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Old 09-06-2007, 01:00 PM   #31
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I have a relatively small aerodynamic car with a long overdrive. At 70mph its turning just under 2500rpms. Peak torque is 2400rpms. I have found that the slower you go the better mileage you get. In rolling country hills two lane roads I can average about 60mpg. I limit minimum speed to 35mph up hill.

I can get around 50mpg at 60-65mph. I can get 60mpg at 42mph. Last tank was 62mpg scangauge reported 29mph average. Thats the nature of the beast.

I find that on a busy freeway I can ease in with a large group of semis doing a 2.5-3 second draft and get 50 mpg at 68-72mph. I was also able to drive out of KS city last weekend for 200 miles at 70+mph and held 49mpg for over 200 miles no drafting. It was a tailwind.

Conclusion, although driving slow puts the engine well outside its most effiecent range I still net better FE.
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Old 09-06-2007, 04:06 PM   #32
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PurpleHaze, you'll just have to test it some more.
If 75 mph really does get you great FE in that car you'll see it happen regularly if you keep your speed up.

I suspect it's otherwise - but you'll see how it goes.
Any time I run my car faster than usual I see the FE drop the next time I fill up.
ymmv, as always.

Assuming no changes to your car, I suspect a good tailwind may have helped you going Detroit to Findlay, Ohio. I think you were going mostly East, and the prevailing winds are west to east so it's quite possible. A 15 mph tailwind would give you the air resistance at 75 that you'd nornally get at 60. Which is a huge change really.

Keep tracking those numbers.
Did you start a gaslog here?
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 09-06-2007, 11:41 PM   #33
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Top speed is relevant but so is how you attain that speed. Driving 'like a grandmother' you accelerate slower and spend longer doing so. Driving 'spiritedly' you accelerate quicker and spend less time doing so.
It's possible to use less fuel by getting to speed quicker even if you put the foot down to do so. (Sorry if that's obvious to a lot of people but it's worth throwing in!)
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Old 09-07-2007, 05:39 AM   #34
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Folks, shoulda-woulda-coulda comments are all well and good, but real world trumps theory every time. In theory a bumblebee can't fly because its wings are too small, but it buzzes from flower to flower anyway. Luckily bees are not engineers, otherwise they'd starve to death. The man says he saw better fuel economy at a higher speed. I believe it's possible based on my own driving, and feel that he has something to pursue here. One tank is not that accurate an indicator, but if he can duplicate the same results over 3 tanks, or preferrably 5 tanks, then it's a fact.

My own results with speed vs economy were found over months of driving, not one shining tank. The Camaro that gave me 30MPG at 80MPH was over the course of 6 months of driving from Maryland to South Carolina and back every weekend, all interstate. My beloved, betrayed 79 GMC's results were an average over 5 years and 3 different transmissions.

Fact is, all engines have their sweet spot, and two otherwise identical engines will have different efficiencies for reasons that I don't know. In theory, two identically built engines should have the same power and fuel economy ratios, but in reality one will blow the other one off the road and net 5MPG better doing it and a third identical engine is even worse. You will generally get the best fuel economy when running the engine at its preferred RPM in high gear regardless of speed, to the point that wind resistance just pushes it down. The only way to find this spot is to experiment, once you find it you can make hard part changes to adjust the speed/RPM down to where you are running that RPM at the speed you run the most. If your car gets its best mileage at 80MPH, regearing the car so you hit that RPM at 60MPH will show a mileage increase because you are then running the car at its most efficient RPM, with less wind to push.

Best way I can see to actually determine the engine's best efficiency RPM would be to strap it down to a dyno, then measure fuel usage at different speeds. You'd need to load the car down to simulate road resistance, but that would be steady. Once you find that efficient RPM, gear the car to run at that RPM at your most used speed, might see a 10MPG jump off that alone on a small car, provided it isn't already geared close to that.
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Old 09-07-2007, 09:43 AM   #35
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Originally Posted by Telco View Post
Folks, shoulda-woulda-coulda comments are all well and good, but real world trumps theory every time. In theory a bumblebee can't fly because its wings are too small, but it buzzes from flower to flower anyway. Luckily bees are not engineers, otherwise they'd starve to death.
What theory are you talking about?
No one here is stating anything about theory AFAIK. We're just relating to other posters based on our experience and known confounds. W/o consistent runs w/ little to no variation it's hard to pick up what does what, however if we isolate what we're trying to track down and test it, then maybe we can figure out what does what provided it's within our ability to detect it. Saying that speed hurts efficiency ain't no theory, this is documented observation. Every single vehicle I've ever driven, or seen tested, has a drop in mpg compared to speed in top gear at highway speeds. If you don't think yours does, test it, in a controlled environment, and post the documented results up. I'm sure we'll all be interested in it, since I for one haven't seen any vehicles documented that increase mileage with speed at highway speeds.


