14 Degree Octane?

I lived at 7300 ft and certainly experienced better fuel economy up there. But alas, also reduced power. Win some, lose some.

I think you've hit all the effects I could think of. Lower density at altitude has two effects: reduced air drag and higher throttle opening at a given power level, thus reduction of "pumping loss" .

The up/down being better than flat theory sounds reasonable, especially if the engine does fuel cutoff. But I bet the two density effects above are the big hitters.

Found one on the web. Air density at 5,000 ft is 14% less. That is clearly not enough to explain a difference from say 30 to 35 mpg but it helps. If we assume we get half of it in reduced fuel consumption (just a wild guess, air resistance is just part of the work), it would add a couple of mpg. (The SciAm note has

Another might be reduced rolling resistance due to effectively more tire pressure. About 8% more effective pressure. Also, tires might be a bit hotter due to reduced air cooling, further reducing resistance. Another 1 mpg together, maybe?

But note that as far as air resistance is concerned, pumping losses dilute the effect: it causes us not to get the full benefit of the reduced power requirement.

Oh it does at the slightest provocation. I know, since it makes tuning the link ECU in the high vacuum region almost impossible. You can come back slightly and carefully on the power, but anything normal and there goes the fuel. It is a wildcard, I really do not know how much mpg to give to it.

And random variation, probably. People might remember a 35 mpg high-altitude trip better than a 33 mpg one that they made at the same altitude.

Taking that into account I would tend to think that the numbers add up. Maybe even with some spirited driving thrown in too. :)

Leon

Which is, "premium unleaded fuel" for my 2002, though regular is OK if necessary.

This, of course, is for a stock, non-S/C application ;-)

Dana

Only if the engine does fuel cutoff. Without it, to first approximation, increases and decreases will average away. More precisely, fuel consumption will increase, since according to the example graph in the Bosch Automotive Handbook, the fuel versus power curve is concave upwards.

Fuel cut-off will change the game however, since it will put the engine on a different, much lower, fuel consumption curve.

And as I said, the engine cuts of fuel readily. As an example, see

I'm surprised. Usually brake specific fuel consumption decreases with load, so I would expect the curve to be concave down, right? I'll try to remember to check my Bosch at work.

Anyway, I think we've beaten this one into submission.

regards, Dave