engine RPM vs MPG experiment

[clare at snyder dot ontario dot canada]

Where are you? In Mass the pumps say, "Contains 10% Ethanol", but when I
go to Vermont the pumps say "Contains Up To 10% Ethanol".

Now, I thought the premium grades would have the less amounts of Ethanol,
but I may be wrong. It seems to me I recall hearing that Ethanol is an
octane booster, so I'd be willing to bet the 93 octane has the most.

Maybe someone can clarify...

Don't know about other companies in other markets, but Shell in
Ontario puts 10% in regular and none in Premium, making the mid grade
E5 (5%). I know a lot of other companies are reported to be doing the
same.

 

[Butch]

2001 Forester 2.5 AT
Gas mileage measured by trip computer on the highway driving with
constant speed for at least 15 miles.
The best gas meleage was achieved on the lowest RPM.
See table:
RPM MPH MPG
1500 37 39
2000 50 34
2500 62 28
2750 68 26
3520 81 21
4200 105 13

 

[Rick Courtright]

2001 Forester 2.5 AT
Gas mileage measured by trip computer on the highway driving with

Hi,

Ya gotta love trip computers! A buddy has a Ford F-250 Super Duty, V-10
gas engine, four wheel drive, blah, blah--in other words, NOT a fuel
economy champ. But he can get 99 mpg according to his trip computer. All
he has to do is coast down one particular hill that's about seven miles
long at whatever speed fifth gear and foot off the throttle allows...
however, if he turns around and pushes it UP the same hill, the trip
computer can register as low as 4 mpg at one point!

But when he starts w/ a full tank, goes 170 miles (up and back down this
particular hill) and it takes 10 gal to fill the tank at the other end
of the "trip" he's ecstatic cuz that's the best it ever does. No matter
WHAT the trip computer said, he ACTUALLY got 17 mpg for the trip. Fuel
economy's a function of averages... over lots and lots of miles, IME.

Rick

 

[DK]

2001 Forester 2.5 AT
Gas mileage measured by trip computer on the highway driving with
constant speed for at least 15 miles.
The best gas meleage was achieved on the lowest RPM.
See table:
RPM MPH MPG
1500 37 39
2000 50 34
2500 62 28
2750 68 26
3520 81 21
4200 105 13

That's not fair comparison at all! You varied not just RPM but
also speed. Of course the milage is lower at lowers speeds.

When I compared ~ 2 vs ~ 3K RPM, I was driving at about
the same speeds, determined by speed limits along my regular
routes.

DK

 

[Carl 1 Lucky Texan]

Hi,

Ya gotta love trip computers! A buddy has a Ford F-250 Super Duty, V-10
gas engine, four wheel drive, blah, blah--in other words, NOT a fuel
economy champ. But he can get 99 mpg according to his trip computer. All
he has to do is coast down one particular hill that's about seven miles
long at whatever speed fifth gear and foot off the throttle allows...
however, if he turns around and pushes it UP the same hill, the trip
computer can register as low as 4 mpg at one point!

But when he starts w/ a full tank, goes 170 miles (up and back down this
particular hill) and it takes 10 gal to fill the tank at the other end
of the "trip" he's ecstatic cuz that's the best it ever does. No matter
WHAT the trip computer said, he ACTUALLY got 17 mpg for the trip. Fuel
economy's a function of averages... over lots and lots of miles, IME.

Rick

Another issue is whether chronic use of the car in high gears at low
RPMs will overheat/or wear engine internals faster.

Carl

 

[John O]

That's not fair comparison at all! You varied not just RPM but
also speed. Of course the milage is lower at lowers speeds.

When I compared ~ 2 vs ~ 3K RPM, I was driving at about
the same speeds, determined by speed limits along my regular
routes.

DK

Does that mean your gas mileage went down when driving in 5th gear, and went
up in 4th, while driving at the same speed?

-John

 

[DK]

Does that mean your gas mileage went down when driving in 5th gear, and went
up in 4th, while driving at the same speed?

Here is my original post:

Exclusively city driving, average trip about 5-7 miles. Was trying to
drive keeping RPMs close to 2,000 or to 3,000. For the most part
in practice it meant choice between 4th and 3rd gear.

Same pump, fuel contains "up to" 10% ethanol, same weather, two
full tanks for each "condition". The milage was consistent between
two tanks in each case within 0.3 MPG. Results:

~ 1,800-2,300 RPM = 26.7 MPG
~ 2,500-3,200 RPM = 28.1 MPG

DK

 

[clare at snyder dot ontario dot canada]

Here is my original post:

Exclusively city driving, average trip about 5-7 miles. Was trying to
drive keeping RPMs close to 2,000 or to 3,000. For the most part
in practice it meant choice between 4th and 3rd gear.

