Speedo inaccuracy/safety factor Vs. Odo reading

[mike_nofx]

It's well known that most vehicles speedometers are not 100% accurate and have upto 10% inaccuracy (or purpose built safety factor?) usually reading higher than actual speed. (Assuming using correct factory tyre size)

Though I have heard that odometers are not purposely built with the same inaccuracies, and are closer to the mark than the speedo. Ie. doing exactly 100kmh on the speedo for exactly one hour doesn't necessarily equate to 100km on the Odo. OR is the odometer directly linked to the speedo for its reading??

Does anyone know any facts on the subject?

I read just recently in the auto section of a newspaper where a reader was concerned that if the speedo is 10% inaccurate, then are you racking up the odo 10% faster and servicing too often etc. but I just don't think that's quite correct.

[Yellow_Festiva]

I think the speedo and the Odo are directly linked when ticking over.

10% is a little high, it is more like around 5%. I did this very test just last week using the GPS. 100km/h indicated on the dash was 97km/h in reality. Now my tyres are well worn 205/55/16's and I think that if they were new the speedo would be almost 100% accurate? That is, adding 6-8mm to the diameter.

I'm soon to put on a new set of tyres / wheels, but they will be 215/55/16's. Will do the test again on the GPS to see what the accuracy is.

Regarding servicing.... I don't see the concern??? Either way, do people actually prop off their cars smack on 10, 20 30k km etc? I always have a difference of +/- 3-500km at time of service anyway so worrying about that side of things is pointless.

[hendrixhc]

Interesting point.

My citroen work van is out when i set the cruise control to 100 on the display the gps says i am doing 94. This leads to abuse on the freeway. I checked with 3 gps all the same. Checked with the the other 2 vans at work - the same. Spoke to the dealership and they said its within tolerance. Set my cruise to 105 and it sits on 100.

Never thoought bout distance travelled being effected tho.

[hendrixhc]

I put new tyres on the van a couple of months ago and noticed it changed by bout 1km/hr on the gps.

[Yellow_Festiva]

Originally Posted by hendrixhc

Interesting point.

My citroen work van is out when i set the cruise control to 100 on the display the gps says i am doing 94. This leads to abuse on the freeway. I checked with 3 gps all the same. Checked with the the other 2 vans at work - the same. Spoke to the dealership and they said its within tolerance. Set my cruise to 105 and it sits on 100.

Never thoought bout distance travelled being effected tho.

The reason why it is within tolerance is that often cars will be available with 2-3 tyre options. Then you have the fact that as a tyre wears it gets smaller. Lose 3-4mm in tread depth and that doubles to make your diameter smaller. This could be the difference between 2 sets of tyre sizes.

eg: LV Focus tyre placard:

Option 1: 195/65/15 = Diameter of 63.5cm
Option 2: 205/55/16 = Diameter of 63.2cm
Option 3: 205/50/17 = Diameter of 63.7cm

They probably use the base option (1) for there calibration and then anything else will be slightly out straight away. Which is why they need a tolerance. So, looking at the above 3 options, a well worn 17 inch tyre will probably be the same diameter as a new 16inch tyre.

I wonder if there are any cars that have electronic odometers that take tyre wear into consideration? Or link the GPS to the speedo to always be 100% accurate?? Doubt it!

[Jim Goose]

My AUII wagon reads about 9% from actual.... so sitting at 110kmh on highway is what i do.. or a bit more lol.

As for milage... sadly the only milage signs where you can check your odometer is halfway to Ayr. (speedo check signs). Each time I have passed those signs i have been overtaking people as its in an overtaking lane grrr...

[Vincenzo]

I've just completed 10,000km through seven States and Territories. Every now and again they have signs that say "check your speed" and each had it's own version of odo check. Zero the odo and every kilometre for five kilometres there is a sign. I was always reading somewhere between 20metres and 50 metres more than the last sign. I am still yet to work out how it helps you check your speed.

Cheers Vincenzo

[Silver Ghia]

There will always be some discrepancy between the odometer and the speedo, its instrument error.

As the policing here in Vic is dependent on speed cameras, its imperative to know what the actual vehicle speed is at the various speeds, by comparing to a GPS. My FG digital speedo over-reads by less than 0.5km/hr (never use the analogue one), so together with the cruise control set at 1kph increments and also displayed on the dash, I can confidently drive at the speed limit without being pinged, but also know that anyone driving faster than me possibly will get pinged for speeding, if driving past a camera.

