Why you should never buy a car without a tachometer

Is it 40% of the red line, or of the max RPM?

The Maxima engine has a redline of 6600, and a max of 8000.

40% of redline makes more sense to me.

Dave C. wrote:
> > The reason I'm questioning your statement is that the effect of wind
> > resistance on milage increases faster than speed. (It isn't arithmetic)
> > I can see this effect on my cars during a 375 mile trip that is
> > repeated several times a year to North Carolina. Driving at 60mph
> > results in 33mpg while driving at 80mph results in 26mpg for the same
> > distance. I'm puzzled about that you can have such different results.
> >
>
> You shouldn't be puzzled at all. You are missing certain facts that would
> clear things up quickly for you. Does your car have a tachometer? If so,
> what is the highest number that the tachometer reads? Multiply that times
> .4. For example, if your tach goes up to 9000, multiply by .4 to get 3600.
> NOW, assuming your tach goes up to 9 grand, and your 40% level is 3600, you
> will probably find that 3600RPM is somewhere below 80MPH in your car. 33MPG
> at 60MPH is not bad, as far as fuel economy goes, and I don't even KNOW what
> kind of car you drive. BUT, based on the fact that you get 33MPG at 60 but
> only 26MPG at 80, it's easy to (LOGICALLY) conclude that, YOUR engine is
> running closer to 40% at 60MPH than it is at 80MPH. This is just a total
> guess, but 80MPH for your engine might be 50% or higher, but definitely
> higher than the optimum 40% RPM speed. -Dave

Wish the world of fuel efficiency were that simple as applying a
constant to rpms.

At 60mph the engine is turning 2000 rpm. What's the next guess.....

"Ted B." > wrote:
>Over the years I've owned my current ride, I have observed that it is most
>fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
>observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
>or lower. My experience with my current vehicle is similar to several other
>vehicles I've owned. In every case, I've found that the best fuel economy
>is achieved somewhere above 70MPH.

This discussion pops up every so often on r.a.d., and once
again, I feel the need to chime in... I don't understand
why a modern vehicle should behave like yours does, but
what do I know. FWIW, I have never observed similar
behavior in any vehicle I've driven for extended periods.

[...]
>I've always known that every engine has a "sweet spot", but didn't know the
>exact scientific explanation for my personal observations until just
>yesterday. During research on various engine types, I happened to discover
>that Otto Cycle engines are most fuel-efficient at RPMs near (not
>necessarily at, but near) 40% of redline. I had no reason to disbelieve
>that particular scientific fact.

I suspect it is far from a scientific fact. At best, it's a
very rough correlation. Even then, rough correlations can
be useful, but I think this one has little relevance to
maximizing cruising fuel economy in a street car or truck.

Did your research discuss part-throttle fuel efficiency or
only wide-open throttle (WOT) fuel efficiency? Street
vehicles don't cruise at WOT, so without access to a map of
BSFC plotted against engine load *and* rpm, the only way to
plot fuel consumption vs speed is by accurate measurement.

FWIW, cruising around for a while at roughly a certain speed
and then measuring fuel usage some time later is *not* an
accurate measurement technique. Without *instrumentation*,
either built-in or added-on, any attempt at correlating mpg
to speed is (at best) a guess.

>I did the math for my own car, and found
>that (surprise, surprise) I SHOULD achieve maximum fuel economy at ABOUT
>3200RPM, with my own Otto Cycle engine. This agrees almost exactly with my
>own observations that 3500RPM (43% of redline) is very fuel efficient, and
>3000RPM or less (37% of redline or lower) is not as fuel efficient.
>Obviously, my engine is very fuel-efficient near 40% of redline, AS IT
>SHOULD BE, and it is less fuel-efficient at lower RPM levels. Note that I
>had no idea that my engine SHOULD be most efficient at about 40% of redline
>before I observed that it WAS most efficient at 3500RPM. So the results I
>observed were not results that I "expected" to observe. The facts are what
>the facts are, and now I have the scientific explanation stating YES, it
>should be that way.

If _I_ use the 40% "rule", my car (4th-gen Corvette 6-speed)
should deliver best fuel economy at about *100 mph*. It
does not. In fact, it delivers best mpg at about 40 mph,
and mpg decreases continuously with higher speeds. At 100
mph/40% redline, mpg is roughly half of the maximum.

[...]
>On a side note, it's easy to understand why manual tranny vehicles are more
>fuel-efficient for use in City driving. You have to get the engine RPMs UP
>to maximize fuel economy, and you just can't do that at low speeds in a
>slush-box.

No way. Increasing rpm (at a given speed) is an excellent
way to *reduce* fuel economy. The engine output does not
increase (constant speed), but the internal friction power
required by the engine increases. IOW, the engine requires
more power consumption to do the same work.

Furthermore, increasing cruising rpm at constant road speed
will force a reduction in throttle angle (more rpm, same
power output), which increases pumping losses. All in all,
running higher rpm than required to maintain cruising speed
is *bad* for fuel economy.

