The all-new 2016 Toyota Mirai Fuel Cell Sedan is making its North American debut this week at the 2014 LA Auto Show, where journalists and consumers will be given their first real chance to inspect Toyota’s plans for transitioning from gasoline to hydrogen fuel and perhaps even put the Toyota Mirai through its paces on the test track.

But while Toyota is finally able to talk about the Mirai’s specifications — including its 5 kilogram compressed hydrogen capacity, 113 kilowatt electric motor and 9-second 0-60 mph time — Toyota hasn’t yet released any kind of fuel efficiency figures for the hydrogen fuel cell electric car.

Luckily, our friends over at *Autobloggreen *have done some back-of-the-napkin maths, and think they have a realistic ballpark figure of 60 MPGe — or miles per gallon equivalent — fuel economy reading for the all-new fuel sell sedan.

As *Autobloggreen* points out, the U.S. Department of Energy states that one kilogram of hydrogen is equivalent to around one gallon of gasoline, which means that it’s just about possible to divide the total number of miles travelled on a tank of hydrogen in a fuel cell vehicle by the capacity of the fuel cell vehicle’s on-board hydrogen tanks.

To test its theory on cars with known fuel economy figures, *Autobloggreen *tried its theory out on hydrogen fuel cell cars with known EPA-approved fuel economies. The 2015 Hyundai Tuscon Fuel Cell vehicle for example, has a 5.64 kilogram tank of hydrogen, and a range of 265 miles per fill. That equates to a fuel economy of 47 miles per kilogram, a little less than the 49 miles per kilogram combined figure from the EPA and Hyundai’s ‘official’ 50 Miles Per Gallon equivalent quote.

Similarly, the 2014 Honda FCX Clarity — a vehicle we drove a few weeks back — works out to an efficiency of 59 miles per kilo using *Autobloggreen’s *maths, which as we’ve already explained, is roughly equivalent to 59 MPGe.

Put Toyota’s own figures for range and hydrogen tank capacity (300 miles and 5 kilograms respectively) and we’re left with a ballpark figure of 60 MPGe.

Naturally, these aren’t official figures in any way, shape or form. But given automakers tend to be optimistic ahead of official EPA figures on range and efficiency, we’re fairly confident that our colleagues over at *Autobloggreen *are right*.*

If they are, that means the Toyota Mirai is about as energy-efficient as the promised next-generation Toyota Prius hybrid will be, and nearly a third less energy efficient on paper than the Tesla Model S electric car.

Unfortunately however, it’s not that easy. To make one kilogram of hydrogen from electrolysis of distilled water — a process that’s only about 70 percent efficient — you need nine litres of water and 56 kilowatt-hours of electricity. To make the entire 5 kilogram tank of hydrogen on which the Toyota Mirai can theoretically travel 300 miles, you need to use 280 kilowatt-hours of electricity.

If we take that 280 kilowatt-hours and put it in the Tesla Model S — which itself is one of the least energy-efficient plug-in cars on the market today due to its large size — you’d be able to travel 800 miles.

Fuel efficiency figures — as always — aren’t as straight forward as they might first appear.

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