 103 kph to achieve 3 mins, that will give you the % error that, assuming it is linear, can be applied across the speed range, viz, +3%.
One of my vehicles is +4% (this went from +5 to +4 when I changed to a new set of tyres), another +3% and a third (the LC with D694 265/75R16 tyres) is -4%
If you can't find the 5km markers, main roads often have small 1km markers, often a white post with black numbers set back behind the shoulder and used by the road authority. These are more accurate than the 5km markers.
Cheers
Andrew

hi wato35
its quiet a common fault for speedo's to read several k's fast my vw golf reads 6k's fast so i use the gps
also the vw speedo doesnt have a defined 110k marking if i drive by the vw speedo other driviers are overtaking me continually like im a sunday driver

my mazda bravo reads 4k's fast and various other vehicles / navara /ford /subaru
i have owned have also been 4ks fast
except for a nissan pulsar that my daughter had was the only vehicle i have seen that was exact at all speeds
i have been through countless speed cameras and radars/multinovas while driving on gps speed and have never been booked
i lock the cruise controll on the gps reading and sit back and enjoy the drive
stop worrying over a common problem
cheers barry

Hi! wato35,

I have two Garmin Gps units I use either or both in the car and boat.

Also a dash board unit in the car. Another Furuno sounder in the boat.

All of them are very compatable with the car and boat speed.

The boat speed does vary with the Gps. because of the effects of wind and tide.

The Furuno sounder is very compatable with the Electronic speedo in the boat dashboard as both register speed through the water, not over the ground.

There can be many variables you just have to work it all out.

Have fun Haji-Baba

You are correct in your belief GPS is correct, your speedo is not 100%.

i think your speedo is reading land speed and your GPS is reading air speed - due to the earths rotation. this is made worse when travelling in an east west direction.
for accurate gps measurement, drive in a north/south direction.
sometimes the effect of airspeed is greater when driving into a headwind - youll notice you've used more fuel - but in effect - you've covered more air distance.


good luck.
i'm no rocket surgeon.

I was sitting at are red light the other day
the gps said i was doing 7 kph

tony

Wow, what a variety of answers.
Your GPS will show the exact speed, unless it's a real cheapie.

To cover themselves, vehicle manufacturers err on the side of "safety" and calibrate the speedo to read 3k's or so higher than actual road speed.
Any reasonable quality GPS will show the correct reading.

My wife's last company car (an imported model) actually read the exact speed as confirmed by two different GPS devices I own.
Both my vehicle and my wife's current vehicle read 3 kilometres faster than the GPS units show.

Here is the rule Plain and simple

From 1 July 2006 newly introduced models of a vehicle available on the market must comply with ADR 18/03. Also, from 1 July 2007 any newly manufactured vehicle (excluding mopeds) must comply with this rule.

This new rule requires that the speedo must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 percent plus 4 km/h.

That is, at a true vehicle speed of 100km/h the speedo must read between 100km/h and 114km/h. An alternative way to look at it is; at an indicated speed of 100km/h, the vehicle may actually be travelling at between 86 km/h and 100km/h.

Wato,
over many years and comparing GPS readings in my vehicles and talking to some old mates in trucks. Here are my readings. Little car 109kph on speedo and this is really 100kph. Cruiser reading is speedo 96kph, real speed is 100kph.

Proof no.... but I push the limit all the time at 109kph and all I have I receive is the odd flash from a candy car. When I stuff up and that has been twice in the last 5 years, I have both times been booked at 111kph or 1 point in Qld. I am happy that the GPS is accurate for me.

Have a safe one,

RA.

I've found that TomTom GPS units gave me the most accurate reading I've ever seen.

Whilst in the police over in the UK (I was there for just over 5 years) I tested my TomTom against Police speed checking devices (various types including calibrated speedo's and VASCAR units etc, and I found that the TomTom was accurate as hell, the only difference with it was that there was about a half to 1 second delay with it's reading of the accurate speed over what the vehicle mounted units were reading, though this is obviously because of the time to send a ping to a satellite and back.

The TomTom was tested to 140 MPH and was accurate the entire way up and down!




That said, I got a speeding fine for doing 73 in a 60 kph zone in Alice Springs a couple of months back. My speedo in my 1989 Nissan Pathfinder was playing up - but I haven't used my TomTom for 2 years because essentially we've got 4 roads out here out of town, so I don't need it!
I have worked out that while my car was doing 73 kph and getting the speeding fine I was only aware of it doing 54 to 56 kph because my speedo was THAT far out... It showed mid 80's when I was doing 101 on the TomTom!
Scary thought really... and it's worth checking your vehicles speedo v a satnav unit.

All vehicles up until some time in the early 2000 years were expected to be up to 10% out on their speedo (reading high), thus why the police MUST give a 10% LEEWAY when giving out fines.
So if the speed limit is 70 kph you shouldn't be fined until you EXCEED 77 kph if you have one of these earlier vehicles, but later vehicles are supposed to have more accurate speedo's and you'll get a fine a lot quicker.

Oh, on the note of GPS units, I tested a GARMIN v my TomTom, the Garmin was supplied by the force that I was working with. I found that not only did it MIS-CALCULATE routes (which wasn't acceptable when trying to get somewhere on blue lights and sirens), it's speed readings would fluctuate sometimes by as much as 5 to 10 MPH for no reason at all, while the TomTom kept a steady speed reading that was found to be accurate.

SO, not only can you not always trust your speedo and it's a good idea to check and replace it if it's not reading correctly, you can't actually use a GPS unit as a defence in court as it is NOT an APPROVED speed checking device.

Anyway, hopefully this has added more light to this discussion and not muddied the waters too far...

Simon