How do you calculate the gas consumption of an electric car? It seems like a very silly question, but many engineers are confused on how to indicate to drivers the gas consumption and the cost of driving a rechargeable hybrid. 

A silly question it is no more. For the Volt and eventually an entire generation of rechargeable hybrids, many drivers may only use it in electric mode. In this case, there would be no gas consumption per say. 

In the US, the Environmental Protection Agency (EPA) determines new car consumption figures. For now, the method to calculate the consumption of rechargeable hybrids is not yet finalized. The EPA would like to keep one consumption unit, miles per gallon, for a technology that really uses two separate energy sources, including one that is measured in kWh.

We could calculate only the consumption of the gas engine. In other words, how many litres of gas does this engine need to travel 100km using only the gas engine. In this case, it would be very difficult to compare vehicles with varied electric only range. 

For instance, GM determined that the Chevrolet Volt uses 1 litre of fuel for 100km. The Volt can travel 64km in electric mode only, without using any gas. Would this mean that the 1 litre of gas is only used on the last 36km? If it is the case, the actual gas consumption would almost triple and reach 2.8L/100km.

GM focused on city driving, as opposed to highway driving, but doesn't specify the methodology behind the calculation. In city driving at slow speeds, the Volt's electric motor performs better. Therefore, the gas consumption announced by GM is not representative of the gas consumption of a car driven both in the city and on the highway. GM's management defends itself by indicating that it has used an EPA-developed calculation method.

With this in mind, it will be interesting to see what Toyota's figures will be when the first rechargeable Prius comes to the market. Toyota will put 500 Prius vehicles to the test later in the year and plans mass-production as of 2012. It is said that the electric range would be 30km at the most and the gas consumption would be approximately 3,5L/100km.

Of course, one could feel compelled to compare the results from the rechargeable Prius to those of the Chevrolet Volt. However, the formula leading to these results are not known, it is impossible to do so.