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Discussion Starter #1
I know this is probably one of the least sought-after adjectives when referring to our cars. Yes, they're a 4-cylinder, but no one really buys a WRX for its fuel efficiency. However, I drive A LOT of highway miles (about 120 miles/day), and I tend to almost always average above 30 mpg (not per the car's computer, but actually dividing my miles by the # of gallons a fill-up). The car's computer usually tells me I'm getting around 34-35 mpg, but I know that's incorrect.

My question is this - has there been any studying of our cars' engines to determine the "sweet spot" at which the car gets the best mileage at highway speeds? I know a shit-ton of government studies have determined that somewhere around 35 mph is the most economical speed to drive in terms of maximizing mileage. And that makes sense. In most cars - especially automatic transmissions - you're in top gear and minimizing the drag created by wind resistance. Any slower and you're either not in top gear and/or bogging down the engine; any faster and wind starts to seriously work against you.

But let's forget that and focus on the real world, where people drive at least 60 mph on the interstate/highway. For my car (2015 WRX Limited), the owner's manual tells me I can shift into top gear (6th on my manual transmission) at 50 mph. Let's say for the sake of argument that I don't want to bog down my engine right at the recommended shift point, so we'll set the cruise control at 55 mph. Wind resistance is making a fairly significant impact on my mileage now! But I also live in Colorado, which means my highway is not flat. In fact, my chief complaint about CO drivers is that they don't bother to give their vehicles more gas when going uphill, therefore creating a slowdown that literally backs up traffic for miles just because stupid assholes can't be bothered to be proactive drivers and think about the others with which they share the roads!!!:mad::mad::mad::mad::mad::mad:

Sorry, I digress....

Anyway, back to my rambling point: With the roadway ups & downs, the engine has to sometimes work harder to maintain 55 mph. I don't know our engines well, but if it's maintaining revs for 60 mph, does it not have to work that hard because it's already at a slightly higher rev than trying to maintain 55? What about at 65 mph? 70? In between? I know the faster we go, the more the wind works against us, so I'm sure the sweet spot is probably not at higher speeds.

Like I said, where's the sweet spot combining the best of speed AND efficiency? What's that band look like? I'm just curious if anyone with a dyno has studied that. I'm very curious about the results. Thanks for reading my rambling! :)
 

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This is going to sound oddly specific, but the highest load possible at the lowest RPM possible generally returns the best efficiency.
 

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Discussion Starter #3
Right, which is what I was alluding to when I said "50 mph in 6th gear." You are absolutely correct. I guess I should have been more specific when I mentioned the power curve... To be more clear, I guess I mean what's the more efficient speeds above that point where we're traveling faster, but not so fast that we're at very high revs and experiencing very high wind resistance?

In my mind - and I know I may be WAY off base - it makes sense that the engine perhaps isn't working as hard to maintain speed (perhaps uphill?) at say 67 mph (because it's at slightly higher revs) than it does at say 63 mph.... But certainly not at 83 mph; it's a moot point because you're well above 3k rpms and air resistance is shoving you back like a bouncer at a titty bar...

I dunno, that's how my mind thinks, but like I said, maybe I'm way off.
 

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You bought the wrong car for your needs
Nah, there is nothing wrong with trying to be as efficient as possible when you aren't tapping into the performance.

I do it too, with an STI no less.

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I've been thinking about this because fuel consumption is most relevant performance parameter in the 21st century (virtually anyone can make power, that's not relevant any more).

I've looked at Directive 90/C81/01 (issued 1999 by what was then presumably the EEC) and there are issues vs. real-world performance. As people point out the testing permits disconnecting ancillaries from the motor and it permits evolved lubricants rather than insisting on everything on, and no special fluids.

Nonetheless is seems that universally urban is worst, followed by 120kph contant, as one might expect.

Best is 70kph constant speed.

The problem with cruise control is that it maintains speed, rather than maintaining throttle. I had a car that maintained throttle once upon a time; in that car, speed varied by load -- throttle was constant, not speed. To save the most fuel, turn off cruise control, set your speed at 70kph on flat ground manually, and then maintain that throttle position regardless of load. This is feasible on salt flats and space centre runways.
 

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The thing is, there are a LOT of factors that have to be considered to "numerically solve" the question you've posed. I'm sure it could be done, but you have to consider the aspiration of the engine, the engine's volumetric efficiency and BSFC at EACH load, the aerodynamics of the car, the gearing and inefficiencies, and the list keeps going.

It is so much easier to answer your question empirically with data that YOU collect yourself! For my STI, it tends to return the best real-world efficiency at around 55mph.
 

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The problem with cruise control is that it maintains speed, rather than maintaining throttle. I had a car that maintained throttle once upon a time; in that car, speed varied by load -- throttle was constant, not speed. To save the most fuel, turn off cruise control, set your speed at 70kph on flat ground manually, and then maintain that throttle position regardless of load. This is feasible on salt flats and space centre runways.
And also the way that throttle locks work on Motorcycles.
 

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Discussion Starter #11
The thing is, there are a LOT of factors that have to be considered to "numerically solve" the question you've posed. I'm sure it could be done, but you have to consider the aspiration of the engine, the engine's volumetric efficiency and BSFC at EACH load, the aerodynamics of the car, the gearing and inefficiencies, and the list keeps going.

It is so much easier to answer your question empirically with data that YOU collect yourself! For my STI, it tends to return the best real-world efficiency at around 55mph.
Exactly! I figured that's probably best for me, as well. However, I don't like getting flipped off constantly because I'm going 20 mph under the speed limit (and 30 mph less than most drivers!!) just for the sake of economy. :)
 

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stupid assholes can't be bothered to be proactive drivers and think about the others with which they share the roads!!!:mad::mad::mad::mad::mad::mad:
This could be dropped in just about any traffic complaint & fairly close to a thought that pops into my head on nearly every outing.


I don't have much to add as I don't watch my mpg beyond what my dash tells me. I can say it makes a lot of sense to me to try for better mpg, especially in a car that isn't super efficient. I don't drive my sti super often & I'm completely guilty of winding it out here & there but I notice my mpg is better if I stay out of boost & hang around 2500rpm.
(edit) After driving around today that 2500 is more like 2100rpm.
 

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Well my truck gets 13-14 MPG around town. At least it burns regular (87 octane). My car is worse (11-12MPG). The bus does a smidge better at 14-15MPG. I honestly don't know what that under-powered German 4 pot gets around town. It did 30MPG on our trip to COTA in December.

I always drive very conservatively. To quote Dan Gurney; "At no time did we exceed 175MPH."
 

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What should I have gotten instead? Why is a WRX an incorrect choice?
I think the argument is that there are better choices for long commutes, e.g. more efficient, more comfortable, more reliable.

All true, BUT that decision is deeply personal. There's no shame in sacrificing some things for pleasure.

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