Inaccurate speedo

[Buddman (Adam)]

Hi,

I just got back home from a driving trip from the Sunshine Coast to Sydney and back and I have found that the speedo is wrong in my new car (1 year old).

My wife has no sense of direction, even up and down can confuse her (I hope that she doesn't read that last bit). She is always complaining about getting lost so I bought her an in car navigation thingy for xmas. On the drive back home I was using it to show her how to use it - getting her to type in addresses to find, listening to the directions etc. when I noticed that it gives the speed of your car as you travel. If I was doing 100 km/h by the speedo the navigator consistently gave our speed as 93 km/h.

I calculated that at 100 clicks I should do 1 km in 36 seconds (the traffic was really heavy and I had a few hours spare) and using the odometer I confirmed that my speedo had to read about 108 to actually get 100 km/h.

Is this common? I really hate people who travel below the speed limit (I'm not into travelling over the speed limit these days though) and now I find that I have been doing exactly that. It really explains the pissed off people that have been overtaking me lately.

Has anyone else found this?

If they can make the km travelled correct why can't they make the speedo correct?

Ahh, I have a lot of time to contemplate this and now I feel a bit better. Sorry about the rant.

Adam

[edwardsdj (Doug)]

My understanding is that by law speedometers only need to be accurate to + or - 10% in Australia.

[erick (Eric)]

Yep, my new GPS confirmed what the overhead advisory speed detectors had already told me - my speedo reads 5kph high up around 100-110kph.

[Tandum (Robin)]

However in Victoria I believe a 5% overspeed results in a fine.
I guess they need the money.

[erick (Eric)]

Yes, 63kph in a 60kph zone will cost you $125 last time I checked.

[edwardsdj (Doug)]

I just found the rules changed in 2006:

http://www.trafficlaw.com.au/speedos.html

It looks like all new vehicle speedometers must indicate a higher speed than the car is actually moving at now.

[Ian Robinson]

Does the car have the proper rims and tyres on it - wrong wheels and tyres may have fitted for some reason by the dealership in which case the speedo may well be inaccurate.

I'd recheck on the open road somewhere , pick a stretch of road where you do 10km uninterrupted , and time it at 100km.

If it's still wrong - I'd be straight back the dealership and expecting the speedo recalibrated immediately .

My nephew who was a highway patrol cop on the Pacific Hw (Forster) told me once they allow up to 10% over the speed limit before booking normally unless they are having a blitz where they allow no such allowance (but it's discretionary) , this is because it is easy to find yourself doing 110kmph on those downhill and long straight stretches , and most your purpetrators do a long faster than 110kmph (not unusual to clock people driving at 160-180kmph) on the good stretches.

[edwardsdj (Doug)]

Showing 100 km/h on the speedo when the actual speed is 93 km/h is completely within current Australian Design Rules - see section 5.3 of:

http://rvcs-prodweb.dot.gov.au/files/ADR%201803.pdf

as explained here:

http://www.trafficlaw.com.au/speedos.html

[Buddman (Adam)]

Originally Posted by Ian Robinson

Does the car have the proper rims and tyres on it - wrong wheels and tyres may have fitted for some reason by the dealership in which case the speedo may well be inaccurate.

I'd recheck on the open road somewhere (pick a stretch of road where you do 10km uninterrupted , and time it at 100km.

If it's still wrong - I'd be straight back the dealership and expecting the speedo recalibrated immediately .

Hi Ian,

The tyres are the same as what were delivered. I checked the odometer using the GPS, registered the same distance over 20 km. The only difference was the speed.

The car's going in for it's second service in about a week and I was going to get the dealer to check it out.

Adam

[erick (Eric)]

Just to update Victoria. I think that there is a mandatory minus 3kph applied to the reading from fixed speed cameras and I haven't heard of anyone pinged for less than 63kph in a 60kph zone, so they were likely travelling at 66kph, or 10% over.

[gary]

Originally Posted by Buddman

The car's going in for it's second service in about a week and I was going to get the dealer to check it out.

