Better gas mileage at higher speeds?

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?
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On 8/11/2017 10:40 AM, Yousuf Khan wrote:

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.
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On 8/11/2017 11:34 AM, John McGaw wrote:

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.
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On 2017-08-11 10:40 AM, Yousuf Khan wrote:

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.
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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.
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On 11/08/2017 12:37 PM, snipped-for-privacy@snyder.on.ca wrote:

Interesting theory, but none of these engines were PZEV.
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On Tue, 29 Aug 2017 00:42:08 -0400, Yousuf Khan

Don't even need to be PZEV for the theory to work. Just more extreme with the PZEV
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On 11/08/2017 12:00 PM, Darryl Johnson wrote:

Yeah, which is why I brought up the question. Knowing that extra speed should result in less fuel economy rather than more, this makes no sense. It's happened over and over again, and there's been very little other variables that could explain it, like unpredicted traffic jams or whatever.
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Yousuf Khan wrote:

So drive twice as fast and get twice the fuel economy. Drive three times as fast for more fuel economy. (wink wink)
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On Fri, 11 Aug 2017 10:40:25 -0400, Yousuf Khan

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 grades.
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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.
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On 8/11/17 2:32 PM, dsi1 wrote:

Being that it's downhill all the way to "town", that's not surprising...
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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.
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Yousuf Khan wrote:

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 engine.
http://www.atbs.com/knowledge-hub/reduce-fuel-spending-by-finding-your-engine%E2%80%99s-swe
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.

https://www.youtube.com/watch?v=aioPrv3zMQU
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.

https://www.youtube.com/watch?v=q5HgthPgzgQ
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).
https://www.youtube.com/watch?v=LYjCt2zF4J4

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., http://www.enhancedstreetperformance.com/dyno/dyno-tuning-pricing ).
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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.
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clare wrote:

I'm guessing its a 2008 Tribeca, 3.5L H6 from the OP's "hesitation" thread.

Those are fixed points as per spec, not a graph showing across an RPM range and per actual measurement. As you said, knowing the RPM would tell where in the graph the OP was running his vehicle.
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On 8/11/2017 6:05 PM, VanguardLH wrote:

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).
    Yousuf Khan
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