How Does the Weight of Your Car Affect MPG?

The Laws of Physics

It takes energy to accelerate mass. The heavier an object is, the more energy is required to get it moving. In the context of driving, that energy comes directly from burning petrol or diesel. While manufacturers spend billions of pounds developing lightweight aluminum and carbon fiber components to improve MPG, many drivers completely undo that engineering by treating their cars like mobile storage units.

The Boot of Shame

Take a look inside your boot right now. Is there a pushchair you rarely use? A set of heavy golf clubs from last weekend? A crate of tools or a bag of winter rock salt?

According to fuel efficiency experts, every extra 50kg of weight in your car reduces your fuel economy by roughly 1% to 2%. While that might not sound like a lot, if you carry around 100kg of unnecessary junk for an entire year, you are effectively paying an invisible tax on every single gallon of fuel you buy.

The Full Tank Debate

Interestingly, fuel itself is very heavy. A full 50-litre tank of petrol weighs roughly 37kg. Some extreme hypermilers advocate for only ever filling the tank halfway, arguing that carrying around a full tank wastes energy to move the fuel itself.

However, the fuel burned driving back and forth to the petrol station twice as often completely negates any minor weight savings. The smartest strategy is to empty your boot, fill your tank completely when you spot a brilliant price on FindPetrol, and enjoy the savings.

Local prices differ far more than most drivers expect. For a worked example, see current petrol prices in Birmingham.

Checking diesel prices near you

Whatever you drive, the price you pay depends far more on which forecourt you use than on anything else. Diesel currently starts at 144.9p a litre across the 7,916 stations we track, against a UK average of 182.6p - a spread worth several pounds on a full tank. Search your postcode to see diesel prices near me ranked cheapest first, or browse fuel prices by area to see how your town compares.