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Meaningful MPG

As I've mentioned elsewhere I have recently moved to a plug-in hybrid car. The other day, when fiddling with the information system I accidentally got to a screen that showed miles per gallon (MPG) and the number - 174 made me think.

Firstly, isn't a bit weird we still refer to miles per gallon when we buy petrol or diesel in litres? The only justification I can come up with is that I have better mental reference of what's a good MPG - maybe more than 40 - than I do MPL - I would have to do the conversion, rather than thinking in semi-metric.

However, the main thing is whether or not this is meaningful. Traditionally MPG would tell you (roughly) how much petrol you would use for a specific journey. I can divide the distance by the MPG and get a feel for the fuel needed. But I will never do a journey where this is the case with an MPG of 174. What is really happening is that all my local journeys are done on electric with a nominally infinite MPG, while my longer journeys are done in hybrid mode where I certainly get more MPG than I would on petrol alone, but nowhere near what 174 suggests.

I was initially inclined to dismiss the number entirely. But thinking about, it does have a kind of value - it does show how much travelling I'm doing against the amount of carbon-producing petrol I'm using, which I suppose is a kind of useful guide to how green I'm being. Confusing? That's the joy of statistics...

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