City Mileage

I’ve known forever that city mileage for cars is worse than highway mileage, but I never knew why. But it’s a bit like riding a bike.

When you ride a bike, you have to put in a lot of work at the beginning, getting up to speed. And after that, you can mostly coast. Assuming you’re on flat ground, you have to pedal a bit because friction is slowing you down, but it’s nowhere near how hard you had to work getting up to speed.

And then you stop at a red light, and you throw away all the energy you had, so that when the light turns green, you have to put in another burst of work getting up to speed. And, of course, in the city you’re stopping like this all the time. Every few blocks, you throw away all your accumulated energy, and have to start over. This applies to cars the same way as to bikes, except that on your bike, you immediately sense this in your legs, not at the end of the week when the tank is empty.

So it’s not that highways somehow enhance your mileage. Rather, it’s cities that make for wasteful driving. It’s a bit embarrassing that it took me this long to figure it out, but better late than never.

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4 Responses to City Mileage

  1. Fez says:

    For ICE (Internal Combustion Engine) cars this is true. For electrics (and hybrids on electric drive) it ends up inverted, primarily due to regenerative braking. The instant torque available at 0rpm in an electric also allows for more efficient acceleration from a stop meaning less energy used over all.

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  2. Steve Watson says:

    The other thing that happens in cities is a lot of time spent idling at traffic lights or in line-ups — gas burned to go exactly nowhere. My last two cars (RAV4 and Prius) have had real-time fuel use monitors, and yes, the instantaneous consumption during even moderate acceleration is easily three times what it is cruising at 110 km/h. Do that all day, and….

    I think that monitor was single most valuable fuel-saving feature on the Prius — just knowing about it makes you ease up a little when accelerating. (Though contrary to Fez’s comment, it still got worse mileage in the city).

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    • arensb says:

      My current set of cars turn off the engine when it’s idle, to avoid burning too much fuel at red lights. But this is a slightly different source of inefficiency from what I described above: stop-and-wait-and-go driving is even less efficient than stop-and-go driving.

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