One of the great things with the FG, the digital speedo together with the terrific cruise control. So many times cars overtake me, then further on slow down to around 10 kph less than the limit while passing a speed camera, I just tootle along with my cc engaged at the limit. I've never been pinged by a camera, but I've often had to brake because of these dimwits.

[Nashy86]

Try this: Sit on the freeway with the cruise control set right on 100km/h, then on your trip computer bring up the "average speed" and re-set it.
Assuming your speed stays the same (hence cruise control must be on) then the measurement for average speed will be the "real speed"

A few years ago when I was working for Ford, I had a brand new Territory Ghia for a long drive over a weekend and I tried the trick above. When the speedo needle said 100km/h the "average" measurement said 96km/h which reflected what the freeway overhead speed check told me, and also reflects the 4% deviation that is programmed in from factory.

Based on that, I would suggest that if the "average speed" is measuring rel speed, then the ODO would be measuring the real distance.

[falconnut]

Yeah i found this too.

I never knew they put it in on purpose though.

[new2ford]

A few weeks ago I had a highway patrol car follow me from Bega to Merimbula in a 100 zone so I set the cruise control by the GPS at 100 km/h. During the journey the speedo was reading around 105-110, the GPS stayed around 100. I wasn't booked! I found the same thing in the Skoda in Europe except the error margin was bigger as I was driving much faster. At about 160 (speedo) there was about a 15 km/h discrepancy between the GPS and the speedo!

[SumoDog68]

Originally Posted by Yellow_Festiva

The reason why it is within tolerance is that often cars will be available with 2-3 tyre options. Then you have the fact that as a tyre wears it gets smaller. Lose 3-4mm in tread depth and that doubles to make your diameter smaller. This could be the difference between 2 sets of tyre sizes.

eg: LV Focus tyre placard:

Option 1: 195/65/15 = Diameter of 63.5cm
Option 2: 205/55/16 = Diameter of 63.2cm
Option 3: 205/50/17 = Diameter of 63.7cm

They probably use the base option (1) for there calibration and then anything else will be slightly out straight away. Which is why they need a tolerance. So, looking at the above 3 options, a well worn 17 inch tyre will probably be the
same diameter as a new 16inch tyre.

I wonder if there are any cars that have electronic odometers that take tyre wear into consideration? Or link the GPS to the speedo to always be 100% accurate?? Doubt it!

As far as tyre wear You have to remember that circumference difference Between other two is only 15.7 mm which wont Make a lot of difference in travel as circumference is approx 2m.

[usernametaken]

Originally Posted by new2ford

A few weeks ago I had a highway patrol car follow me from Bega to Merimbula in a 100 zone so I set the cruise control by the GPS at 100 km/h. During the journey the speedo was reading around 105-110, the GPS stayed around 100. I wasn't booked!!

Ballsy

[shaness8]

A few years ago had TJ Magna, speedo was always fast out 7kph @ 100kph and this also effected the odo.

A return trip to work was 38.2km in the magna same trip in the SS was 37km speedo in the SS was spot on means racking up kms quicker in the magna.

Car speedo & odo and tripcomputer was out by about 7% what a joke.

Got a Aurion even thats out by 4%.

I wish the car manufactures could get this right really annoys me.

[Yellow_Festiva]

Originally Posted by usernametaken

Ballsy

Don't forget, a police radar will also have its own tolerance to work with. You will need to do a fraction over the limit before you can get done, some say it's 5% but really who knows?

Ballsy is the people who also know this, so set their speed by GPS a few km over the limit.

[aquahead2001]

I drive a lot of hire cars and have found that FG Falcons have very accurate speedos, usually within 1km/h of indicated. VE Commodores are close but some vary by 2-3km/h. My old PJ Ranger was out by 9km/h at 100km/h indicated, my new PX Wildtrak is out by 5km/h. Our SY Territory is also out by 4km/h.

Checks on the odometer of the PJ Ranger showed that although the speedo was out by such a large margin, the odometer was pretty accurate, only out by a couple of kms over 100kms on the GPS.

