Best speed for best milage?

[Andrew Hess]

For whatever reason, I've always assumed that the best gas milage on a
car was at about 75mph, or 3k rpm. I really have nothing to base this
on, though. For the subies, in particular the my04 Forester 2.5x, what
is the best mph for best mpg? I know there are probably lots of
variables that enter into this (load, wind, etc), but in general, how
fast?

Andrew

 

[TG]

I have not done this on a Forrester but most consumer cars and trucks get
the best at a much lower speed...55-65MPG. Let us know what you find if it
doesn't bore you to death doing the experiment. TG

 

[Rodrigo Diaz]

Warp Factor 7.

 

[Bruce Hoult]

For whatever reason, I've always assumed that the best gas milage on a
car was at about 75mph, or 3k rpm.

It's hard for me to find out, since 75 mph is well over the speed limit
here in NZ, but FWIW when I travel between Wellington and Auckland I
have a choice of doing 80 - 105 km/h in heavy traffic on the wide and
fairly straight SH1, or finding a hillier, more twisting, narrow
tertiary road and taking a lot of corners at 80 km/h and doing 120+ km/h
on the straights.

I seem to get about the same MPG either way, or maybe even slightly
better on the back roads.

It's *very* noticable that hills that require a bootfull of gas in top
gear (or even a downchange) at 100 km/h can be taken with only the
lightest of pressure at 120+, so I can well imagine that 120+ might be
more economical as well.

Unfortunatly, with only a 4 speed auto, forcing it into 3rd all the time
isn't really an answer, since you end up at 4000 RPM, not 3000, and that
doesn't feel very economical at all (it would be 160 km/h in 4th).

-- Bruce

 

[Hawksoob]

For whatever reason, I've always assumed that the best gas milage on a
car was at about 75mph, or 3k rpm. I really have nothing to base this
on, though. For the subies, in particular the my04 Forester 2.5x, what
is the best mph for best mpg? I know there are probably lots of
variables that enter into this (load, wind, etc), but in general, how
fast?

Andrew

Best mileage would be zero. ;-)

According to the cartalk guys, the speed you're going right after
you've shifted into the highest gear is the most fuel efficient.
Typically around 45mph. If you could conceivably drive that speed in
that gear for the majority of the time, you would be getting the best
mileage.

 

[Bruce Hoult]

Best mileage would be zero. ;-)

No, that would be the worst possible, assuming that the engine is
actually running.

According to the cartalk guys, the speed you're going right after
you've shifted into the highest gear is the most fuel efficient.
Typically around 45mph. If you could conceivably drive that speed in
that gear for the majority of the time, you would be getting the best
mileage.

Sometimes I shift into top gear at 50 km/h, and sometimes at 150 km/h.
Which should I pick?

I probably shift into top gear most often at 109 km/h, that being the
highest speed consistent with not getting a speeding ticket in a 100
km/h zone. If not for that constraint, I'd probably shift at 120 km/h,
in line with the previous poster's posited 75 mph.

-- Bruce

 

[Cam Penner]

According to the cartalk guys, the speed you're going right after
you've shifted into the highest gear is the most fuel efficient.
Typically around 45mph. If you could conceivably drive that speed in
that gear for the majority of the time, you would be getting the best
mileage.

I've heard that the best economy on a four stroke internal
combustion engine is when the engine is being lugged at low
RPM. Lots of throttle, low RPM. This is the best
efficiency for the ENGINE to make power, and has nothing to
do with wind resistance. In theory, lugging the engine at
low RPM in top gearing would produce the best MPG. The
engine is producing the most power for the least gas, and
running that power through the highest gear in the
transmission.

Unfortunately, lugging an engine at low RPM with full
throttle is also supposed to be one of the hardest things
you can do to the engine, so your real economy may vary.

 

[Bruce Hoult]

Cam Penner

I've heard that the best economy on a four stroke internal
combustion engine is when the engine is being lugged at low
RPM. Lots of throttle, low RPM. This is the best
efficiency for the ENGINE to make power, and has nothing to
do with wind resistance. In theory, lugging the engine at
low RPM in top gearing would produce the best MPG. The
engine is producing the most power for the least gas, and
running that power through the highest gear in the
transmission.

That's true, but only if the engine in this condition produces exactly
the right amount of power to maintain speed on a flat road. For the
vehicles we're talking about that is nowhere near being the case ... the
engine will be producing far more power than required at that speed.
Given the size of engines we have (e.g. 2.5L), and assuming an engine
speed of 2000 RPM, I'd guess that a subie would be capable of pushing a
gear giving around 160 km/h (100 mph) at 2000 rpm, which is twice as
tall as normal gearing.

Unfortunately, lugging an engine at low RPM with full
throttle is also supposed to be one of the hardest things
you can do to the engine, so your real economy may vary.

That's true as well. You'd have to build it stronger than they are now.

-- Bruce

 

[Ross E.]