Adam

Hi Adam,

There will be nothing wrong with your speedometer.

All new cars on the Australian market now have speedometers calibrated so that
the speed indicated is higher than the actual speed you are traveling.

The two documents cited by Doug, including the Australian Design Rules, provide
the mathematical relationship.

Your indicated speed of 100km/h and your actual speed of 93km/h is
not uncommon on late model cars and is intentional.

It is intentional for two reasons -
1) It falls within the Australian Design Rules standard
2) It help protects a car maker from litigation in the event of a driver receiving
a speeding fine or being involved in an accident where excessive speed was
a factor.

Since the advent of GPS systems that can estimate and display the actual
speed of the vehicle, the fact that speedometers on newer cars will tend
to indicate the speed to be faster than what it actually is has become
evident to motorists who were previously unaware of this convention.

At lower speeds (i.e. 0 to 60 km/h) you will note that there is typically
less of a discrepancy than in the range of speeds higher than this.

The good news is that most new cars are calibrated in much the same way
so that if you are in a 100km/h zone on a freeway, adjust your speed so
that the indicated speed is 100k/h and then engage cruise control, other
motorists who have done the same in late model cars will tend to be going at
nearly the same actual speed as you.

Your dealer will most probably provide you with effectively the same advice
as above and recommend that you leave the speedometer at its factory
calibrated setting. Your GPS unit will then provide you with a different estimate.

Happy motoring!

[rat156]

Hi Doug,

Nothing new there, speedos have been reading under for many years.

It really should be addressed. In Victoria you can be done for 3kms over the speed limit, minus the 3km they allow for error, in a 110kmh zone that's an allowance of 5.5%. There are overhead speed check devices and GPS units are commonplace these days for you to check the accuracy of your speedo, so no excuses for not knowing what it has to read to be doing the speed limit, if you found that your speedo was reading slow (probably after changing your wheel size), you could get it fixed.

What really annoys me is that I like to travel at the speed limit. I have many years experience driving, and drive a race car at significantly higher speeds that I'm allowed to on the road. But you have people tooling along in the right hand lane of the freeway doing their 93kmh (probably on cruise, as everyone else is doing that speed), who refuse to move over or speed up, in fact some will actively try and block you from doing a speed that they deem to be faster than they like.

Now I don't mind these people doing their 93kmh on the freeway, but do it in the left hand lane, not the one next to it, and never in the right hand lane. If you're on a highway, with only one lane each way, don't just blindly follow the car in front at your 93kmh, drop back to allow other cars to overtake more safely, it means that cars will not need to have a huge break in the traffic coming the other way in order to overtake multiples of cars.

I'm no hoon, but I like driving fast, safely. People need to learn to share the road, no one of us owns the road, so try to think of other road users when you're out there.

As you can probably tell this is one of my rant topics, I can talk for hours on road safety and the various rubbish we are feed by the government (Speed Kills, and the braking distances spring to mind) to justify the use of speed cameras as taxation devices. How many people each year are killed by hitting stationary objects (like trees, power poles, sign posts) that are placed on the side of the road without the proper protection, sometimes even by the autorities that are supposed to be protecting us.

That's my 2c (well maybe slightly more) worth.

Cheers
Stuart

[Ian Robinson]

Golden rule that my dad told me when I was learning to drive - stay with the flow of the traffic and ignore the speedo.

If everyone is travelling at a bit over the speed limit , the cops will do nothing so long as the traffic is moving well and safely.

[gman (Grant)]

Originally Posted by Ian Robinson

Golden rule that my dad told me when I was learning to drive - stay with the flow of the traffic and ignore the speedo.

If everyone is travelling at a bit over the speed limit , the cops will do nothing so long as the traffic is moving well and safely.

Unless you are in Victoria.

4kph over the designated speed limit regardless and your gone.
Not to mention the multiple speed limit changes on the same stretch of road with speed cameras waiting to fill the state coffers..... actually I meant to say keep the roads safe of those menacing speeders.

