Some graphs of temperature vs. MPG

[instarx]

Originally Posted by PaleMelanesian

There is a definite leveling off of any gains past about 80 degrees. I don't use AC, so that's not the reason. I'm guessing 80-90 is the optimum intake temperature for my car.

I have very little data at the cold end, as you can see, so that's somewhat suspect. Most of this winter's points were on the high end there, while last winter had the low points. I think early results skewed the data. That, or just unfamiliarity with winter driving for mileage.

Pale,
Wow, lots of data points - what a nice change. However, don't you thinkk the distribution of those data points indicate that huge variables other than temperature are effecting your mpg? I'm not sure it is possible to pull the temperature component out like you have done. Your r* value of .16 tends to bear this out by indicating a very low correlation between the x and y axes. (For those who may not know, an r* (r-squared) value of 1 = perfect correlation, while 0 = no correlation).

For example, at 71F your data shows a high of 80 mpg and a low of 35 mpg (and everything in between). That's a difference of 45 mpg! Clearly some unknown variables are having a very large effect on your mpg even when temperature is constant. Therefore, the smoothed regression line calculated from data with an r* of 0.16 has to be taken with a huge grain of salt. A problem with Excel charts is that they often imply stronger correlation than is actually there because Excel will draw a nice regression line even when one may not really be appropriate.

Another problem in accepting that this line accurately represents temperature's effect on mpg is that seasonal variables independent of temperature are not controlled for. For example, is some of your winter commute in the dark when the days are shorter? Traffic generally moves more slowly in the dark than in daylight and would reduce your mpgs. Or does the lower winter Sun shine in driver's eyes, slowing traffic? Neither of these confounding variables would have anything to do with ambient temperature per se, yet would produce an apparent correlation when T and mpg are plotted against each other on a graph. This is the bugaboo that catches so many researchers - their study actually measures an unrelated correlation rather than the cause and effect they were thinking about.

[PaleMelanesian]

Of course there are other variables involved. This is real-world data, not test conditions. The higher probability of frost at 28F, for example, is included here. There's a huge human factor (me) that skews things. Wind is a big one, and rain too. Rigorous statistical analysis this isn't. I have data collected on these other factors, as well. This chart is simply a data-dump, not counting for any of the other factors.

Those points at 71F, for example. The 80 mpg one was with a warmed-up engine from earlier trips, and with a 15 mph tailwind. The 35 mpg was a 1.0 mile trip - should probably not be included in the data.

I debated whether to include the trend line. Clearly that was a mistake. This discussion is detracting from the value of the data itself.

[instarx]

Pale, I'm not saying putting a trend line in was a mistake, just pointing out how easy it is to give the line too much weight as a descriptor of a temperature/mpg relationship. I didn't mean my post to be a gotcha kind of post - just a discussion of your data that I thought might be interesting. Sorry if it came across as anything other than musings about interpreting data.

[MetroMPG]

Interesting plots, guys.

Andrew: re-label your chart with "month" instead of "temp". Then it's unassailable.

One only has to isolate a single variable pulled out experimentally to see the significant effect of temp on efficiency. This is a coastdown test of rolling resistance at various temps:



Source: MARS projects - Crr vs Temperature

The data is for a bicycle/trike, but applies generally to auto tires as well. Anyone who pays close attention to efficiency knows they have to adjust their glide points on familiar routes as ambient temperatures swing, even with tire pressure and road conditions held constant.

[PaleMelanesian]

I can play that game too.

A certain stretch on my commute. Hit the 50 mph sign at exactly 50 mph, and coast until the spot where the pavement changes. Record the final speed. This was last spring, but I stopped bothering to record this data when it showed to be so very linear.



It would be even closer, but a couple of those low points showed me that my tires were a few psi low. Once I fixed that, the data got back in line.

[MetroMPG]

Nice! Mind if I use that graph at some point in a MetroMPG post?

[MetroMPG]

I'm such a nerd for the data that seeing all the pretty graphs almost makes me wish I was a commuter, so I could play too. :P

[PaleMelanesian]

Go ahead and use it. I'm happy to provide data.

[McTimson]

I've been figuring that the low temperature must affect coasting as well, but never took measurements of it. Thanks for the graph, PaleMelanesian, that's a pretty big difference in speeds. Just another reason for summer to get here soon

[DIMS]

It does not get more straight forward than this on temp vs mpg