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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