What peeves me about all this is manufacturers actually can produce accurate speedos and choose to put an error in them for some reason. I suspect it's to fudge fuel economy readings as there can be a reasonable reduction in fuel use by dropping 5 to 8 km/h in actual speed...

[mike_nofx]

Worth noting is the variation in tyre sizes by different tyre manufacturers, even those with the same size written on the sidewall.

When I had 4x4s you could buy 33" diameter tyres (as indicated on the sidewall) which varied anywhere from actually measuring 31.5" to 34.5" depending on manufacturer. That's a huge difference!

Though I have never accurately measured car tyres, I suspect there would be size differences too.

For this reason, maybe that's why car manufacturers build in a safety factor??

[irish2]

Originally Posted by aquahead2001

I drive a lot of hire cars and have found that FG Falcons have very accurate speedos, usually within 1km/h of indicated. VE Commodores are close but some vary by 2-3km/h. My old PJ Ranger was out by 9km/h at 100km/h indicated, my new PX Wildtrak is out by 5km/h. Our SY Territory is also out by 4km/h.

Checks on the odometer of the PJ Ranger showed that although the speedo was out by such a large margin, the odometer was pretty accurate, only out by a couple of kms over 100kms on the GPS.

What peeves me about all this is manufacturers actually can produce accurate speedos and choose to put an error in them for some reason. I suspect it's to fudge fuel economy readings as there can be a reasonable reduction in fuel use by dropping 5 to 8 km/h in actual speed...

By law, (ADR1803) a speedometer can not read slower than actual speed. That is why they build in a slight tolerance because of things like varying tyre size, pressure, and just plain instrument inaccuracies. No scientific tool states it is accurate, they all have a tolerance.

[2011G6E]

Up until, I think, 2005 it was allowable under ADR's for a speedo to be "up to" 10% out, and "preferably" read on the slow side...but they could read on the high side. That's a section of the ADRs that people should print out and keep for if they get pinged in a state that has a ludicrously low tolerance over the speed limit (is it only 3kph in NSW?) in an old car.

Our Falcon is the most accurate speedo we've ever had. Before GPS existed I had no real idea what "true" speed my cars were doing...you would compare it to a mates car, but you didn't really know if his was out too. Our G6E according to the sat nav is only about 1 to 2kph slow on what the speedo says...100 by the speedo is about 98/99 on the GPS.
My 2008 Suzuki GSX1400 is shocking...after I bought it, I would be sitting almost dead on 100 or 110 kph indicated by the speedo, and people would keep catching up and overtaking me...I thought "Surely not everyone is speeding", and so I took the phone I had at the time...a Nokia N9 with built in GPS...wrapped it in cling wrap (so the tape wouldn't stick yo the phone surface), set up the GPS for the "dashboard view" which had a nice big speedo reading, and taped it to the handlebars, and went for a ride. I found that 60kph was dead on, but after that it got way worse...for a genuine 80kph, I had to sit on nearly an indicated 90, for 100, I had to sit on 110, and for 110, I had to sit on an indicated 119/120 by the speedo. No wonder I kept getting overtaken...people were probably abusing "that slow idiot on that great big bike!"...
Couple of small yellow dots of paint on the edge of the speedo glass now indicates the true reading. I've since double checked it with a proper GPS suckered onto the fuel tank (after I got new tyres just in case) and found that my marks are pretty much exactly right still.

Our old 1982 Celica reads a bit slow, but only a few kph out, maybe five slow at 100/110.

[prydey]

Originally Posted by Silver Ghia

There will always be some discrepancy between the odometer and the speedo, its instrument error.

As the policing here in Vic is dependent on speed cameras, its imperative to know what the actual vehicle speed is at the various speeds, by comparing to a GPS. My FG digital speedo over-reads by less than 0.5km/hr (never use the analogue one), so together with the cruise control set at 1kph increments and also displayed on the dash, I can confidently drive at the speed limit without being pinged, but also know that anyone driving faster than me possibly will get pinged for speeding, if driving past a camera.

One of the great things with the FG, the digital speedo together with the terrific cruise control. So many times cars overtake me, then further on slow down to around 10 kph less than the limit while passing a speed camera, I just tootle along with my cc engaged at the limit. I've never been pinged by a camera, but I've often had to brake because of these dimwits.

this is almost what i would write word for word

love the cruise in the FG. i've put 18's on mine (factory G6ET) and my digital speedo is 100% accurate to GPS. i use cruise all the time, even in 50 zones and like you, it bugs me when people hit the brakes simply because there is a camera. to many its just a standard reaction, even if they are already going slow.

as for servicing too often i guess if you owned it for over 1000 years you might clock up 1 extra service than was required....

