# Car Fuel Efficiency

**URL:** <https://boards.straightdope.com/t/car-fuel-efficiency/573874>\
**Category:** Factual Questions\
**Created:** [March 8, 2011, 2:13am UTC](https://boards.straightdope.com/t/car-fuel-efficiency/573874 "2011-03-08T02:13:02Z")\
**Posts on this page:** 3\
**Page:** 2

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**Author:** ![YamatoTwinkie](https://avatars.discourse-cdn.com/v4/letter/y/cc9497/32.png) [@YamatoTwinkie](https://boards.straightdope.com/u/YamatoTwinkie)\
**Post date:** [March 8, 2011, 10:34pm UTC](https://boards.straightdope.com/t/car-fuel-efficiency/573874/21 "2011-03-08T22:34:09Z")

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> [@lazybratsche](#):
>
> Sure, if you’re looking for the most efficient way to get lots of torque. In a car that’s relevant for acceleration. But for cruising, you don’t need much torque to overcome rolling and air resistance. You just want to select the gear where the engine provides sufficient torque at the most efficient RPM. If I’m reading the diagram correctly, that’s somewhere between 1500 and 2000 RPM for the example you linked. And because the rolling resistance and air resistance increases with speed, the most efficient cruising RPM is probably closer to 1500 RPM at low speeds and perhaps 2500 RPM at highway speeds.

True, but I think hypothetically, if you could cycle between hard acceleration (targeting the “bullseye” of the BSFC map) and coasting, you might have even better overall mileage than staying in the lower RPM band with constant throttle.

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**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\
**Post date:** [March 8, 2011, 11:20pm UTC](https://boards.straightdope.com/t/car-fuel-efficiency/573874/22 "2011-03-08T23:20:40Z")

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> [@YamatoTwinkie](#):
>
> True, but I think hypothetically, if you could cycle between hard acceleration (targeting the “bullseye” of the BSFC map) and coasting, you might have even better overall mileage than staying in the lower RPM band with constant throttle.

In practice though, hard acceleration wastes energy in many ways. The rear of the car squats, the rear tires deform, fricion increases within the drive train, etc.

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**Author:** ![Lord\_Mondegreen](https://avatars.discourse-cdn.com/v4/letter/l/91b2a8/32.png) [@Lord\_Mondegreen](https://boards.straightdope.com/u/Lord_Mondegreen)\
**Post date:** [March 8, 2011, 11:35pm UTC](https://boards.straightdope.com/t/car-fuel-efficiency/573874/23 "2011-03-08T23:35:01Z")

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> [@Rick](#):
>
> \*I’ll leave it as an exercise for the Air Canada fuel truck driver to convert kilos to gallons.

Well played. 😃

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