How the hell do they calculate mpg for a PHEV?

[lornemalvo]

I've been looking at PHEVs on autotrader and can't understand how they calculate combined mpg, when usage can be so different for so many people. Surely it depends on how long journeys are, how frequent they plug their cars in etc. Are the numbers as random as I suspect they are?

[sherman]

Mpg in the brouchure is calculated on a rig in a lab. The car will never achieve it in the real world.

[Evanivitch]

Best on the test cycle, same as everything else...

But just like not everyone drives their ICE like the test cycle, neither do PHEV drivers. Usually a car review will tell you what they achieved when travelling well beyond the battery range.

My Ampera does 30-40 miles on electric (approx 4m/kWh) then about 40-45 MPG on the ICE. But my average in my ownership is 108 MPG.

[Foss62]

Quoting an mpg figure clearly makes no sense at all. Some people never plug in their PHEVs and others will never put petrol in them. The former only have one because of the company car tax rate, whereas the latter have one mainly for short electric journeys but like the idea of a back up for the occasional long journey.
For what it’s worth my Skoda Superb Estate does about 65 miles for every gallon of petrol that goes in.

[donkmeister]

In summary, it's a load of bks for people incapable of doing maths.

[HelldogBE]

Doesn't help that the testing changes over time (WLTP/NEDC) and that a bigger battery results in a lower MPG.
On youtube you'll find reviewers trying longer journeys, sometimes starting off with an empty battery and steady state consumption at different speeds.

There's also a big difference between buying a PHEV privately because you're going to maximize the electric range for your commute and charge once or multiple times per day or having one as a company car for the low BIK and rarely charging it not seldom in combination with doing high mileage basically using it as a hybrid electric vehicle with a weight disadvantage because of the big battery.


Source: International Council of Clean Transportation: Real-world usage of plug-in hybrid electric vehicles