[Spanrz (Brett)]

My speedo is out by 7k's (shows 7k's over). But that's with aftermarket wheels.
Tyres and wheels sizes play a significant part in speedo's.
Go out and read what your tyre size says on the sidewall and correlate it to your handbook. You might be surprised.

The original wheels and correctly spec'd tyre size for my Falcon BA, produced a 4-5km difference over, to what I was doing.
I went from 16" rims and spec'd tyres to 18" rims with 235/40's.
I previously tested 3 times, before I did a real test.
Some places around some cities, might have those gantry speedo checks on certain fwy's. I know there is 2 in Melbourne, but they are more like 40-50k's outside of Melbourne.
They have to be correct.
Test 1. Speedo was about 4-5k's
Test 2 was use 2 separate GPS Units, just to be sure. Again 4-5 k's out.
Test 3 was the real test. Did 104-105k's through a speed camera. NO FINE.

I did the same with the new package and it showed about 107k's, was the actual 100K limit. Proved it many times going through Eastlink Speed Camera's without a fine. And I know I was doing 100k's.
To be absolute 100% sure of this (mathmatically correct in a court of law), I did a speed test with time measurements and distance.
On Eastlink, if you have a tag, it gives you the time you entered a toll gate and an exit time (in minutes and seconds).
I used GPS units and Odometer of the car to reflect the readings.

After the time and distance calculations, I found 100% sure, I was at 107k's on the speedo for an actual 100K speed limit.

Now I did a measurement of the difference, to ascertain what my speedo would do, the new package changed the diameter (total diameter of wheel and tyre to inflated pressure) by a few mm.
Remember, PI (3.14) x D = Circumference. So for each approx 3 mm of diameter change, you have 10mm difference of travel per revolution.
With the new wheels and tyres it was slightly smaller in diameter (around 3mm), which made it smaller travel approx 10mm.
That made the difference of a couple kmph difference.

So the next time, someone asks you to change tyres, recheck your speedo readings and ask the tyre guy "What will this tyre choice do to my speedo readings?", if you agree to change the tyre size.

Remember, you can have so many tiyre dimensions (profiles) for a given rim size.
10 different tyre profiles that will fit a standard 16" rim, will give different Kph readings.

I still go under the speed camera on Eastlink at under 107k's (no faster), still have not received a fine.
So it pays to do your sums

[rat156]

So Brett,

How do you account for the wear? There's about 10mm of tread on new tyres (at least 5mm each side of the diameter).

It is a good idea to try to match the rolling circumference of your new tyres to the old though.

Cheers
Stuart

[norm]

This maybe off the topic, but why the hell don't they make parts of the F3 120-140kph zones.

Surely there are stretches where its safe to travel at those speeds. Even many country back rds in NSW are capable of travelling 110-120kph safely.

Norm

[Smirnoff]

The Hume Highway in VIc is flat and straight for practically the whole way, it should be 130kmh atleast, along with the Princes highway to Geelong.

What needs to be done is link Adelaide - Melbourne - Sydney - Brisbane with 4 lane freeway and have it at 130kmh.
There are ofcourse a ton of other "highways" that need to be converted to dual carriageway. Many of the highways in this coutnry linking major cities and towns are still only 2 lanes with no shoulders and sometimes with ridiculous layouts within the towns they pass through. The highway infrastructure in Australia is a joke.

[Glenhuon (Bill)]

Just come back from a 4 1/2 hour run up the Hume H'way in NSW. My speedo is 5 km above true speed. I've known this since I bought the Starwagon. Who cares ! It has probably saved me heaps in fines. That few minutes later you get there is well worth it if it saves you $$$$'s. On a run that long speeding will probably save you 20 mins max. The one that made time made a lot of it ( and you get there in one piece)

Bill

[Smirnoff]

I tend to avoid speeding, even by a small amount, as IMO ending up wasting $165 or more of ya hard earned on something completely avoidable is silly.