Same pump, fuel contains "up to" 10% ethanol, same weather, two
full tanks for each "condition". The milage was consistent between
two tanks in each case within 0.3 MPG. Results:

~ 1,800-2,300 RPM = 26.7 MPG
~ 2,500-3,200 RPM = 28.1 MPG

DK

Sounds right to me. The engine is rnning more efficiently at 2800 than
at 1800, for sure.

 

[Butch]

Hi,

Ya gotta love trip computers! A buddy has a Ford F-250 Super Duty, V-10
gas engine, four wheel drive, blah, blah--in other words, NOT a fuel
economy champ. But he can get 99 mpg according to his trip computer. All
he has to do is coast down one particular hill that's about seven miles
long at whatever speed fifth gear and foot off the throttle allows...
however, if he turns around and pushes it UP the same hill, the trip
computer can register as low as 4 mpg at one point!

But when he starts w/ a full tank, goes 170 miles (up and back down this
particular hill) and it takes 10 gal to fill the tank at the other end
of the "trip" he's ecstatic cuz that's the best it ever does. No matter
WHAT the trip computer said, he ACTUALLY got 17 mpg for the trip. Fuel
economy's a function of averages... over lots and lots of miles, IME.

Rick

Trip computer is installed as additional feature. (can be installed on
any vehicle by connecting to injector and speed sensor).
I have calibrated it by measuring of actual consumed fuel.
It always shows correct values. For example:
Trip - 10 miles
Consumed gas - 1.1 litres

 

[Butch]

That's not fair comparison at all! You varied not just RPM but
also speed. Of course the milage is lower at lowers speeds.

When I compared ~ 2 vs ~ 3K RPM, I was driving at about
the same speeds, determined by speed limits along my regular
routes.

DK

I also measured gas mileage during daily driving.
1-st mode: 1,2,3,4 gears city driving (1500 - 2000 RPM)
2-nd mode: 1,2,3 gear driving (2000-3000 RPM)

fuel economy is actully better in the 1-st mode by about 2-4 mpg

I have additionally measured gas mileage at same speed, same gear but
different RPM. See results:
RPM MPH MPG
1500 37 39
1800 37 34 Note: (cruise control mode)

 

[Rick Courtright]

I have additionally measured gas mileage at same speed, same gear but
different RPM. See results:
RPM MPH MPG
1500 37 39
1800 37 34 Note: (cruise control mode)

Hi,

You have an automatic?

IF it's a stick, speed/rpm ratios are FIXED, so you CAN'T go the same
speed in the same gear at different engine RPMs (assuming your foot's
OFF the clutch, of course!)

OTOH, w/ an auto, there's "slippage" in there under various conditions,
so it IS possible to go the same speed at different RPMs. Hence your
1800 rpm/34 mpg measurement makes sense if you're going the same 37 mph
you were doing at 1500 rpm/39 mpg part of the test: you're "wasting" 300
rpm thru slippage, creating heat and burning extra gas. This is the main
reason behind the development of "lock up" torque converters some years
back: if the t/c and trans can be made to act like a stick, w/ minimal
to zero slippage at certain speeds, MPG increases. In the case of autos,
there are times that what we "think" we see can be deceptive...

Rick

 

[John O]

Sounds right to me. The engine is rnning more efficiently at 2800 than
at 1800, for sure.

At city speeds wind resistance is a non-factor, so you're right, and this is
an interesting experiment. Once you add a wind load however (above ~45 mph),
this engine efficiency benefit falls apart, rather dramatically. Efficiency
goes up fractionally for a while, but wind loads increase by factors of two
and four as speeds increase!

-John O

 

[clare at snyder dot ontario dot canada]

At city speeds wind resistance is a non-factor, so you're right, and this is
an interesting experiment. Once you add a wind load however (above ~45 mph),
this engine efficiency benefit falls apart, rather dramatically. Efficiency
goes up fractionally for a while, but wind loads increase by factors of two
and four as speeds increase!

