Originally Posted By: friendly_jacek
Ekpolk,
I'm heapy for your MPG. No doubt 0W20 helped some, but not the 33-39 jump. Lets see, it was raining hard, you slowed down some, didn't you? Then it was cooler, and AC didn't work as hard. And then, maybe a wind difference, like you alluded to. Here is you difference.
There is no way MPG goes up (in highway driving and hot weather) by 20% just by decreased oil viscosity. Simply, because the amount of energy absorbed by oil in a form of friction is less than 20% in highway driving. And that is physics. In EPA studies, change to lower friction lubricants accounts for only 0.5% decrease in CO2 emissions (and fuel consumption). And that is science. What you have is just an anecdote, and not a good one.
When I switched to all synthetic fluids (and not just engine oil, ATF, and diffs x2) in my Subary, I saw no change in MPG whatsoever. That is my anecdote.
"Heapy", what's "heapy"??? That conjures some potentially ugly images...
Seriously though, I agree with you, in part, but with some "howevers". First, the rain involved was southern summer rain. The kind that lasts for five or ten minutes, like a wall of water, and then is abruptly gone. I would estimate that on the drive home, it was raining for maybe 20 min of the total 3-plus hours. Yes, I could see the obvious air cooling from the rain, and yes, I did slow to more reasonable speeds, appropriate to the moment. The rest of the time, it was my usual, which I have to admit, is right below 80 mph.
Second, and I should put this into a separate posting in this thread, but I also noticed from my ScanGauge-II that despite the temps being about the same, the engine was running somewhat cooler. With the GC, on the highway, I'd normally see something between 190-195F, with occasional deviations in either direction. I'd only rarely see north of 200F, but I'd occasionally see that while in motion. Yesterday, I was consistently seeing 185-190F. This seems strange, since with a healthy, modern thermostat, and a computer controlled engine, such a disparity shouldn't be. But there it was. Perhaps, and to be perfectly clear, this is supposition, maybe the engine was simply not working as hard to maintain a given speed, and that was showing up in the coolant temp.
Third, yes, there could have been a tailwind, but I'd think that would apply for only part of the trip, given the unstable weather. Moreover, a steady east-to-west tailwind would seen improbable. Possible, but improbable.
Now, as for this part,
Quote:
There is no way MPG goes up (in highway driving and hot weather) by 20% just by decreased oil viscosity. Simply, because the amount of energy absorbed by oil in a form of friction is less than 20% in highway driving. And that is physics. In EPA studies, change to lower friction lubricants accounts for only 0.5% decrease in CO2 emissions (and fuel consumption). And that is science...
I must disagree (again, in part, but more "parts" here). You are overlooking several things. Your 20% figure reflects only one point in the "energy absorption continuum". Obviously, the amount of energy oils absorb through fluid friction varies with the grade involved. That's physics too. The real question is by how much. I'd also bet a fair chunk of change that the EPA studies you cite were performed on a limited range of vehicles. I'm pretty well convinced that one of the side effects of the hybrid drive train (at least the Toyota version) is that it effectively magnifies the differences involved, by layering on other factors that are not present with conventional cars. For example, these cars combine Atkinson-cycle running of the ICE with the hybrid system. That's going to distort the outcome some. Also, even in highway driving, the car continually cycles between some electric boost (and charges the TB while slowing or going downhill). More use of electric boost would also distort things.
And I didn't present this as anything but an anecdote, but I think it's not such a bad one. Really, how often have we see somebody do a back-to-back comparison with a near-4 cSt change in oil vis?
*** Darn, I have to run to a meeting. More later...