Does engine RPM affect gas mileage?

[mugaliens]

I get up to 29 MPG average in my 'vette by using cruise control.

And I averaged 47 mpg in my 1979 VW diesel. Back in 1979. That was more than thirty years ago for those who can't count.

We're fooling ourselves if we think we're somehow making "progress" on this front. Please stop fooling yourselves. That milage is better than what you'll enjoy from a Toyota Prius.

If you want to make progress, http://www.aptera.com/".

Grow a brain, folks! Here on PF, we actually HAVE brains, so please use them, and let's begin with 100 mpg or better, ok? Thanks. We owe it to both ourselves as well as the other several billion.

 

[Jasongreat]

I guess I will have to go take the one out my truck cause its not in the SAE manual then. On a diesel, measuring the heat of the exhaust gasses tells you how much fuel is being injected. Higher the temp, the more fuel you are burning.

 

[mugaliens]

I guess I will have to go take the one out my truck cause its not in the SAE manual then. On a diesel, measuring the heat of the exhaust gasses tells you how much fuel is being injected. Higher the temp, the more fuel you are burning.

Sounds like you're talking about an EGT gauge, to me. (exhaust gas temperature) Pyrometer is an archaic usage, so if you have an archaic diesel, you might want to leave it in!

Just kidding with you...

On a more serious note, other "-ometers" originating in and around the 1700s include barometer, ceilometer, chronometer, cyclometer, ergometer, Fathometer, gasometer, geometer, hydrometer, hygrometer, kilometer, manometer, micrometer, odometer, pedometer, photometer, rheometer, seismometer, spectrometer, speedometer, tachometer, thermometer, viscometer...

And some of those we still use, so the idea of someone calling an EGT gauge a "pyrometer" is by no means archaic. Just different than what I'm used to hearing.

 

[Yalcrab]

My car gets does exactly 2000 RPM at 50 MPH and 3000 RPM at 75 MPH.
For a 75 mile non-stop trip using cruise control the engine does same number of revolutions at either speed.
It just takes less time at 75 MPH, so the amount of fuel should be the same.
It also does 4000 at 100 MPH, but I'm not that eager to get a ticket.

 

[FlexGunship]

My car gets does exactly 2000 RPM at 50 MPH and 3000 RPM at 75 MPH.
For a 75 mile non-stop trip using cruise control the engine does same number of revolutions at either speed.
It just takes less time at 75 MPH, so the amount of fuel should be the same.

I don't know of any cars that use a "constant" fuel curve. Usually the injector pulse width gets wider through the power band. It's likely that you're burning more gas at higher RPM. Furthermore, wind resistance will play a significant role at those speeds.

EDIT: Hmm, did I encourage a necropost? It's been 9 months.

 

[brewnog]

He started the necropost. Yalcrab's statement that the car 'should' use the same amount of fuel regardless of speed is incorrect, for two important reasons:

- The amount of power required to push the car forwards varies with speed (but not linearly)
- The amount of fuel injected per stroke is not constant (nor does it vary linearly with engine speed)