Sorry 'bout the hotlink Metro.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 09-07-2007, 10:07 AM   #36
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"Saying that speed hurts efficiency ain't no theory, this is documented observation."

I think everyone here is talking about optimal spots in the MPG envelope... if you extend all those lines on the graph below 50mph, at some point they'll drop as well.

In "real world" driving conditions around where I live with the cruise on, I was very surprised to find lower FE at 80kph (50mph) than at 100kph (62mph), 90/55 was best, and I agree totally with you on the theory aspect. The power to overcome aerodynamic drag is exponential. That's a fact nobody can dispute.
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Old 09-07-2007, 10:08 AM   #37
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No prob - I already linked that image up at post #21.
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Old 09-07-2007, 10:16 AM   #38
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Originally Posted by 2TonJellyBean View Post
if you extend all those lines on the graph below 50mph, at some point they'll drop as well.
I don't think they will - as long as the car stays in top gear above engine lugging RPM. But the context for that to be true is: measuring FE under constant load (e.g. steady speed, level road).

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Old 09-07-2007, 10:28 AM   #39
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Originally Posted by omgwtfbyobbq View Post
What theory are you talking about?
No one here is stating anything about theory AFAIK. We're just relating to other posters based on our experience and known confounds. W/o consistent runs w/ little to no variation it's hard to pick up what does what, however if we isolate what we're trying to track down and test it, then maybe we can figure out what does what provided it's within our ability to detect it. Saying that speed hurts efficiency ain't no theory, this is documented observation. Every single vehicle I've ever driven, or seen tested, has a drop in mpg compared to speed in top gear at highway speeds. If you don't think yours does, test it, in a controlled environment, and post the documented results up. I'm sure we'll all be interested in it, since I for one haven't seen any vehicles documented that increase mileage with speed at highway speeds.


Sorry 'bout the hotlink Metro.
As I stated in my earlier post, I have consistent runs with little variation over thousands of miles and am getting about the same mpg at an average of 75mph as I was at 65mph. This is about as stable results as we can get in real world driving. Your response was I was not getting consistent runs due to differences in traffic/weather etc. If this is the case then we should be disputing nearly every mpg claim on this site. You choose not to believe my results and personally, I could care less. I know what I am seeing at the gas pump. On the same token, I could tape up my front grill, claim a 10% increase in mpg, and because the general belief here that that grill blocks increase FE, I'd be amazed if anyone here would make similar claims that my results were caused by inconsistent testing.

In general, it cannot be disputed that at higher speeds the FE will decrease, but there can be exceptions to the rule and not every car on the road may have smooth curves as in the ones shown in the plot Metrompg posted. The curve also could be a little misleading since other then the Prius (a hybrid) all other cars on that chart are turbocharged or supercharged. A normally aspirated, non variable valve timing engine will run more efficiently at the rpm determined by the intake and cam timing. Depending on how aerodynamic the car is and the differences in the torque curves, it is possible that at small differences in speed, a car's mpg could increase or remain the same. I'm sure if I averaged 85 or 95, my mpg would fall of dramatically, or at 45 or 55, it probably would improve, but from 65-75, I see no difference.
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Old 09-07-2007, 10:45 AM   #40
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Originally Posted by mrmad View Post
As I stated in my earlier post, I have consistent runs with little variation over thousands of miles and am getting about the same mpg at an average of 75mph as I was at 65mph. This is about as stable results as we can get in real world driving. Your response was I was not getting consistent runs due to differences in traffic/weather etc. If this is the case then we should be disputing nearly every mpg claim on this site.
The difference is, our mpg claims aren't comparisons. They're just claims. We all deal with different traffic and road conditions in different vehicles. What you're claiming is that going 65mph compared to 75mph doesn't produce a difference in mileage, which it may not for you because of your driving habits, conditions, vehicle, etc... But it does in every single controlled test I've seen. It's tantamount to someone who drives nothing but highway saying that killing the engine at idle doesn't increase mileage. Sure, it may not increase mileage for them because they seldom idle, but it does compared to idling, and if a driver idles enough, they'll see an increase.

If you really want to test it, I have some great two lane low traffic roads up here we can do bidirectional runs on. All you need is yerself, yer car, and a SM. Until then, because every mileage/speed test I've ever seen, and every BSFC map compared to glider/gearing indicates that mileage will drop as highway speed in top gear increases, I don't believe your assertion that your car shows no drop in mileage from 75mpg to 65mph. It may show no drop for you due to your driving conditions, but unless you test it w/o confounds, no way meng, Iyam a skeptic.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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