Re: Cruise Control Freeway Mileage in 06HCH (Experiment)
 

1 meter is 3.28 feet.

Re: Cruise Control Freeway Mileage in 06HCH (Experiment)
 

Sorry, I meant to say 3 meters, and funny enough, I multiplied 3 x 3.28 to arrive at 9.8 feet anyway. I did notice in a quick check in this article it states that WAAS is for "better than three meter" accuracy 95% of the time. I guess that explains why I often saw accuracy listed as less than 9 feet.

Re: Cruise Control Freeway Mileage in 06HCH (Experiment)
 

I finally got a good opportunity to try the 80 MPH test again. Traffic conditions have prevented me from trying it during my normal commute, but today my wife and I went to a friend's house who lives very close to where I work, so I took advantage of the wide open Sunday freeways.

Day 3 (80 MPH)

Close to Work -> Home

Trip Mileage: 35.9 MPG, 24.0 miles
Set Speed: 80 MPH
Temp: 47 (start), 40 (end)
SoC: 88% (start), 88% (end)
Notes: Cruise control set to 80 MPH the whole time.

As you can see, the temperature was lower than my previous tests, so that probably brought my average down a bit. However, the car was driven 2 miles before the start of the test (unlike my other tests, which started cold), so it was at least partially warmed up. Also, after the 70 MPH test I pumped up my tires to 40 PSI, so that should have offset the lower temperatures somewhat. Since I did the original 60 MPH test, I have done it again and gotten 55.2 MPG (I'm guessing the improvement coming from the higher tire pressure and the car being more broken in?)

The exponential drag really seemed to be taking its toll at 80 MPH. One thing I noticed is that on the same gradual downhills where the ICE usually shuts down @ 60 and 70 MPH, I was only getting 50-70 MPG (depending on the degree on decline) at 80 MPH. The ICE had to work to keep pushing the car through that 80 MPH wind, even downhill.

So, to summarize:

Day 1 (60 mph) = 51.7 mpg
Day 2 (70 mph) = 44.9 mpg (6.8 worse than @ 60)
Day 3 (80 mph) = 35.9 mpg (15.8 worse than @ 60)

I guess the moral of the story is that speed kills!

Re: Cruise Control Freeway Mileage in 06HCH (Experiment)
 

Wow, I drive 80 mph all the time and at the moment I tend to get 40-41 mpg round-trip maintaining that speed, though without cruise-control. But that's at 70-degree temperatures.

Logically, the decrease in FE should be smaller with each jump in speed. So if going from 60 to 70 mph makes a 7 mpg difference, going from 70 to 80 should make less than a 7 mpg difference. And 80 to 90 should be an even smaller difference.

Re: Cruise Control Freeway Mileage in 06HCH (Experiment)
 

So everything else being equal (which of course it isn't!), that means a 25 degree temperature drop results in a 4-5 MPG hit. Based on other reports I have seen from HCH drivers in colder climates, that seems feasible.

Why is that? Given that the drag is increasing exponentially, it makes sense that the car has to expend more energy to go from 70 to 80 than it does to go from 60 to 70. Other effects may alleviate that somewhat at lower speeds, but when you have a force working against you exponentially vs. speed, eventually it starts overwhelming everything else.

Re: Cruise Control Freeway Mileage in 06HCH (Experiment)
 

I'm in San Diego too and drive 80+ MPH all the time so this is a bit dissapointing but what you said about the FE to speed relationship is backwards.

The relationship between drag and speed is nonlinear like most real systems. In the range of interest for us in this case, I believe that drag increase exponentially wrt speed. i.e. for an unit increase in speed you get a more that a unit increase in drag.

When you think about it a street car like a civic with 100 hp can get up to 110mph, but just tripling the power output to 300 hp doesnt get you past 300mph. (in fact you wont even get past 200mph)

Re: Cruise Control Freeway Mileage in 06HCH (Experiment)
 

As another SD resident, I can say that driving around San Diego tends to ruin any chance I have had so far for a 50+ mpg tank. Our hills have been brutal to my average fuel economy.

I was getting 30 mpg with my 2001 Civic EX driving 80 mph all the time on our highways. With my 2006 Hybrid, I drive far more gently, and get 45 or less. I wonder what my EX would get with similarly gentle driving he Seems the battery should be helping me out a bit more often.

Re: Cruise Control Freeway Mileage in 06HCH (Experiment)
 

I recall that you pointed out in another post that driving faster will get you to your destination faster, which is true, but the time savings is directly proportional to the speed increase. As your speed increases linearly, your time to destination decreases linearly, but at the same time the drag working against you is increasing exponentially, so the effects of drag will overshadow the time savings, and the difference gets worse the faster you go.

Re: Cruise Control Freeway Mileage in 06HCH (Experiment)
 

No, I didn't get it backwards. If the MPG dropped increased then you'd hit 0 mpg at 100 mph or so, which obviously is ridiculous. Specifically, since MPG = MPH / GPH, you'll have to consume an infinite amount of fuel.

It is true that drag increases nonlinearly with speed. If it increased at the same rate as speed then your MPG would remain constant; it wouldn't decrease at all. The fact that it does decrease is a testament to the fact that drag increases faster than speed.

Apparently if NASA gets 52 mpg at 60 mph then he's using 1.15 gallons of gas per hour. If he's getting 45 mpg at 70 mph then he's using 1.55 gallons of gas per hour. That's a 0.4 GPH difference. If the GPH for doing 80 mph increases by the same amount, to 1.95, then at 80 mph one would get 41 mpg, and that is exactly what I have been getting. No this is not scientific but it's a good guesstimate. I do think one's GPH would be more likely to increase a little, but who knows.

Re: Cruise Control Freeway Mileage in 06HCH (Experiment)
 

There's no need to guesstimate. The aerodynamic drag force increases with the square of speed.

If you get 52 mpg at 60 MPH you should get, according to the formula, 38 MPG at 70 MPH, and 29 MPG at 80 MPH. The reason the drop isn't so bad, probably, is that there are other drag forces such as road friction and drivetrain friction that don't increase with the square of speed but more linearly with speed. And the energy spent to climb a hill, for example, is the same regardless of speed.