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Old 06-19-2008, 04:11 PM   #1
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Better highway gas mileage at high speed

I'm baffled. Plain and simple. Here are the numbers and then I'll go from there.


Averaging 60 mph highway with some playing around in 30 miles total of twisties and approximately 8 miles total of driving less than 30 mph on a dirt road. There was about a 15 mph head wind however.

317 miles total
11.855 gallons used
26.73 mpg


Averaging 70-75 mph for 160 miles (I was tired and wanted to get back home ASAP) with about a 10 mph side wind, and then for 168 miles just my daily commute. My daily commute consists of 22 miles each way in rush hour traffic in the Dallas/Fort Worth metroplex. I go through 24 stop lights in each direction, my top speed is usually 60 mph on 2 seperate 5 mile stretches of road. In the mornings, traffic isn't too bad, but in the afternoons, sometimes I sit at some of the stop lights for 2-3 cycles before I finally get through.

328 miles
11.876 gallons used
27.62 mpg


Now, in my old 1991 Cavalier, it would get 42 mpg at 80 mph, but only get 35 mpg at 60 mph! However, it was a 4 cylinder 5 speed manual coupe. My current 1991 Cavalier is a 3.2L V6 with a 5 speed manual and it is a station wagon. It also has a turbocharger and a nice Crower cam.

At 50 mph the car is at 2000 rpm in 5th gear, at 65 mph it is at 2500 rpm in 5th, and at 3000 rpm the car is at 80 mph with a redline on the motor of 6000 rpm.

I wonder if it's possible that the turbocharger and cam together help to improve the car's VE as the rpms climb therefore increasing mpg. Cruising at 60 mph, I was seeing an average of 10-12 inches of vacuum. Cruising at 75 mph, I saw an average of 14-16 inches of vacuum.

Does anyone have an explanation for this?
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Old 06-19-2008, 07:47 PM   #2
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Sometimes the increase in engine efficiency by running the engine a little harder and faster to get the higher speed outweighs the increased drag on the car.

My ex's 1996 Cavalier would get about 35mpg at 60mph but around 39mpg at 75-80. It had the same OHV 2.2 your old 1991 cav probably did but had over 200k miles. To my suprise it used almost zero oil between changes. The most I ever saw it use was half a quart in 5000 all city miles.

Hate to bring it up again but the Corvette is similar. The engine is so huge and grossly inefficient at making the 10hp needed to keep it at 60mph(still not bad at 28-32mpg for what it is) that taking it up to 100mph and needing 20-30hp doesn't drop your mileage by much at all because of the increased efficiency. Vette owners report that long trips in the triple digits still get above 25mpg if the speed doesn't vary too much(it's still a V8). They also say mileage usually gradually increases til about 85 where it levels off and starts to fall again around 110.
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Old 06-19-2008, 09:02 PM   #3
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It all depends on the car, I believe every car has an optimum speed and this speed will vary from one to the next... But that someone is getting better mpg at 80 vs. 60 I find that a bit harsh, perhaps there exist other reasons such as:

- Is the car 'lugging' in 5th at 60?
> If so it might do better at 64 mph, it isn't so much a matter of a higher speed giving better mpg, it is a matter we went from 60 straight to 80...
Heck why not just compare 15 mph to 90?
I would be for testing various speeds, in between too...

It takes time, but it helps to develop a feel for the car.
Each car is different, but they all have a 'feel' to them.
Once you get this feel, once you nail it down for a car it becomes a whole another twist of a story concerning improved FE.

Things such as 'driving with load' can help spot that feel.
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Old 06-19-2008, 09:35 PM   #4
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isnt it obvious? the faster you travel the further you go. higher gear keeps same rpm as lower gear @ lower speed. so your using roughly same amt. fuel but going alot further. thus huge mpg. right? maybe i missed sumthin though
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Old 06-19-2008, 10:33 PM   #5
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isnt it obvious? the faster you travel the further you go. higher gear keeps same rpm as lower gear @ lower speed. so your using roughly same amt. fuel but going alot further. thus huge mpg. right? maybe i missed sumthin though
ya missed the other posts that explained each car has its peak efficiency speed for mpg. My car is about 68-70mph, depending on grades.
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Old 06-20-2008, 03:31 AM   #6
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Sometimes aero features don't "work" below a certain speed.
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Old 06-20-2008, 04:40 AM   #7
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Originally Posted by brandmattice View Post
isnt it obvious? the faster you travel the further you go. higher gear keeps same rpm as lower gear @ lower speed. so your using roughly same amt. fuel but going alot further. thus huge mpg. right? maybe i missed sumthin though
He is still in top gear at 55mph or 80mph, so that point is invalid.
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Old 06-20-2008, 10:39 AM   #8
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Another thing to consider is the BSFC/Load curve. As the load on the engine increases, pumping losses decrease (wider throttle opening), and BSFC falls. The "sweet" spot on any car is a function of how quickly drag increases and how much BSFC falls at higher loads. I stands to reason that a low-drag car would have a that "sweet spot" at a higher speed. Or a car with an engine having a relatively flat BSFC/Load curve would have the "sweet spot" at a lower speed.
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Old 06-20-2008, 11:05 AM   #9
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8307c4, yes my 4 cylinder Cavalier would lug quit often while traveling at 60 mph in 5th gear. My Wagonstein though, it doesn't know what the word lug means! I can put the car into 5th gear at 30 mph and smoothly, almost efortlessly accelerate up to whatever speed I need to be at.


I did notice that the throttle position was more open while traveling at higher speeds, but the vacuum level in the intake manifold was higher.


I've tried searching in the past to find the stock Cd for this car but I've had no luck.
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Old 06-20-2008, 11:16 AM   #10
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Speaking of not knowing what the word "lug" means, I was surprised when I found out. I suspect that even your 4 cylinder Cavalier wasn't actually lugging, just being a little growly.
http://www.gassavers.org/showpost.ph...0&postcount=13

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Lugging is bad, but what you're describing is probably not lugging.

http://www.standardshift.com/forum/v...p=91271#p91271
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I'm pretty sure that most people who think they have been lugging their engine really have not been lugging it at all. I'm not even sure that modern ECUs and knock sensors would let the engine lug. My experience with the Jetta so far has been that it either runs or stalls. I have not heared it lug (even when I accidentally took a corner in 4th instead of 2nd).

Putting a load on an engine at low RPM will make it growl, but growling is not lugging. Lugging is irregular. If it growls and goes it is not lugging (at least in my understanding of the matter).
http://www.standardshift.com/forum/v...ed7710#p172612
Quote:
It isn't lugging. You'll never forget the sound of lugging once you do it. Sounds like a bunch of metal sh*t exploding under your hood. Or like a jackhammer.
Based on what I've read there, growling is acceptable, but lugging is very terrible.
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