[Sprint]

10% by factory
2% tollerance by police,go figure!

[jpd80]

LINK
The accuracy of vehicle speedos is covered by Australian Design Rule 18. Until July 2006 this rule specified an accuracy of +/- 10 percent of the vehicle’s true speed when the vehicle was travelling above 40km/h.

That is, at a true vehicle speed of 100km/h the speedo was allowed to indicate between 90km/h and 110km/h.

An odometer accuracy of +/- 4 percent was also a requirement.

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. Significantly, this change means that speedos must always read 'safe', meaning that the vehicle's true speed must not be higher than the speed indicated by the speedo.

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's true speed must be between 87.3 km/h and 100km/h.

Significantly, this change means that speedos must always read ‘safe’, meaning that they are not permitted to read lower than the actual speed of the vehicle.

Additionally, there is now no requirement to have an odometer, and therefore there is no accuracy requirement.

This change was made to align Australian vehicle rules with those already in place in Europe.

Note that some vehicle manufacturers chose to comply with the new rule before 1 July 2006. This is acceptable.

In any case, it is the driver's responsibility to ensure that speedo is accurately calibrated,
driving above the posted speed limit is getting hazardous to your license

[2011G6E]

Originally Posted by jpd80

In any case, it is the driver's responsibility to ensure that speedo is accurately calibrated,
driving above the posted speed limit is getting hazardous to your license

I love it when the authorities just wheel out that excuse, that it's up to the driver to "ensure" their speedo is accurate.

Really. How do you do that? Even GPS isn't 100% accurate. I asked a copper once who told me to ensure my speedo is accurate "Where do I get that done, sir?". He suggested the VDO shop in town which did instrument repairs. I did ask them, but the price would have been an unknown, somewhere in the range of "Pehaps sir would like to sit down before I tell him how much the bill is" territory.
Taxis apparently used to have to have their speedos calibrated, but that was many years ago...not sure if they do now.

My car was engineered by a factory which presumably put a lot of effort into getting it to meet the ADR's...that should be it, full stop. It shouldn't be up to me to keep modifying and making sure it's calibrated to continue meeting them. In fact one engineer I talked to a few years back who worked in the industry said that any new car more than six months old would most likely not still meet a raft of ADR's once things got worn in a bit with use.

[MercurySilver]

my magna tj is about 8kmh slower than it says on he dial
i get a lot of tail gaters

[prydey]

Originally Posted by 2011G6E

Really. How do you do that?

most decent workshops have a rolling road, otherwise known as a dynomometer! its not difficult to get a speedo checked and as mentioned, ignorance is no excuse.

[usernametaken]

Originally Posted by Yellow_Festiva

Don't forget, a police radar will also have its own tolerance to work with. You will need to do a fraction over the limit before you can get done, some say it's 5% but really who knows?

Ballsy is the people who also know this, so set their speed by GPS a few km over the limit.

Agreed, I also set my speedo according to my GPS, but seeing as a GPS reading is inadmissable in court, I would reduce my speed to indicated rather than actual if HWP was on my tail. ;)

hence, Ballsy.

[prydey]

Originally Posted by new2ford

A few weeks ago I had a highway patrol car follow me from Bega to Merimbula in a 100 zone so I set the cruise control by the GPS at 100 km/h. During the journey the speedo was reading around 105-110, the GPS stayed around 100. I wasn't booked!

why would you be booked?? am i missing something?

speed detection measures the speed of the object, not the speed displayed on the dashboard.

[Blue Roo]

Originally Posted by Sprint

10% by factory
2% tollerance by police,go figure!

That's how the police can afford their christmas parties.

[aussie muscle]

Originally Posted by Jim Goose

My AUII wagon reads about 9% from actual.... so sitting at 110kmh on highway is what i do.. or a bit more lol..

all the AU's i've driven are out by about that much too. (at least 3 i can think of).

[xtremerus]

The Odo on my ute reads ~2% LESS than the real Kms.
The speedo is pretty much spot on, just reads a fraction fast.
So the error in one is not the same as in the other.