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Comment by TylerE | Hacker News Reader

You're more or less totally wrong, in a modern car.

Under overrun conditions (e.g. engine braking - driveshaft turning the engine, essentially), a modern engine totally shuts off fueling completely, so engine braking alone is always a MPG win, as zero fuel consumption is less than idle fuel consumption with the clutch in.

Also, brakes these days are so good that you really NEED anti-lock. The limiting factor in stopping is the grip of the tire.

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But when driving a manual car the driver doesn't put the clutch in while braking until almost completely stopped.

If you need to slow down quickly enough that you're not just letting the car coast in the gear you were driving in then using the brakes to bleed some speed off isn't going to be less fuel efficient than down-shifting to a gear that will stop you in time. The engine is engaged for almost exactly the same period of time.

Yeah, in perfect conditions (i.e. unlimited stopping distance) with perfectly matched revs when downshifting you will save fuel.

In the real world, just brake.

Why not a combination of both? Use the engine to slow the car down for the first part, and then use your brakes?

I'm not sure I understand why your revs would need to be perfectly matched to avoid fuel consumption. If you're slipping the clutch, you're wearing the clutch, but what does that have to do with fuel consumption?

Personally, I wouldn't downshift without a rev match because of the clutch slip factor, but I was always under the impression that with the throttle closed and the engine being driven by the wheels (not by the throttle via idle), you were using virtually no fuel.

If you don't rev match then when you down shift your revs will be higher and as such you'll be fuelling again without having to touch the throttle with your foot.

I must be missing something here. Your revs will go up regardless due to the nature of downshifting. It's just a question of whether you get there immediately or if you slowly glide there at the expense of your clutch. It seems to me you'd actually use less fuel if you didn't rev match, since rev matching requires you to blip the throttle. That said, I'll take the throttle blip over the clutch wear any day.

Clutch pedal in, until the revs drop.

If you don't rev match then when you down shift your revs will be higher and as such you'll be fuelling again without having to touch the throttle with your foot.

This is not how modern fuel injected cars work. Fuel consumption is determined by the ECU and is directly proportional (but not linear) to the load required to fulfill the accelerator pedal or maintain idle RPM. When the wheels drive the engine, no fuel is consumed regardless of the RPM.

The only way there won't be an increased load on the powertrain is if the revs are matched. If the revs are mis-matched then the transmission will be at a different point in the powerband (likely out of the powerband) and will result in increased loads experienced.

No. Just no. If there is NEGATIVE load, the engine is not being fueled. Period.

Also if there is no power demand (e.g. engine RPM > transmission RPM, but RPM > idle), again, no fueling.

Right. Perhaps jamesjguthrie isn't seeing the difference between engine noise (mechanical strain), and engine load. "Engine load" is the additional force requested, which will be negative when the wheels are driving the engine. He's also including rev matching in a conversation that is not about clutch or torque converter wear, so there's definitely some confusion.

That's not how cars work ...

In the real world, just brake.

This is good advice for the common case of city and suburban driving, but very poor advice for mountainous terrain.

YMMV

(... sigh, couldn't resist that one)

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