-John O

Not from my considerable experience I've found on several vehicles (my
current PT Cruiser included) that running at low speeds, OTHER than on
dead level terrain, the engine is not working efficiently - and burns
MORE gas than at higher (reasonable) speeds.
Untill the engine "gets on cam" a bit, you are using large throttle
openings to produce small results - and burning more gas in the
process.
I can also consistently beat cruise control for mileage under normal
driving conditions - again except,sibly, on dead level driving - which
I see very little of. The wind load on most of todays vehicles only
really starts to be a serious issue above somewhere around 120kph.
(unless you have a roof rack, tow a trailer, etc)

I've put a LOT of miles onto a lot of cars in over 40 years of driving
- and as a mechanic I do understand the factors that come into play
with RPM, gear ratios, torque curves, etc.

 

[Butch]

Hi,

You have an automatic?

IF it's a stick, speed/rpm ratios are FIXED, so you CAN'T go the same
speed in the same gear at different engine RPMs (assuming your foot's
OFF the clutch, of course!)

OTOH, w/ an auto, there's "slippage" in there under various conditions,
so it IS possible to go the same speed at different RPMs. Hence your
1800 rpm/34 mpg measurement makes sense if you're going the same 37 mph
you were doing at 1500 rpm/39 mpg part of the test: you're "wasting" 300
rpm thru slippage, creating heat and burning extra gas. This is the main
reason behind the development of "lock up" torque converters some years
back: if the t/c and trans can be made to act like a stick, w/ minimal
to zero slippage at certain speeds, MPG increases. In the case of autos,
there are times that what we "think" we see can be deceptive...

Rick

you are right. I have an auto.
37 mph/1500 rpm - 4-th gear with torque converter "lock up", cruise
control mode is OFF
37 mph/1800 rpm - 4-th gear w/o torque converter "lock up", cruise
control mode is ON

at higher speed with gear selector in "D" position, I have the only
"torque converter "lock up" mode at highest 4-th gear position. So I
could perform measurements at same speed but different rpm only if I
set gear selector into "3" or "D" position. For example:
1-st mode: 4-th gear, 50 mph, 2000 rpm, with "torque converter "lock
up" mode
2-nd mode: 3-rd gear, 50 mph, ???? rpm, with "torque converter "lock
up" mode

is it fair comparison?
I will do such comparison soon and post the results.

I'm sure that 1-st mode has better gas mileage.
My statement is: lower RPM -> better gas mileage. There is no any
extremum in mpg / rpm chart.

 

[JD]

in message

On Sat, 6 Sep 2008 20:23:55 -0400, "John O"

I can also consistently beat cruise control for mileage under normal
driving conditions - again except,sibly, on dead level driving - which
I see very little of. The wind load on most of todays vehicles only
really starts to be a serious issue above somewhere around 120kph.
(unless you have a roof rack, tow a trailer, etc)

That isn't totally true. The primary input into wind load (drag) at low
speed is the frontal cross-section area; which is not that different today
than it was 20 years ago. Consequently, the speed at which wind begins to
be a factor is around 80 km/hr, and it goes up exponentially. Once you get
to higher speeds, shape becomes an issue and can reduce the increase in drag
somewhat. With performance cars 120 is probably a safe bet since a
performance car burns a fair bit of gas just doing nothing. But with a
normal passenger car, you would begin to notice the effects certainly by 100
km/hr, and a significant drop in gas mileage by 120.

Air is a fluid and anything moving through it obeys the same set of rules.

 

[Butch]

I will do such comparison soon and post the results.

4-th gear, 50 mph, 2000 rpm - 34mpg
3-rd gear, 50 mph, 3000 rpm - 27mpg

 

[Rick Courtright]

4-th gear, 50 mph, 2000 rpm - 34mpg
3-rd gear, 50 mph, 3000 rpm - 27mpg

Hi,

Are those "measured" or "calculated" figures?

I ask not to challenge, but because I've often experienced situations
where the actual "measured" mileage ran contrary to the "lower
rpm=higher fuel economy" dictum. In other words, measured results were
in opposition to what the trip computer was telling us (somewhat in line
w/ other parts of this thread where the discussion's focusing on engine
efficiency at a given rpm.)

Just curious...

Rick

 

[Butch]

Hi,

Are those "measured" or "calculated" figures?

I ask not to challenge, but because I've often experienced situations
where the actual "measured" mileage ran contrary to the "lower
rpm=higher fuel economy" dictum. In other words, measured results were
in opposition to what the trip computer was telling us (somewhat in line
w/ other parts of this thread where the discussion's focusing on engine
efficiency at a given rpm.)

Just curious...

Rick

1. "calculated", but it always equal to "measured" after fuel the tank
on gas station. +- 1%
2. originally topic was engine RPM vs MPG. What is "efficiency at a
given rpm", MPG, "engine wear" or somthing else?