Cam Penner


That's true, but only if the engine in this condition produces exactly
the right amount of power to maintain speed on a flat road. For the
vehicles we're talking about that is nowhere near being the case ... the
engine will be producing far more power than required at that speed.
Given the size of engines we have (e.g. 2.5L), and assuming an engine
speed of 2000 RPM, I'd guess that a subie would be capable of pushing a
gear giving around 160 km/h (100 mph) at 2000 rpm, which is twice as
tall as normal gearing.



That's true as well. You'd have to build it stronger than they are now.

-- Bruce

I thought I read somewhere ages ago that when the engine is producing it's
max torque it is at it's most efficient point in the rev range.
Of course 3500-4000rpm in top gear would bring wind resistance into the
equation so that wouldn't work.

 

[Hawksoob]

No, that would be the worst possible, assuming that the engine is
actually running.



Sometimes I shift into top gear at 50 km/h, and sometimes at 150 km/h.
Which should I pick?

I probably shift into top gear most often at 109 km/h, that being the
highest speed consistent with not getting a speeding ticket in a 100
km/h zone. If not for that constraint, I'd probably shift at 120 km/h,
in line with the previous poster's posited 75 mph.

-- Bruce

What that means is the fastest the wheels can go with the least amount
of RPMs, which would typically be the slowest speed in the top gear.

YMMV

 

[Edward Hayes]

Seems to be some misconceptions here. Maximum engine efficiency is where the
engine is producing the maximum horse power per gallon of fuel consumed. The
occurs at the maximum BMEP (break mean effective pressure). That usually
occurs near the maximum torque value. The maximum fuel economy occurs where
you get the most miles per gallon. With so many variables it's difficult to
predict but I suspect it consides with the lowest vehicle
speed(aerodynamics) and lowest engine speed where the engine is running
clean and smooth. I have no data but, I am guessing that's about 16-1800 rpm
and ~38 mph on my Forester S (fourth gear and torque converter locked up).
eddie

 

[C. Brunner]

For whatever reason, I've always assumed that the best gas milage on a
car was at about 75mph, or 3k rpm. I really have nothing to base this
on, though. For the subies, in particular the my04 Forester 2.5x, what
is the best mph for best mpg? I know there are probably lots of
variables that enter into this (load, wind, etc), but in general, how
fast?

Here's an EPA (U.S.) guide on fuel economy:
http://www.fueleconomy.gov/feg/FEG2004intro.pdf

There's a graph in that document that indicates the best mpg will be
achieved at a speed of 45 to 55 mph. That's probably the standard
government line, though, not taking all the variables into account.
You can also get the standard EPA mpg figures for U.S. cars of various
makes, models, and years at that site:

http://www.fueleconomy.gov/feg/findacar.htm

C. Brunner

 

[Mickey]

Seems to be some misconceptions here. Maximum engine efficiency is where the
engine is producing the maximum horse power per gallon of fuel consumed. The
occurs at the maximum BMEP (break mean effective pressure). That usually
occurs near the maximum torque value. The maximum fuel economy occurs where
you get the most miles per gallon. With so many variables it's difficult to
predict but I suspect it consides with the lowest vehicle
speed(aerodynamics) and lowest engine speed where the engine is running
clean and smooth. I have no data but, I am guessing that's about 16-1800 rpm
and ~38 mph on my Forester S (fourth gear and torque converter locked up).
eddie

Edward is correct with his comments but the biggest factor has all but not
mentioned and that's wind resistance. From an engineering standpoint, air drag
is a ^3 (cube) function. What that means is it takes 8x power to double the
speed not 2x as some might think. Without a lot of tech work it is hard to say
exactly what speed will provide the best mileage but keeping it under 50-55 mph
would be a good bet.

Mickey

 

[Edward Hayes]

Mickey: I used the word Aerodynamics instead of wind resistance so we're on
the same wavelength. eddie

 

[Rick Courtright]

mentioned and that's wind resistance. From an engineering standpoint, air drag
is a ^3 (cube) function. What that means is it takes 8x power to double the

From a "not an engineer" standpoint, I'd always thought it was a square
(^2) factor, but regardless, wind resistance is THE killer factor.

exactly what speed will provide the best mileage but keeping it under 50-55 mph

Back in the early '70s when the US instituted the 55 mph national speed
limit in response to gas "shortages," I was led to believe that speed
was chosen as the best "average" top economical speed for the "most"
vehicles. Naturally, some were more or less affected by increases in
speed: I drove a Fiat 850 Spyder at the time that got virtually
identical economy between 55 and 70 mph, but others I knew saw widely
differing results depending on what they were driving. I got a laugh at
the time out of the highway patrolmen (several) who chased me in their 6
or 8 mpg Dodge cruisers to give me tickets for "wasting" gas by going
over 55 mph--while I was getting right at 35 mpg out of my Fiat! Oh,
well...

Anyway, in response to the OP's question, only testing will tell for
sure! But as others have suggested, a range of 50-60 mph is probably a
good target zone.

