AC on/off Corvette at 55 mph data right?

Yes, understood. It would be interesting for you to reset your trip odometer next fillup though so that the fillup after THAT you could compare the fuel efficiency calculation to actual gallons burned/miles traveled. I mean, maybe my car is just a lemon in this regard, but it is consistently, grossly wrong about mileage; I’m assuming that calculator is not regulated the way the odometer is, and that the odometer IS accurate. I’m further assuming that the mileage calculator for Toyota hybrids is the same piece of equipment–maybe not, but why design two?

But as I say, maybe I got the one that was assembled by a drunk.

The notion of going to the gas station to fill up before each test seemed counter-productive. The goal was to find a stretch of highway on which I could drive 50 mph consistently for a length of time; if I had to slow down, pull off the exit ramp to a gas station to fill up, there’d be distortion of the average MPH.

Especially at the slow 25 MPH speed, we didn’t want that kind of distortion. Slowing to a stop (especially with the hybrid battery) meant “coasting” for a period of time, no gas but covering distance (“infinite” mpg) which would distort results worse than however the car’s computer was calculating.

And, if we’re talking a fraction of a gallon (at 40 mpg, the 25 MPH run wouldn’t have used a full gallon), the measurement would still be subject to error – how do I know the tank is really “full” when I fill it up?

Finally, if the measurement were consistently 10% too high, the relative results would be the same, n’est-ce pas?

I will try to do a test with measuring the gallons and miles on my own and comparing to the car’s calculation, but I won’t be able to do that until November, we’re travelling during most of October.

You’re correct that if your Camry’s MPG estimator is consistently out of whack in the way mine seems to be it would not, or should not, affect the results in the sense that the relative values should still be of interest. However, maybe not…maybe my car’s MPG estimator thinks I’m getting better mileage than I really am when I run with the AC on for example. I don’t know, I haven’t paid that much attention to the discrepancies under different conditions.

But since you do fill up the car regularly (I assume), I’m just wondering how your quick n’ dirty calculation for one tank, on your car, compares to what your MPG estimator says. Una says her car’s MPG estimator is consistently very accurate; my experience with mine is that it is not; if the Toyota estimator is a POS, one hopes that for the purpose of this limited trial it is at least a consistent POS. Right?

Also don’t you want to know, when it comes time for your next car, how good your mileage really is?

Common sense also suggested to me that removing a pick-up truck’s tailgate would improve its aerodynamics, but Mythbusters showed the opposite (again, with some sophisticated aerodynamic modeling also showing the same effect they measured, so it’s pretty clear it wasn’t just a Mythbuster’s hiccup).

[quote=“Ed_Zotti, post:20, topic:555744”]

Well, I’m sure if a genie came out of a bottle with a magic extra 10 mpg, and allowed a Corvette designer to choose whether that efficiency would apply with the windows up or windows down, the designer would choose windows up. But I don’t think it works that way in real life. Engineering is full of trade-offs, but not always the trade-offs you want-- it’s not like deliberately decreasing efficiency with the windows down will automatically increase efficiency with the windows up. And aerodynamics is funny enough that it may well turn out that maximizing efficiency with the windows up results in a form that’s even more efficient with the windows down. And of course, this being a Corvette we’re talking about, while I’m sure fuel efficiency is valued, I think it is a lower priority than the esthetics of the shape.

To me, it’s surprising enough that I’d want more data to confirm it, and maybe even some modeling to give a little stronger theoretical basis, but not impossible to understand or believe.

• Mythbusters* did this too. They concluded that going less than 50mph it is more efficient to leave the windows down, but going greater than 50mph it is more efficient to use the A/C.

When I first got the car, back in Feb 2007, I did check my calcs against the car’s calcs. I don’t remember the exact results, I do recall that there was very slight disagreement between the two calcs, but not anything that I cared about, like a mile or two per gallon. (And, of course, since it’s never clear that a fill-up actually gets the tank to the same level; and at 40 - 50 mpg, a cupful of gas is 3 miles or so?) Anyhow, my recollection is that it was close enough for practical purposes.

I will check again now that it’s been the subject of a Cecilian Experiment.

I am amazed how much the mileage goes up at 40/50 MPH vs 25 MPH. Is this a gearing thing?

It likely is. At 55 mph the Corvette is in 6th gear loafing at about 1200 rpm or so.

Can it be confirmed that the Corvette uses climate control (And the Camery does not)? If the answer is yes, then it seems likely that phb1’s theory is correct. From there, I believe, it would be a legitimate to conclude that “Yes, modern AC *with climate control * has been improved so much that it is no longer an energy sink.”

I don’t know what you mean by “climate control”, but it was a 93-95 degree day in full sun during the test, and we set the A/C to “60 F” on both sides, and turned the fan to “max”, and set it to inside air.

by “climate control” I meant that the AC doesn’t run at maximum output for the entire duration. With “Climate Control,” like the thermostat in the house, the AC runs at maximum until the desired temperature is achieved and then turns off or runs at 50% maximum until the temperature starts to climb again, at which point it runs at maximum.

By going off of cabin temperature instead of running full out all the time, the AC will use less power.

Like the trucks that shut off 2 cylinders at highway speeds to conserve fuel.

I think we’re talking past each other here. Nobody on our end was surprised about the results when the AC was on, or about AC on/windows up vs. AC off/windows down. What we have no explanation for is the two results with the AC off.

Oh yeah. I suggest buying a plastic model and getting some wind tunnel time.

I’m really confused here. Una has consistently said the opposite

“The only possible explanation is that somehow the aerodynamics improved remarkably with the windows down - but then why was the AC on/windows down case poorer than the AC on/windows up case?”

“That’s what we thought at first - the problem is, windows down with the AC on was worse than windows up with the AC on. The only theory here is that since there was air blowing through the car, the AC compressor had to run constantly to keep temperature, rather than cycling…but that’s just speculation.”

I think it’s simple, there are two effects as a number of people have pointed out. Climate control is more efficient with the windows up, and (strange as it may seem) the Covette is more aerodynamic at certain speeds with the windows down.