eli5: why does a car get 35mpg, but a lawn mower 2?

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A car can take a vehicle that weighs 2000lbs 35 miles on gallon of gas, but a lawn mower that weights 35lbs and has one spinning blade only goes 2 or 3 miles before needing a refill.

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Even a car moving at ~3-4mph for a long time will result in very poor fuel efficiency.

Equipment like farm equipment isn’t designed to be efficient over distance, it’s primary goal is to deliver a lot of power to an attached tool(s)

If you drove the same distance at the same speed with the same load, it would be alot closer. But your lawnmower is also spinning blades and cutting grass. This is a tremendous load for such a small engine. The equivalent to the car would be hard to make but it would be like driving at full throttle all the time. The mileage would be going down tremendously at that load setting as well.

Cars are also fuel injected and optimized for fuel economy. Your lawn mower is basically the same since the 60s. Technology has not changed much during that time for a standard lawnmower.

because the motor is more efficient in a car. This efficiency cost a lot to research and a lot to build motors in such a way. Measured against the price of a car and 5he amount of cars sold ajd the amount of fuel the owner buys in the lifetime of thr car, this is a reasonable investment. Not so much with cheap, relatively sparsely used lawnmowers. Also Mowers should be simpler in construction in order to be more resilient and easy to (self) repair

Cars are designed to travel long distances down the road efficiently, whereas lawnmowers are designed to cut grass. You could just as well note that cars make very poor lawnmowers, despite being much more expensive.

Lawnmowers use most of the energy from burning gasoline to spin the blade and cut the grass. They could be far more efficient at traveling if you dispensed with the blade and used them only on a paved road.

What speed are we talking? A car is fairly streamlined, which means it uses a lot less fuel to push through the air at a higher speed.

A lawnmower has very grippy, knobby tires, which also make for poor fuel mileage on a car. Put hardcore mud tires on a car, and its mileage will drop too.

A lawnmower isn’t geared for speed. It’s at high revs when going maybe 10 mph. Car has better gearing, letting the engine barely pur along at 50 mph.

And overall, every system has its waste energy. Gas engines aren’t very efficient, and you waste the majority of your energy every second that it’s running. A car at 50 mph spends 1/5th the time driving as the lawnmower at 10 mph, so that’s a lot less time it’s wasting fuel.

In case you take that last part wrong, no there is a point where wind resistance causes far more fuel use than you can save by running for less time since velocity gets squared in the drag equation. But drive a car along at a pokey 10 mph, and your mileage will drop too.