By Marcus Wilson 07/08/2019

How accurate are our car speedometers?

That’s well discussed., e.g. on this AA question forum.  If the ‘expert’ here is correct, your car speedometer could over-read by as much as 10% + 4 km/h (which is quite a bit – if you are doing 45 km/h it might read 54.5 km/h, or if you are doing 90 km/h it might read 103 km/h). But it should never under-read speed.

So, when I travel through the Tamahere roadworks  and the large “Your speed is…” sign tells me I am doing 48 km/h, I should not be surprised that my speedometer is reading 55 km/h. (OK, you ask, when can you ever travel through the Tamahere roadworks at 48 km/h? Fair point. Answer: about 9:15 am this morning. Child number two is sick today and I was rather late in to work.)

But the speedometer isn’t the only instrument that tells porkies in my car. I know for sure that my odometer or the ‘fuel economy’ calculator on the car are not telling the truth either. Or maybe both. Here’s how I know:

When I fill up with petrol, I tend to fill the tank up, to where the pump cuts out. Now, fuel pumps might vary a bit as to how full they let your tank get, but assuming they don’t (and in any case I can average over many fill-ups) the volume of petrol I put in tells me the volume I have used since the last fill-up. I also measure the number of kilometers I have travelled since the last fill up with the odometer. So I can calculate my average fuel economy. E.g. litres filled divided by kilometers travelled, multiplied by 100, gives me litres per 100 km.

So, for example, at my last fill-up I put in 29.0 litres into the car. I had done 467 kilometres since the last fill. Thus the car has a fuel efficiency of (29/467)*100 = 6.2 litres per 100 km, which is quite reasonable for a petrol car.

But I also have another measure of fuel efficiency. The car calculates it for me. I reset it every fill-up, so I get the average fuel consumption rate between fill-ups. At the last fill-up it read 5.8 litres per 100 km, a cool 6% less efficient than my calculation.

So what is lying to me?  Is it my odometer, recording more km than I have actually travelled? There are some ways of checking this – not least the era of Google maps makes it easy to know the actual kilometre-length of the route you’re taking. Or is it that inbuilt calculator of efficiency? And what are the industry guidelines for how accurate these can be?

If the odometer is pegged to the speedometer, and thus over-measures the distance I travel because it over-measures speed, then also the fuel efficiency calculator may overestimate my efficiency.  I assume that the fuel pumps themselves are considerably more accurate – these do have to be calibrated to ensure that my 29.02 litres is actually 29.02 litres.

The cynic in me thinks that over-reading distance travelled is a nice way of ensuring you take your car to the garage earlier for your cam-belt replacement, or that manufacturers can claim that your warranty is invalid because you have exceeded the number of kilometers stipulated by it, or that you just think you have a more efficient car than you actually have.


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0 Responses to “The lying dashboard”

  • Bloody instruments. Always lying to us…

    Here, it’s only a problem concerning a car… But what counts as an instrument has greater consequence when it comes to social sciences, and even more when it comes to the social science component of medicine, where a typical unreliable instrument are the claims of a patient and more importantly of its social environment. Reliability of instrumentation is a big deal, and in medicine, it can be a matter of life or death.

  • I’ve got two cars from the same company, twenty-five year age difference between them, the later one being a few generations newer replacement of the original.

    Using the same GPS unit to compare them shows that at 100 kph (on the GPS) the older car is indicating 105 kph, the younger is indicating 95 kph. The difference is even greater at 50 kph. It’s quite significant if considering the possibility of inadvertent speeding offences. My GPS unit is always on and alarms at 4 kph over the speed limits it “knows” for that reason.

    In terms of real world economics (or economies): a return trip of 500 km to visit family costs $110 for fuel in the older car, $45 for the newer one – but that reflects both a newer, more efficient engine, 9 l/100 km to 5.5 l/100km, and a change from 98 octane fuel to diesel. (Others can add in RUC, I can’t be bothered – but diesel cost less for me as well as a smaller carbon footprint)

  • Things that may affect speedo accuracy include tyre size a difference of almost 20mm in circumference between new and just warrantable typically, and so the smaller the tyre, the greater the error.

    Comparing to GPS – unless you have access to military-grade systems – is something ot be wary of. My phone’s gps varies the height of Mt Ruapehu by 10’s of metres some days. Distance as well is variably accurate point by point, but “acceptably” accurate in the long run.

    Petrol pumps rely on a pressure differential signal to automatically shut off. This is impacted by atmospheric pressure, pump flow rate and several other factors. Its accurate enough for its intended purpose, but may be out by 200ml day to day or between pumps.

    Fuel usage computers in the car typically rely on the signal from the fuel injection computer – add up the pulse lengths of injector on time and multiply by injector flow rate and you get an approximation of fuel use. Its only approximate because it relies on the injector being to specification which, of course, is ideal but not real world. I’d assume a 1-3% error in flow rate because of manufacturing tolerances and wear as a matter of course.

    Errors tend to compound, not cancel. I’ll leave everyone to do the math.

  • Reading 95 km/h when you’re actually doing 100 km/h? That’s a bad speedometer, surely?

  • Bad speedometer? Surely one that overestimates speed is just as bad? Especially if it results in a convoy of frustrated drivers behind wondering why you’re traveling so slowly.

    In the cars I’ve owned, testing against a common GPS unit, on the same straight, level road at constant speed, I’ve seen the GPS unit reading 100 kph and the car speedos reading from 95 up to 118 kph. And, yes, those readings can change by about 3-4 kph depending on the state of the tyres.

    I just think that I need to be aware of the actual performance of the car I’m driving compared with what the dashboard tells me. Which neatly gets us back to the lying dashboard…