Do fat people have noticeably worse gas mileage?

Say a 300 pound guy gets in his car, or worse–say a family of four 300 pound people get into their car, en route to their third breakfast at McDonald’s. That’s a lot of weight for a car to be carrying around, I’d say.

Would their gas mileage be noticeable worse than non-fat people?

The Master Speaks (quite recently, in fact.) The conclusion Cecil comes to, as far as gas mileage is concerned, is that the effect on mileage isn’t because of the extra weight in the car per se, it’s because fatter people buy larger cars with poorer mileage.

To quote the gas-saving tips at fueleconomy.gov:

Note the “up to”. In city driving, with lots of acceleration, extra mass is going to harm economy a fair amount. During highway driving, extra mass only adds a little to rolling resistance, so there will be little if any effect on fuel economy.

In real-world terms, an extra 100-150 lbs is only going to be noticeable under carefully controlled conditions. Variations in driving patterns will swamp out an “up to 2%” decrease. An extra 1000 lbs , on the other hand, will be very noticeable. I’ve got a little hatchback, and on a few occasions I’ve carried three (ordinary-sized) passengers, or a trunk completely filled with various stuff. When I do so, the acceleration is pretty pokey, and according to the instrument panel MPG display I lose maybe 5-10% of the fuel economy.

Assuming identical cars in identical mechanical condition and identical driving behaviour, yes the heavier driver will have a worse mileage.

How worse it will depend on the type of commute. Cruising on the freeway at steady speed will have minimal additional impact on the mileage, but in start-and-stop traffic the lighter driver will have a big advantage