There has been a lot of press lately stemming from GM's announcement that the Chevy Volt would get 230 mpg on city driving and Nissan's follow-up announcement that its LEAF EV would get 367 mpg.
These calculations were made though using very different measurements and technically aren't comparable. Indeed, this highlights the fact that the rising tide of pure and partial EVs have the potential to render the current MPG determination standards obsolete. So where do we go from here?
Progressive X Prize is a marketing-neutral organization that is sponsoring a contest that will award $10 million to a company whose vehicle gets more than 100 MPGs. They are promoting a new measurement system called miles per gallon equivalents (MPGe) which is given by the following formula:
MPGe = (miles driven) / [(total energy of all fuels consumed)/(energy of one gallon of gasoline)])
This system levels the playing field by converting all energy sources going into powering a car into a common value (energy) and then determines how many gallon-equivalent of gas it takes to propel the car a mile.
It works well for pure EVs like the LEAF or Tesla Roadster but still is hard to apply to the Volt, because it switches energy sources after 40 miles.
According to X Prize senior adviser John Shore to account for that, "you might want to estimate the Volt’s MPGe at the various trip lengths listed in the table on page 36 of the Competition Guidelines – current version available here. You could then estimate a combined MPGe by taking a weighted average with the distribution weights shown in the table."
The table he refers to is the NHTS statistical distribution of trip lengths driven by US drivers in 2001.
If we do the calculation he recommends and assume the Volt gets 50 MPG in generator mode (GM's initial claim) we get the following results:
Chevrolet Volt: 166 MPGe
Tesla Roadster: 158 MPGe
Nissan LEAF: 142 MPGe