I just recently got back from a long trip, and noticed that my fuel
mileage was much better on the way back than on the way there. On the
way there, I kept an average speed of between 120 to 125 km/h, but on
the way back, I kept an average speed of 130 km/h. I had 0.8 L/100km
better fuel consumption over the entire trip. That's about 1.5 mpg better!
Now, this is not just a fluke, I've noticed this happen before. And it's
not just on one car, but I've seen it happen on my previous car too,
it's happened on my old 2000 Outback H4 5MT, and on my 2008 Tribeca H6
AT. Different models, different engines, different transmissions, the
only commonality is that they were both Subarus, but I doubt that had
anything to do with it (or did it?). What I noticed over the years is
that fuel mileage is good if you stay at 100 km/h or less, and then it
gets bad from there to about 125 km/h, and then above 125 km/h, it
starts to improve again! I haven't gone much above 130, as that's
already pretty illegal for the speed limits around here (100 km/h), but
it's an interesting observation. Why would this be happening?
Sent from Giganews on Thunderbird on my Toshiba laptop
Too many variables beyond speed come into play to say definitively. Was one
direction more uphill than the other? What about the temperature? Tire
pressure? Vehicle loading? Cruise control used more in one direction than
the other? Traffic differences? Air conditioner use? Fuel differences? What
was the trip? Was it generally east-west? If so then prevailing winds can
have a huge effect. I noticed it strongly on my last-foray-before-selling
in my 2006 Miata -- driving the Lincoln highway across the continent
east-to-west gave worse mileage than west-to-east because of a fairly
strong prevailing wind across NA even though the route and conditions were
pretty much identical.
Looking at thread I would think another variable is variations in
fill-up at different stations. I can also remember driving on an
interstate and watching my mileage drop when I ran into a long hold-up
after an accident. Too many variables.
If you had been doing primarily city driving, a decent run at highway
speeds may have "cleaned out" the engine, giving you the slight
increase in fuel economy.
If you regularly notice the same fuel economy improvement regardless
of how the vehicle had been driven for the previous period, it may be
that the combination of gearing and engine speed *does* produce
slightly better economy at the speeds you mention. Hard to imagine it
though: drag increases -- if I remember this correctly -- with the
square of the speed and it would take a significant increase in the
efficiency of the engine to overcome the additional drag at the higher
speed and still produce better fuel economy.
On Fri, 11 Aug 2017 12:00:36 -0400, Darryl Johnson
If it is a PZEV it will be running LEAN at low speeds - quite possibly
lean enough to really effect power output (and rherefore fuel
efficiency) at low engine speeds in the name of polution control. At
highe speeds and power outputs it MAY be running a bit richer, raising
the specific power output of the engine - combined with valve
timing/camming and highway gearing, this CAN make more efficiency
improvements than the effect of more wind drag at higher speeds with
some of today's aerodynamically cleaner designs.
What RPM is the engine running at 100? and how about at 130?
What is the specified "peak torque" RPM of your engine?
The engine itself is most efficient at peak torque RPM If that
ballances out with wind drag etc, yes, a car CAN be more fuel
efficient at higher speeds. (particularly if those higher speeds
provide a better "momentum buffer" when it comes to handling dips and
On Friday, August 11, 2017 at 4:39:52 AM UTC-10, Yousuf Khan wrote:
MPG should always be calculated over a round trip, not portions or legs. Un
less you're a gypsy and planning on not returning home. When I go to town,
I always get better milage on the outbound part. There's any number of reas
ons why that happens and there's not much that can be done about that.
On Friday, August 11, 2017 at 9:02:20 AM UTC-10, Bobby Axelrod wrote:
. Unless you're a gypsy and planning on not returning home. When I go to to
wn, I always get better milage on the outbound part. There's any number of
reasons why that happens and there's not much that can be done about that.
We have to go through a mountain to get to town and back. First we go uphil
l, then we go downhill. It's the same both ways.
An off-the-cuff guess: With everything else being equal (as noted by
McGraw), the tires got hotter at sustained higher speed so they inflated
a bit more (higher temperature = higher pressure), made the tread more
round, so less contact area to the road meaning less friction.
Engines are not linearly efficient and why clare's answer might
correlate to better efficiency when closer to the sweet spot for the
I'm not sure that you'll find any published RPM vs torque sheets for
your non-commercial vehicle. For that, you'd probably have to take it
in to a dyno shop to find out what it looks like for YOUR car. I didn't
go by what the car maker said. I was customizing my "toy" vehicle. So
I'd use the dyno shop to measure actual results at the wheels, not at
the engine. Using a dynometer takes out variations caused by a human
driver. Make sure the dyno shop is familiar with your car, like it
having constant all-wheel drive. Ask your own dealer, trusted car shop,
and others who they trust and used. Expertise seems to vary a lot.
Some of those were the fault of the customize job on the car, not with
the dyno shop. They can't help the customization was fucked. Some of
those dyno tests were performed using junk gear. The dyno shop that I
used was built as a dyno shop, not a coverted gas station with car
repair bays, not portable dynos, and not up on ramps (dyno was built
into the floor) or otherwise elevated (other than a small ramp up to get
on the rollers). I hear the Mustang Dyno (not a car brand but a dyno
brand) is pretty good.
Wouldn't chains be safer than straps? No stretch with chains. Maybe
the car is meant to float up at speed to reduce pressure of tires
against rollers (to reduce friction).
No recollection what a dyno test cost me. I was throwing so much money
at that car that the dyno tests where inconsequential. There might be
cheaper runs for $100 but air-fuel mix and shift point adjusts will
raise the price. Where I had dyno runs performed (many many years ago),
I do not recall that they had a variable fan (or any fan) to simulate
higher air flow rate at higher speed. I was really just trying to
determine if a new change made my toy better or worse or no difference.
Cost depends on what you have them do (e.g.,
The specs are readilly available. Not sure what year this soob is,
but 2014 H6 produces 256HP at 6000 rpm, and 247 ft lb torque at 4400.
An educated guess is the engine is capable ofproducing roughly 230 ft
lb at 2200 RPM for a total of about 96HP, and if the engine is lugging
at that speed (which it almost definitely will be on any grade) the
manifold pressure will be high and it will suck gas.
As for chains or straps, straps are actually safer - and they have
been used for dyno tie-downs for decades. We used straps back in 1970,
and the straps have only gotten better with time.
It was only 5-10 km/h faster, shouldn't result in too much higher
heating. In fact, the additional speed should mean more air blowing past
it to cool it down further.
I was thinking along these lines too, however, as I mentioned I saw the
same behaviour with my old 2000 OBW, two very different eras of cars.
The OBW had a 5-speed MT, and in 5th gear, at 100 km/h, it was just
under 3000 RPM (approx 2900 guestimate), and at 120 or 130 kph it was
just slightly over 3000 below 3500. I used to consider this a bit of a
downside of owning a Subie manual, as I knew that the automatic version
of the same car was 500 RPM down from there, so the 5MT was geared too low.
Now, my Tribeca is hovering around 2500 RPM at 120 - 130 kph. So very
different RPM ranges here, but they both displayed better mileage at the
slightly higher speeds than at the slightly lower speeds.
My guess is that there might have been an aerodynamic effect that
smooths out at a slightly higher RPM. Perhaps the roof rack or
something? Both cars had a roof rack (empty).
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