Fuel Economy vs RPM

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Old 16 Aug 2005, 01:29 pm   #1 (permalink)
nat2k5us@gmail.com
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Default Fuel Economy vs RPM

I have an AT Toyota Comrey 1999.

I want to know why the RPM stays high when I do not press the
accelrator and since fule consumption is directly proportinal to RPM.
When I am going downhill I don't depress the accelrator but still the
RPM stays above 2500 ( 85 MPH). Is the fuel consumption high even in
this case?

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Old 16 Aug 2005, 01:36 pm   #2 (permalink)
karinhall
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Default Re: Fuel Economy vs RPM


<nat2k5us@gmail.com> wrote in message news:1124216950.973794.236600@o13g2000cwo.googlegr oups.com...

>I have an AT Toyota Comrey 1999.
>
> I want to know why the RPM stays high when I do not press the
> accelrator and since fule consumption is directly proportinal to RPM.
> When I am going downhill I don't depress the accelrator but still the
> RPM stays above 2500 ( 85 MPH). Is the fuel consumption high even in
> this case?
>

Huh? ROFL
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Old 16 Aug 2005, 04:22 pm   #3 (permalink)
Jason James
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Default Re: Fuel Economy vs RPM


<nat2k5us@gmail.com> wrote in message
news:1124216950.973794.236600@o13g2000cwo.googlegr oups.com...
> I have an AT Toyota Comrey 1999.
>
> I want to know why the RPM stays high when I do not press the
> accelrator and since fule consumption is directly proportinal to RPM.
> When I am going downhill I don't depress the accelrator but still the
> RPM stays above 2500 ( 85 MPH). Is the fuel consumption high even in
> this case?



First, hi RPM does cause a degradation in economy, but not as much as to be
a great problem. If you are towing or have a hill to climb, your engine
needs RPM to deliver torque.

At 85 mph. 2500 rpm on a down-hill grade is to be expected. I'm surprised
its not higher.

Jason


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Old 17 Aug 2005, 09:23 am   #4 (permalink)
nat2k5us@gmail.com
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Default Re: Fuel Economy vs RPM

I also read about a fuel cut off when going downhill .. i.e only the
gas required to keep the oxygen sensors warm is pumped.

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Old 17 Aug 2005, 10:10 pm   #5 (permalink)
Merritt Mullen
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Default Re: Fuel Economy vs RPM

In article <1124288639.062696.72640@o13g2000cwo.googlegroups. com>,
nat2k5us@gmail.com wrote:

> I also read about a fuel cut off when going downhill .. i.e only the
> gas required to keep the oxygen sensors warm is pumped.


Normally, that is called the throttle.

If you cut off fuel completely, the engine, of course, stops running (even
though the drive shaft may be causing the parts to rotate) and must be
restarted.

Merritt
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Old 18 Aug 2005, 09:56 am   #6 (permalink)
Daniel
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Default Re: Fuel Economy vs RPM

The computer controlled electronic fuel injection system maximizes fuel
economy.
If you're coasting downhill, regardless of engine speed, with the
throttle closed, fuel consumption is minimal.
Fuel consumption occurs when fuel is sprayed through the injectors.
Injector "pulse width" (how long they are open) is controlled (in
addition to engine speed) by temperature, throttle position and engine
load (intake manifold vacuum).
To go one step further, with the throttle closed, manifold vacuum is
high, so fuel consumption would be low. At larger throttle openings,
vacuum drops, fuel consumption increases.
At highway speeds, the lockup torque converter engages to improve fuel
economy by eliminating drive line inefficiency. That is why the engine
speed (RPM) is constant coasting downhill at 85 mph.

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Old 18 Aug 2005, 11:01 am   #7 (permalink)
nat2k5us@gmail.com
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Default Re: Fuel Economy vs RPM

Thanks Daniel and Merritt .. just by not keeping the a leg on the gas
when the car is travelling with the desired speed I was able to increse
my mileage by 20%.

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