The length of my journey from home to work, as recorded by my odometer this morning, was 24.7 km. The length as recorded by Google Maps is 25.2 km. So, my odometer **underreads**. Or it did this morning, anyway. But the car overreads my speed. If it gets the speed from revolutions-of-the-wheels data, one might expect that an overreading speedometer implies that the speed is being calculated based on a larger tyre circumference than actually exists. That makes sense with my current tyres, which are getting close to needing replacement. (Yes, they are still legal).

But then, assuming the odometer also goes off wheel revolutions, I’d expect that to overread too. But it showed less than Google Maps.

Now, also, I note that the ‘average speed’ indicator agreed with the odomometer and clock. The 24.7 km (as the odometer read) was done in 32 minutes which means an average speed of 24.7 x 32/60 = 46.3 km/h. The ‘average speed’ indicator read 46 km/h. At least this is consistent. But, not so fast…We can (and should) do a bit better with this assessment. Given I measured the 32 minutes off my car clock, which reads only to a minute accuracy, it could actually have been 31 – 33 minutes. At 31 minutes we’d get an average speed of 47.8 km/h, or at 33 a speed of 44.9 km/h.

Now, if we instead take Google Maps’ 25.2 km, we get average speeds of 45.8 – 48.8 km/h. That also includes the 46 km/h speed indicated by the computer. So, the average speed reading is consistent with both Google Maps AND the odometer measures of distance. I could do better by measuring my journey time in seconds, but that involves fumbling for a mobile phone while starting and stopping.

So, the speed information doesn’t help me work out this puzzle (why the odometer underreads but the speedometer overreads). Is the speedometer deliberately set to overread, then?

**The post The lying dashboard (part 2) appeared first on Physics Stop.**

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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