Rick

 

[Bruce Hoult]

From a "not an engineer" standpoint, I'd always thought it was a square
(^2) factor, but regardless, wind resistance is THE killer factor.

Drag force is proportional to the square of the speed. Power required
is proportional to the cube of the speed.

Back in the early '70s when the US instituted the 55 mph national speed
limit in response to gas "shortages," I was led to believe that speed
was chosen as the best "average" top economical speed for the "most"
vehicles.

Right. But it's completely dependent on design.

Land barges in the 70's probably *did* do best at 55 mph, but there's
nothing magical about that number. A Boeing 747 has its best fuel
consumption (best range on a tankfull) at something like 500 mph.
Different designs in terms of aerodynamics and amount of power available
could produce a most economical speed anywhere in between (or outside)
those numbers. As a bicyclist, my best range is probably at around 10 -
12 mph.

There is no scientific reason whatsoever to doubt that certain modern
cars might well get their best fuel economy at 120 km/h (75 mph) or
higher.

-- Bruce

 

[Rick Courtright]

Drag force is proportional to the square of the speed. Power required
is proportional to the cube of the speed.

Thank you! I knew there was a square factor in there somewhere...

There is no scientific reason whatsoever to doubt that certain modern
cars might well get their best fuel economy at 120 km/h (75 mph) or

'Tis very possible... when you look at drag coefficients on cars
today--regular passenger cars--which are not far off what race car
designers were shooting for not that many years ago, the game has
changed considerably. Plus gearing, engine output parameters, fuel
delivery, tires, etc. can all be manipulated to produce a variety of
results hardly dreamed of 20 or 30 years ago. Still, only hands on
testing can tell us what a particular car can or will do, though. And
even that's at least somewhat dependent on the driver: I've driven cross
country in informal tests with other drivers, all going as close to the
same speed as possible during their shifts at the wheel, where fuel
economy varied by over 10% from one driver to another. So I might be
able to get the same economy at 70 mph that another driver could only
get at 60, which makes it difficult to say what's best for a given car!
I'm thinking one would have to run many thousands of miles at speeds
ranging from perhaps as low as 45 mph to 75 or better to establish the
ranges and make a valid determination based on the combination of both
car and driver. As they say, YMMV!

Rick

 

[Edward Hayes]

Many years ago when I worked for Argonne labs electric vehicle program we
used a coast-down test to determine the horse power required vs. a vehicle
speed. You take the vehicle to the highest speed and decouple the engine
(declutch) and measure time vs. vehicle speed to establish a "coast-down
curve. I cant remember the books name but I think it was "Vehicle dynamics:
by Torboric. Wish I could find a copy as so much good stuff like comparing
AWD vs. 2 wheel drive etc. Anyone familiar with this book?? eddie

 

[Guest]

: Many years ago when I worked for Argonne labs electric vehicle program we
: used a coast-down test to determine the horse power required vs. a vehicle
: speed. You take the vehicle to the highest speed and decouple the engine
: (declutch) and measure time vs. vehicle speed to establish a "coast-down
: curve. I cant remember the books name but I think it was "Vehicle dynamics:
: by Torboric. Wish I could find a copy as so much good stuff like comparing
: AWD vs. 2 wheel drive etc. Anyone familiar with this book?? eddie

I got my Masters degree working on a hybrid electric vehicle, and we did the same
coast-down test. Bottom line is (as mentioned earlier) drag force goes with the square,
so drag power goes as the cube. The rolling resistance force stays roughly constant with
speed so for a given weight the power is linear with speed. Add those two, and you get
something dominated by a linear term at low speed and cubic term at high speed. In older
cars (>10 years old), these two values are equal at around 45 mph. In newer cars, it's
more like 55-60 mph. That doesn't mean that's where it's most efficient, but rather where
it goes much less efficient quickly.

Given that, ignoring the engine size/gear ratio choices for a specific car, the
best mileage is at the slowest speed. Problem is engines/gear ratios are generally sized
for giggle-factor, not economy.

-Cory

--
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* (Just my 20 USm$) *
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[Jim D]

Here's an EPA (U.S.) guide on fuel economy:
http://www.fueleconomy.gov/feg/FEG2004intro.pdf

There's a graph in that document that indicates the best mpg will be
achieved at a speed of 45 to 55 mph. That's probably the standard
government line, though, not taking all the variables into account.
You can also get the standard EPA mpg figures for U.S. cars of various
makes, models, and years at that site:

http://www.fueleconomy.gov/feg/findacar.htm

C. Brunner

By experience in my 2000 Subaru Outback, with the 4-cylinder boxer engine,
my best gas mileage occurs when I'm driving between 45 and 70 MPH. Over 70
and it drops off noticeably. Under 45, and I'm usually in stop-and-go
traffic, which gives so-so gas mileage. I just changed jobs, and my commute
went from 22 miles down to 8 miles, one way. My miles-per-gallon also went
down, from 25 to 20, cause the old commute was on Interstate highway and the
new commute is in town.