Brake usage on cars with auto vs manual transmission

[RAMd®d]

but when I rent a newer manual car or when I drive my Dad's car. the fuel consumption goes to 0.0 (0 L per 100 Km) when downshifting.

I don't understand this.

With an automatic transmission, you take your foot of the gas pedal to use the brake pedal (normally). Butterflies close, acceleration goes to 0, but gas is fuel is still consumed, the rate of consumption decreasing as the rpm stabilize at idle.

It *seems* to me that downshifting raises the rpm above what would be seen if the car/truck were left in a higher gear, at least until the rpm dropped to idle.

So where does the increased fuel economy/0 fuel consumption occur? I assume we're still talking about compression braking versus manual braking, right?

I drive an A/T car for the sheer convenience now. Given the weather, my general feeling is "Sticks are for kids", with appropriate exceptions.

However I do down shift for longish down hill runs. Easier on the brakes, less chance of fade, and for years, cars have idled at higher RPM than I like. (My feeling is a car shouldn't accelerate to +15mph on level ground just by taking your foot of the gas, but it happens.)


Everyone should learn how to double-clutch.

Uh, no.

[Pat]

Taking the auto out of O/D on my pickup will allow it to downshift and provide engine braking while decelerating and I assume prolong brake life. I use this in town and when coming off the freeway. In O/D it will just coast and the brakes are the sole stopping power. So, a possible cause for more wear over a manual transmission.

I'm sure the logic is not the same on all transmissions and some are probably programmed to downshift based on speed/rpm regardless of gear selector position.

[mrlynn]

Haven't had a manual tranny for a while (my big vans have been A/T, and my wife prefers A/T on her smaller cars. But when I did I liked to double-clutch to match downshift RPMs. Not necessary with synchromesh, I know, but still fun to do.

Got to get a stick again. When I get my convertible. . .

/Mr Lynn

[DRR]

Black wrote:
I missed that DRR initially stated that downshifting increases fuel economy....

I never stated that. Downshifting as a matter of course for slowing decreases fuel economy by increasing engine RPM while in a lower gear, and keeping the injectors working. That increases your fuel consumption.

[space-time]

DRR wrote:
[quote=Black]
I missed that DRR initially stated that downshifting increases fuel economy....

I never stated that. Downshifting as a matter of course for slowing decreases fuel economy by increasing engine RPM while in a lower gear, and keeping the injectors working. That increases your fuel consumption.
I believe on modern cars the injectors do not pump gas in cylinders in cases like these, this is why I see 99.9 MPG in some cars in USA (infinite gas mileage) and 0.0 L/km when driving in Europe (no gas consumption).

[Speedy]

space-time wrote: I believe on modern cars the injectors do not pump gas in cylinders in cases like these, this is why I see 99.9 MPG in some cars in USA (infinite gas mileage) and 0.0 L/km when driving in Europe (no gas consumption).

Always a little fuel going in to keep the engine running.

I will put my Diesel cars with auto trans into neutral as I approach a stop, then place in drive as I get close enough to let the high compression engines slow the cars nearly to a stop.

[sekker]

Fun conversation. I have a true museum-piece - a manual hybrid. Regenerative braking collects the kinetic energy when slowing down by charging the battery.

The energy system also helps me know which driving method uses gas. I don't think my Honda uses gas when downshifting and I'm not pushing the accelerator.

I've spent the weekend teaching my son how to drive a stick. It's been a little jerky, but he's getting the hang of it.

[M A V I C]

DRR wrote:
[quote=Black]
I missed that DRR initially stated that downshifting increases fuel economy....

I never stated that. Downshifting as a matter of course for slowing decreases fuel economy by increasing engine RPM while in a lower gear, and keeping the injectors working. That increases your fuel consumption.
You didn't? What did you mean by:

DRR wrote:
As for downshifting instead of braking, just know that that puts additional stress on the motor, the transmission, and increases fuel economy

(Emphasis mine)

RAMd®d wrote:
but when I rent a newer manual car or when I drive my Dad's car. the fuel consumption goes to 0.0 (0 L per 100 Km) when downshifting.

I don't understand this.

With an automatic transmission, you take your foot of the gas pedal to use the brake pedal (normally). Butterflies close, acceleration goes to 0, but gas is fuel is still consumed, the rate of consumption decreasing as the rpm stabilize at idle.

Speedy wrote:
[quote=space-time]I believe on modern cars the injectors do not pump gas in cylinders in cases like these, this is why I see 99.9 MPG in some cars in USA (infinite gas mileage) and 0.0 L/km when driving in Europe (no gas consumption).

Always a little fuel going in to keep the engine running.

I will put my Diesel cars with auto trans into neutral as I approach a stop, then place in drive as I get close enough to let the high compression engines slow the cars nearly to a stop.
space-time has it correct. As mentioned by another poster above, most (all?) modern petrol cars cut fuel when the car is coasting and the RPMs are above a certain level. By putting the car in neutral, your car is actually consuming more fuel since it needs fuel to keep from stalling. Whereas when it's coasting it doesn't. I'm not sure if diesels do this or not.

Some newer cars even cut the engine at stoplights and start it back up when you push on the gas.

[DRR]

M A V I C wrote:
[quote=DRR]
[quote=Black]
I missed that DRR initially stated that downshifting increases fuel economy....

I never stated that. Downshifting as a matter of course for slowing decreases fuel economy by increasing engine RPM while in a lower gear, and keeping the injectors working. That increases your fuel consumption.
You didn't? What did you mean by:

DRR wrote:
As for downshifting instead of braking, just know that that puts additional stress on the motor, the transmission, and increases fuel economy

(Emphasis mine)
Apologies to you and Black, one of those cases where I typed exactly the opposite of what I meant.

[M A V I C]

DRR wrote:
[quote=M A V I C]
[quote=DRR]
[quote=Black]
I missed that DRR initially stated that downshifting increases fuel economy....

I never stated that. Downshifting as a matter of course for slowing decreases fuel economy by increasing engine RPM while in a lower gear, and keeping the injectors working. That increases your fuel consumption.
You didn't? What did you mean by:

DRR wrote:
As for downshifting instead of braking, just know that that puts additional stress on the motor, the transmission, and increases fuel economy

(Emphasis mine)
Apologies to you and Black, one of those cases where I typed exactly the opposite of what I meant.
but you were originally correct