Gas usage at the same RPM question... - Fuelly Forums

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Old 01-04-2008, 08:11 PM   #1
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Gas usage at the same RPM question...

Hi,

I was watching the instantaneous mpg gauge (the one that bops all around while you drive) and I was travelling on a slightly up and down hill road using cruise control. What I noticed surprised me is that even though the rpm stayed the same and I stayed in the same gear, same speed, the MPG gauge was going up and down. I guess I always thought that the engine would use the same amount of fuel at a specific RPM, but now I am thinking this is obviously a wrong assumption.

So, would it be true that an engine that is being pulled to a specific RPM by the transmission is using much less fuel than an engine that is pushing the transmission at the same RPM ?

Thanks,

Alan
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Old 01-04-2008, 08:26 PM   #2
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Hey Alan

I just read this same post on SL . . . try lightly resting your foot on the gas pedal when using the cruise control and you will feel it moving up and down as you go down and up the hills. If you want to see a constant MPG reading then you have to lock the throttle position and let the speed vary as you go up and down hills and even then you will see the MPG vary. The throttle is still moving and varying the intake vacuum and fuel being injected which will vary the power that the engine will produce even if the RPM stays constant.
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Old 01-04-2008, 08:29 PM   #3
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Hi,

Yep, I posted it over there a little earlier. I thought I'd try here as well so see what people think about it!

Thanks,

Alan
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Old 01-04-2008, 08:39 PM   #4
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Welcome fellow xB owner to GasSavers and get your car setup in the garage and start tracking your mileage! And you can ignore the SL post where the guy says the MPG gauge is really a vacuum gauge - it really isn't.
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Old 01-05-2008, 05:55 AM   #5
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LOAD is the culprit...throotle position changes to compensate for load.
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Old 01-05-2008, 01:00 PM   #6
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The fuel economy is affected by engine speed as you say, but also load, as shown :

[FUEL CONSUMPTION] = [ENGINE SPEED] x [ENGINE LOAD]

obviously there is an adjustment factor there too. Also if you floor it, the engine injects too much fuel to protect the engine from overheating/detonating.

The more you push your foot down on the accelerator pedal, more air is let into the engine, and so, more fuel is also injected, so that the air/fuel mixture is balanced.

The best way to save fuel (when starting with fuel-efficient driving anyway), is to get up to speed using about 50-70% throttle, keeping the revs low, changing at 3000rpm or below, then cruising in 5th gear / top gear, with as little accelerator pedal as possible.
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Old 01-05-2008, 04:13 PM   #7
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Load on the engine is what is changing the volume of fuel used.

If the car is EFI then the computer will open the injectors for a longer period to add more fuel to handle the added load. The RPM may not change.

Carbs have vacuum sensors which perform the same function but mechanically.

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Old 01-05-2008, 05:38 PM   #8
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Hi,

Thanks for the info everyone, I got my ScanGauge today and it has been fun to play with so far!

Alan
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Old 01-26-2008, 11:50 AM   #9
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Quote:
Originally Posted by alank2 View Post
Hi,

Thanks for the info everyone, I got my ScanGauge today and it has been fun to play with so far!

Alan
Yes - try your cruise control experiment again, but this time watch TPS on your ScangaugeII. You should see it increase up the hills, and decrease down the hills, due to load.
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