We look at vehicle fuel and energy consumption, and put some questions to owners of hybrid cars.

In an article posted on 30 June 2018, we asked if the pricing of domestic gas by the kWh rather than the joule makes ‘metric sense’. In this article we look at an area where little makes sense – the measurement of fuel consumption.

An American staying with me last week was heard to remark: “You buy your gas (ie petrol) in litres but you give your fuel consumption in miles per gallon. Is this practical?” I did not dare to confuse him further by pointing out that identical cars will achieve 20% more mpg in Britain than in the US as the Imperial gallon larger than the US gallon.

The official measure of fuel consumption is litres per 100 kilometres (L/100 km). This makes sense elsewhere in the world (except in the US, of course) as calculating total fuel consumption on a journey then involves multiplication not division. But it is unhelpful in the UK as our road signs remain in an Imperial time warp.

But how is fuel consumption measured for hybrid cars? Owners please advise.

One option would be to treat the car as two separate vehicles, and consider fuel and energy consumption figures for the two different modes: mpg or L/100 km for one part of the journey and kWh/100 km or MJ/100 km for the remainder. And then rely on an in-board computer to work out average and total energy consumption for the journey.

Owners of hybrid cars: is this how it works?

And alternative would be to price fuel by its energy content. Typical figures for the latter are: diesel 39 MJ/L, petrol 35 MJ/L, LPG 22 MJ/L. Imagine pulling into the petrol station and seeing petrol priced at 3.7p/MJ. Hmmm, that sounds cheaper than electricity, currently priced around 15p/kWh or 4.2p/MJ. But how efficient is the car’s petrol engine?

Owners of hybrid cars: how do you compare the costs of running on petrol and electricity?

There is one consolation. British thermal units (BTUs), therms and foot pounds are unlikely to figure in this discussion.

In our final article on the energy measurement muddle, we shall look at food labelling.

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