FWIW, if I drop from 6th to 5th in my car at 65-75 mph, mpg
(observed on the mpg readout) will drop by ~20%, even though
5th gear puts the engine at the magic 40% at 67 mph.

I have no way of knowing whether your vehicle does what you
think it does, but I *know* that not all vehicles behave
similarly.
--
Chuck Tomlinson

> Ted B. wrote:
> clipped it - no need to copy all that again.

So "Dave" or "Ted" or "Alice" what ever name you are posting under now.
When are you going to tell us the details of your milage test. We're
waiting for the specifics on how you determined in the real world that
a car going at 80 mph get's better milage than one going 50 or 60 mph.
Anything is possible if you ignore enough of the real world, like wind
resistance and inefficiency of the motor at high speeds. Yeah, just
take the high RPM marker on the tach and multiply by .40 to get the
most efficient engine and therefore car speed. Why don't you send that
in for the Car Guys next show...that'll be great material for saturday
morning laughs!

I'm sitting here thinking about my brother-in-law and his 1995 vette.
As I remember it is loafing along at about 1500 rpm or maybe a little
slower at 60mph. By your formula he should be getting great milage
somewhere above 100mph right? Wonder why it doesn't work that way.

Ted B. wrote:

> Over the years I've owned my current ride, I have observed that it is most
> fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
> observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
> or lower. My experience with my current vehicle is similar to several other
> vehicles I've owned. In every case, I've found that the best fuel economy
> is achieved somewhere above 70MPH.

Several of your OTHER vehicles have also had the mysterious ability to
get better MPG while producing more power at higher RPMs?

NOW we know you're really making this stuff up. Some people always seem
to be abducted by aliens while others never see them.

C.T.

http://www.geocities.com/aggressive_driving/

John S. wrote:

> If I understand you correctly you are saying that your car gets better
> milage at 78mph (3500rpm) in high gear than it got at lower increments
> say: 70mph, 60mph and 50mph?
>
> Were you able to hold all the variables constant for a long enough time
> to get a reasonably accurate reading for each increment?
>
> Did you use the car computer to measure the miles per gallon?

Ted/Dave has stayed consistently vague about those details. His car is
a 1995 Mitsubish Galant, and here are its published MPG figures:

http://www.fueleconomy.gov/feg/noframes/11897.shtml

MPG (city) 23
MPG (highway) 30
MPG (combined) 26

I think this all got started when someone created a thread suggesting
3,000 RPM mandate would save fuel. Dave/Ted said no way, I get better
MPG at 3,500 RPM, and he offered no legit reasons for it. He even made
a crazy analogy to a human arm lifting a weight so slowly that it
fatigued from static stress (engines suffer no such weakness).

What we have here is a chronic speeder inventing more ways to justify
speeding. He's really just full of it and all these polite responses
amuse me.

C.T.

http://www.geocities.com/aggressive_driving/

Chuck Tomlinson wrote:

> "Ted B." > wrote:
> >Over the years I've owned my current ride, I have observed that it is most
> >fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
> >observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
> >or lower. My experience with my current vehicle is similar to several other
> >vehicles I've owned. In every case, I've found that the best fuel economy
> >is achieved somewhere above 70MPH.
>
> This discussion pops up every so often on r.a.d., and once
> again, I feel the need to chime in... I don't understand
> why a modern vehicle should behave like yours does, but
> what do I know. FWIW, I have never observed similar
> behavior in any vehicle I've driven for extended periods.

No need to be that tactful. His claims are mostly a ruse to justify
speeding. I don't know if it's deliberate or the happy coincidence of
faulty measurements, since he never quite cites his methods. Here's the
official word on his car:

http://www.fueleconomy.gov/feg/noframes/11897.shtml

> If _I_ use the 40% "rule", my car (4th-gen Corvette 6-speed)
> should deliver best fuel economy at about *100 mph*. It
> does not. In fact, it delivers best mpg at about 40 mph,
> and mpg decreases continuously with higher speeds. At 100
> mph/40% redline, mpg is roughly half of the maximum.

That agrees perfectly with the findings of this and other links on the
topic.

http://auto.howstuffworks.com/question477.htm

There's no free "power lunch" and fuel economy is the price to be paid.
There will always be people who can't handle the truth when it
conflicts with a lifestyle. My whole crusade on r.a.d. was/is about
exposing that selfish aspect of human nature. I'll never win, I know.
I'm fighting millions of years of mind-game evolution!

> Furthermore, increasing cruising rpm at constant road speed
> will force a reduction in throttle angle (more rpm, same
> power output), which increases pumping losses. All in all,
> running higher rpm than required to maintain cruising speed
> is *bad* for fuel economy.

Put more simply, once you're in top gear you burn more fuel the faster
you go. Except in a gravity-free vacuum, keeping a mass moving at a
higher speed requires more power, and more power means more air and
fuel passing through the cylinders. Good old physics, right Chuck?

We're in a spot where we need to conserve oil more than ever but some
people won't give up their fast ways. The lengths they'll go to don't
surprise me, given the history of this newsgroup. How about you, old
sage? You might even be able to squeeze 30 MPG out of that Corvette if
you hold down your highway speed in 6th with a tailwind.

