Speedometers - how fast are you going?

19:36, Mar 16 2014
GPS-based speedometer
GPS-BASED SPEEDOMETER: Among other things an app like this can give you a much more accurate reeadout of your speed, and in some versions can offer a heads-up display (right).

Over the extended two-month "trial" of a 4km an hour cushion for speeders on the open road over the Christmas holidays, talk-back phone lines ran hot.

As is usual with this kind of broadcasting, facts and well-reasoned argument took a backseat to anecdote and hearsay.

Before long, the accepted view was that "just 4kmh" was just not enough leeway with our speedometers, which can never be spot on. It appears the law just wasn't fair.

SPEED READOUT: A car's speedometer can often read over by 12kmh or more, so there's even less excuse for being caought.

That the law was a success has also become accepted currency, as newsrooms in that no-news period quoted verbatim from press releases, not bothering to look at just why it might have been success.

Could it be that during the period that the laws of physics, for the sake of the speeding campaign, had to change in order for it to be dangerous at 104kmh, the extra police numbers brought out to nab the transgressors may also have had an effect on driver behaviour?

Perhaps if the old laws of physics that accepted it was dangerous at 109kmh and up had been retained and then policed by the same numbers, the result would have been much the same.


An answer can possibly be found in the decision not to extend the 4kmh cushion into a year-round scheme. It's probable that it is thought that merely changing the speed numbers wouldn't work on its own and there simply isn't the money to pay the overtime or recruit the required traffic officers to make the scheme tenable over 12 months instead of just two months and some holiday weekends.

Fairness doesn't really come into it if you look closely at most people's speedo readings.

In the past, when a test car's odometer seemed to be "off", I'd check it against a measured kilometre on State Highway 1 just north of Dunsandel in Canterbury.

But for 18 months now it has been made much easier for me, by the use of a SpeedTracker application on my iPhone, which can be set to project a heads-up readout of the car's velocity on the windscreen, delivering a quicker and more accurate comparison with the instruments.

In all that time, I've never come across a speedo that reads a lower speed than the real velocity at which the car is travelling.

In fact, the closest to being spot on was the Rolls-Royce Wraith and even that was out by about 1.5kmh, allowing of course for the thickness of car's instrument's needle-tip.

The ute I'm driving at the moment reads 100kmh when the GPS insists it is 91kmh. So in order to be travelling at 104kmh, a driver using the vehicle would need to have the speedometer indicating 112kmh or more.

The majority of cars' odometers I have checked against my trusty speed tracker show between 5kmh and 8kmh more than the actual speed when the app is indicating 100kmh.

Which suggests that the 4kmh cushion is no less "fair" than one of 8kmh or 9kmh, the fact being that when triggering the settings on a speed camera, every car's speedometer will have been reading well over the actual speed limit anyway.

We should count ourselves lucky that we don't have the hair's width cushion used by Victoria, which will have you for 102kmh if you're unlucky.

Of course, there will be those who claim that their changes of wheel and tyre diameters "confused" their odometer reading, while others say they can't be checking the dash when they should be checking the road ahead.

The fact is it's up to any car owner to know where they stand with their odometer, so I'd advise that they either find a measured kilometre in their area, or, better still, cough up a few dollars for an app similar to mine.

Then they can have no excuse for either taking their eyes off the windscreen or their speedo reading.

The GPS app also tells you more accurately the actual distance travelled, and allows you to properly work out your fuel consumption. It probably won't be as good as you thought, as the car won't have travelled as far as its odometer suggested. Just think, instead of doing 9L/100km, it could be 10L/100km or more in my ute. Now THAT'S unfair!


As well as telling what speed you're doing at any one time, the app also tells you about other important information.

What distance you've covered, how much time did you spend travelling from work to home or from any place to another, and with its mapping function it'll help work out what route to use.

Just firing up the application automatically records your velocity, time, distance, heading, elevation and many more.

The latest version also has a heads-up display which can display the most accurate speed right on the windscreen.

The unit also has a built-in GPS navigator and you can switch to navigation mode and check your current position on the map and the route you've already taken. A Trip Log function records and saves the information within the application and you can even share on Facebook, Twitter or via email.

Another good thing is you can watch your speed with greater precision however you're travelling, even on a plane, train, car, bike, bicycle or on foot.

It has to be said that GPS use does reduce device battery life and that GPS is not always accurate due to device hardware sensor limitations, but we found when checked against a known measured kilometre, SpeedTracker was absolutely on the button.

Basic SpeedTracker for iOS and Android is free, but more advanced versions like SpeedTracker Pro ask $2.59 on app store.