Doug wrote:
If gas were consumed on a linear scale, meaning that the fuel/air ratio
was 100% constant throughout both the entire RPM range and also under
all driving conditions, then yes, the slowest engine speed possible
would consume the least amount of fuel. But your argument does not
address those other points as I have.

I never said that the entire drivetrain wasn't part of the equation...
don't you remember earlier in the thread where I advocated getting his
gearing to match up with his engine's peak output? Gears that are too
low would spin the engine much faster than it would need to in order to
effectively drive the vehicle forward, which can of course, hurt fuel
economy. Gears that are too high put an unnecessary load on the engine
which can also hurt fuel economy.

If the engine makes peak torque at 2000rpm, then your best gas mileage
in 1st gear would be at 2000rpm. As soon as you shift to 2nd, engine
RPM drops, load increases, fuel economy drops a bit until you get the
engine up to it's peak output range again.

In that case, running 2000rpm in 5th gear, with little load on the
engine would move the vehicles the longest distance per drop of fuel.

Depends on what we're talking about. The engine has an ideal operating
range (rpm) at which it'll be most efficient in terms of both fuel
comsumption and power output.

In terms of distance per drop of fuel, it's a function of keeping the
engine at it's ideal operating range through gearing and driving
technique.

Simply put, when the engine is at it's peak output (making the most
power per drop of fuel) then you will get the most mileage in any
particular gear. More miles in 5th gear than 3rd obviously... But
still, you'll go further in the same gear if the engine is at its most
efficient. Gas consumption is *not* a linear scale that's directly
linked to RPM. The amount of load placed on the engine will affect the
fuel/air ratio. That's how all fuel systems on cars work, be they
carbed or fuel-injected, naturally aspirated or forced induction.

No... the fuel/air mix will be richer because you floored it, meaning
more gas comsumed to go the same distance. In many cases, more gas
will be consumed than if you'd down-shifted and spun the engine at a
higher RPM.

I used to try to stay in the highest gear possible at all times. I
stopped doing that. My fuel economy has increased. The onboard
computer doesn't lie, the gas pump doesn't lie, and my bank statements
don't lie. I have increased my mileage in terrible rush-hour driving
conditions from 23mpg (average for the week) to 28, sometimes 29+mpg.
I did this by paying attention to the load placed on the engine. A
vacuum/boost gauge is a good indicator of how hard you're pushing the
engine. I watch that gauge like a hawk now when I'm in
bumper-to-bumper traffic. It makes a difference. More vacuum means
I'm keeping my foot out of the gas pedal, and that equates to better
fuel economy.

The fact that you directly link fuel consumption to engine RPM shows
your lack of understanding on the subject.

I didn't present a link because I don't consider it to be factual info
just because I pulled it off some website. I don't need to present a
link to defeat your argument. That link you sent has truth in there
about the effect wind resistance plays in fuel economy. But it's only
part of the equation.

I'm not saying it's not "good", just that it's not complete. It
doesn't tell the whole story, which is what I'm trying to do here. You
seem to have a problem with that.

Which makes it funny that you state that *I* am not seeing the whole
picture. I keep filling in the other pieces of the picture while you
keep focusing on engine RPM only. You don't consider load or any of
the things that will affect the engine's load such as vehicle weight,
aerodynamics and wind resistance, increased rolling resistance from the
OP's use of mud tires over street tires, driving on any surface other
than a level grade, etc...

All engines have an optimal operating range for power and efficiency.
If engine efficiency was constant, and fuel consumption were tied
directly to engine RPM as you believe, then under equal driving
conditions spinning the engine at a lower RPM would indeed result in
lower gas consumption as you state. Unfortunately, that's not the
case. Lugging along in 4th gear below 1300rpm at 30mph in my car, or
just about any car, will put a much heavier load on the engine than
running in 3rd gear at a higher RPM. That's just the plain truth of
it.

You are refusing to address the fundamental principles on which an
engine operates, while ignoring real-world driving conditions. The
earth is not flat!

Really... because you seem to have an awful hard time understanding it.

Ah yes... Those damn automotive engineers and their conspiracies...
trying to make the rest of us spend more on gas!

So an engine produces more power as it turns faster? Interesting. Do
you wonder then why both torque and HP will begin to drop after a
certain point? It's because the engine is past its most efficient
operating range.

Ever wonder why we even need accelerators? If we didn't need to
increase the engine speed then we could all just drive around with our
cars idling.

That depends on many factors. My 4cyl turbo-charged car uses a whole
lot less gas and moves a whole lot faster than my Blazer with a 350.
The 350 burns a lot more fuel (energy), yet...I'm going slower...

Hmm...

That's a given...but I never mentioned anywhere that gunning it was the
way to better gas mileage. You on the other hand, did...

Why leave that out? The fundamentals do not change whatsoever. At any
given RPM more fuel is needed to generate more power. Forced induction
just means you can push more fuel and air into the engine thus
increasing its volumetric efficiency. It still burns fuel and air.

I've already told you the principles of driving with forced induction.
Where most engines reach atmospheric manifold pressure when the gas
pedal is on the floor, a turbo-charged engine will increase the
pressure above atmospheric, which is measured in pounds of boost. If
air, and only air were being added, the engine would just starve for
fuel by being run too lean. The carb or fuel-injection setup adds fuel
accordingly and the car makes more power...pretty simple.

Why don't you revisit that thread you participated in regarding diesel
engines...

Here's a link for you...

http://groups.google.com/group/rec.a...d74bbc2830f0/#

Read your messages and the responses by Willem-Jan Markerink and
myself. Think about it.

I'm starting to think you're a troll...

~jp

Efficiency is usually defined as a ratio. My ratio is MILES per GALLON.
You want specific and scientific, there it is.

A mile is a distance. A gallon is a certain volume of fuel.

EFF = miles/gallon

What is your definition of efficient?

That's one way of looking at it... You're talking about the overall
fuel efficiency of a vehicle.

The whole point of the thread is to increase MPG...or the overall fuel
efficiency of a vehicle. To do that you must understand the efficiency
of the engine, which, at this point I'm giving up trying to explain to
you. You obviously don't get it and aren't going to anytime soon.

You continue to avoid discussion of the inner workings of an engine or
even part of one (the fuel system) which indicates a lack of knowledge
on the subject. And your link dealing with how wind resistance affects
gas mileage did more to reinforce my argument (wind resistance =
increased load) than it did to support yours. We haven't even
scratched the surface of drivetrain losses and how gearing affects
power actually delivered to the wheels.

The fact that you're not addressing, rebutting, or even acknowledging
the points raised in my previous posts further leads me to believe that
you don't know what you're talking about and are indeed just a troll
trying to stir something up.

FYI, efficiency is not usually "defined as a ratio"...which would make
no sense. Efficiency is "expressed" as a percentage. For example, if
a given engine converts 30% of the heat content of the fuel (expressed
in BTUs) to usable force, that would make it 30% efficient. The same
engine with a good fuel system may extract 95% of the heat content from
the fuel. That means it's 95% efficient in terms of converting fuel to
torque, and only 5% of the fuel is wasted. But still, only 30% of the
fuel's energy comes out the crankshaft.

A ratio such as Miles per Gallon can be used as a handy way of gauging
the fuel consumption of one car over another, but it's hardly an
accurate means of describing true efficiency.

~jp

Doug wrote:

Ok, I get it. That's cool....