How is plug-in hybrid MPG calculated?

Own a plug-in hybrid and curious how its MPG is calculated? This handy guide explains everything, including how to calculate it yourself.

Understanding fuel efficiency for plug-in hybrids can be confusing. Unlike regular cars, PHEVs use both electricity and petrol, so their MPG works differently. If you’re wondering how plug-in hybrid MPG is calculated, this guide will explain the plug-in hybrid MPG calculation in simple terms, helping you see what the figures really mean for your driving.

Remember, you can buy a brand new or used plug-in-hybrid right here on Carwow. We’re here to help you through every step of your car-changing journey.

What is MPG?

MPG stands for miles per gallon and is the standard measure of a vehicle’s fuel efficiency, showing how many miles you can travel on a single gallon of petrol or diesel. A higher MPG number means the car is more fuel-efficient, helping you go further on less fuel and save money.

For plug-in hybrid cars, though, MPG can be misleading. Some models are advertised with figures over 200mpg, but these are based on ideal test conditions and short electric-only trips. In everyday driving, where petrol is often used more frequently, such numbers are almost impossible to achieve.

Why plug-in hybrids have MPG claims that aren’t reflected in the real world

There are a few reasons plug-in hybrids are advertised with extremely high MPG figures, even though most drivers won’t see anything close to that in real life. This includes:

1. Tests assume a fully charged battery

The car is run from a full battery until it’s depleted, then tested again with a completely flat battery, and the results are mathematically weighted according to the car’s electric range. This weighting gives a more realistic MPG figure, which reflects ideal use of the electric range rather than typical real-world driving.

2. Real-world driving is very different

In everyday use, journeys are often longer than the test cycle. Once the battery runs low, the petrol engine takes over – and fuel consumption rises sharply. Conditions such as motorway driving, cold weather, traffic and fast acceleration also reduce efficiency.

3. Not everyone charges regularly

Plug-in hybrids only deliver very high efficiency if they’re charged frequently. If they aren’t plugged in often, they behave more like petrol cars. This means they’ll use much more fuel than the advertised MPG suggests.

4. They’re heavier than petrol cars

PHEVs carry both a battery and petrol engine. When the battery is empty, the engine has to move that extra weight, which reduces fuel economy further.

How is plug-in hybrid MPG calculated?

Plug-in hybrid MPG is calculated using the Worldwide Harmonised Light Vehicle Test Procedure (WLTP). This is a standardised lab test designed to produce comparable fuel economy figures across all cars. However, the way it works favours plug-in hybrids. Here’s what happens.

1. The test starts with a full battery

The car begins the WLTP cycle fully charged. It’s driven in electric mode until the battery is depleted. As the car is using electricity rather than petrol for much of this phase, fuel consumption appears extremely low – which pushes the MPG figure very high.

2. Then it’s tested as a petrol hybrid

Once the battery is empty, it’s tested again with the car operating like a conventional hybrid, using its petrol engine (and some energy regeneration). The fuel used in this phase is measured normally.

3. The results are combined

The final official MPG is a weighted average of:

• Electric driving (using little or no petrol)
• Petrol-powered driving after the battery is depleted

Because the electric portion makes up a large part of the short cycle test, the combined result can produce figures of 150-200mpg.

Why electric range makes such a big difference

A plug-in hybrid with a longer electric range can complete more of the test without using petrol. The more miles it covers electrically during the test, the higher the final MPG figure will be.

This is why two similar cars with different battery sizes can have dramatically different official MPG numbers, even if their petrol engines are almost identical.

Can you calculate a realistic MPG for your driving?

There are so many variables involved in the calculation of any MPG estimate, from the type of roads you travel on to your own driving style. It gets even more complex in plug-in hybrids, because you also have to factor in the electric battery range and whether you actually charge it, and how often.

I spoke to Carwow’s deputy reviews editor, Tom Wiltshire, who drives and reviews dozens of cars every year, to get an idea of what you can expect from different journey types.

He explained that PHEVs get really uneconomical once the battery is run down, in part because you’re carrying around the extra weight of the batteries. On journeys that take place with a dead battery you can typically expect about 30-40mpg.

PHEVs also don’t like long motorway journeys, because they either run the battery out quickly, or you can force the car to use the petrol engine – either way economy is not ideal.

However, where PHEVs work really well is if most of your journeys are local and you have access to home charging. In this case, if you really don’t do long trips you could feasibly see in excess of 100mpg.

Most people do a mix of the above, and this is where plug-in hybrids can show a benefit. Managing editor Darren Cassey does exactly this – mostly local journeys with a 100-mile office commute twice a week, regularly topping up the battery at home. Over the past 1,000 miles his Volvo V60 plug-in hybrid has achieved 67mpg, which is much better than you would expect from a non-hybrid model.

Car change? Carwow!

Looking for a new set of wheels? With Carwow you can sell your car quickly and for a fair price – as well as find great offers on your next one. Whether you’re looking to buy a car brand new, are after something used or you want to explore car leasing options, Carwow is your one stop shop for new car deals.

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