C.T.

http://www.geocities.com/aggressive_driving/

N8N wrote:
> wrote:
> > Valve float is typically the limiting factor. To keep friction low you
> > want as light a spring as you can, but that lowers the red line.
> > Piston speed relates more to connecting rod strength, but they can take
> > considerable rpm, well above the redline or the typical spot where
> > you'll want to put the horsepower peak. Basically the power game is to
> > get the horsepower peak where you want it, then use as light a valve
> > spring as you can. The result is you usually end up with the redline
> > not too far over the horsepower peak. Indeed, there's bound to be a
> > safety factor built in. They'll lower the redline a bit from what will
> > probably be fine. I ran my engine a good 500-600 rpm past the redline
> > and the whole engine sound changed. Most likely valve float there.
> > :-) I don't recommend trying that though of course..
> >
> > Todd Wasson
> > Performance Simulations
> > http://www.PerformanceSimulations.com
> > http://www.PerformanceSimulations.co.../ToddSim9a.wmv
>
> Sometimes, sometimes not. I once had to over-rev an old Rabbit GTI,
> the valves did indeed float about 700 RPM over redline (was turning
> left, oncoming driver ran a red light and was headed right toward me,
> didn't feel like taking the time to shift.) I've repeatedly
> over-revved my GTI 1.8T however, as the only indication that you've
> exceeded the redline is the actual tachometer. Power doesn't fall off,
> valves don't float, it just pulls like a freight train. I miss that
> car (sold it to my mom, if you can believe that. She loves it.) I
> have no idea what consideration the engineers used to set the redline;
> the earlier 16V NA engines (same basic block) had a much higher redline
> so it couldn't have been piston speed or bearing issues. Maybe just a
> little CYA?
>
> Now my old cars - all Studebakers - yeah, the valvetrain is the
> limiting factor. 5200 RPM - float city, it's all over. I'm working on
> rectifying that situation
>
> Haven't blown up an engine yet (knock on wood) save for an old Bimmer
> that I didn't abuse but I suspect that the PO did...
>
> nate

I was referring more to the mechanical limitations that dictate the
real red line; i.e., the point where you can be pretty sure something
is going to break. Valve float in the 'real redline' context is
generally going to be the limiting factor in a production car. Of
course, the red line that's stuck on your tach is going to be set lower
than that to give some safety margin. Who knows, maybe there are cars
out there where manufacturers got a good deal on tachs that had a
redline that was well under the limit, but still over the power peak
and just decided to throw them in ;-)

Dave C. wrote:
> > >
> > > I didn't calculate it. Through research, I learned that the Otto Cycle
> > > engine is most fuel-efficient at about 40% of redline RPM. That's not
> my
> > > calculation, it's just how the Otto Cycle engine works. -Dave
> >
> >
> > >From your original post:
> >
> > "During research on various engine types, I happened to discover
> > that Otto Cycle engines are most fuel-efficient at RPMs near (not
> > necessarily at, but near) 40% of redline. I had no reason to
> > disbelieve
> > that particular scientific fact. I did the math for my own car, and
> > found
> > that (surprise, surprise) I SHOULD achieve maximum fuel economy at
> > ABOUT
> > 3200RPM, with my own Otto Cycle engine. "
> >
> > Was curious what math you said you did there for your own car. No big
> > deal really, was just curious. I've written a couple of engine
> > simulations and it hasn't yet dawned on me how one would use the Otto
> > Cycle to derive any conclusions like this. I'm not arguing or saying
> > it's wrong, after all, the best fuel efficiency isn't going to be at 1
> > mph, nor 300 mph, so it'd have to be somewhere in between :P
> >
> > Todd Wasson
> > Performance Simulations
>
> OH! I guess I misunderstood the question. My own car tops out at 8000 RPM,
> so if the maximum fuel efficiency RPM is ABOUT 40% (for any Otto Cycle
> engine, not just mine) of that, then 8000 X .4 equals 3200. So, 3200RPM is
> ABOUT where my own engine should be most fuel efficient. I already knew
> that 3500RPM was the most fuel-efficient speed for my engine, so when I
> learned that it should be about 3200RPM, I wasn't surprised, at all. Yes,
> that computes. -Dave

Oh, ok. Actually, after I'd written that I reread your earlier post
and realized that's what you'd done.

I understand you're talking in approximate terms here, but that rule is
really not quite right. If you're indeed getting the best fuel economy
at that speed and rpm, I'd take the 40% bit as a mere coincidence :-)

>
> I understand you're talking in approximate terms here, but that rule is
> really not quite right. If you're indeed getting the best fuel economy
> at that speed and rpm, I'd take the 40% bit as a mere coincidence :-)
>

But it's not coincidence at all. An Otto Cycle engine (any Otto Cycle
engine, not just my own) will be most fuel-efficient near 40% of redline
RPM. So the fact that my own Otto Cycle engine is indeed most
fuel-efficient near 40% of redline is hardly a coincidence. -Dave