Quote:
Originally Posted by M3Armand
I assume that mpg is better at higher altitudes where the air is thinner. Thinner air = less air mass = the computer send less fuel = your HP sucks, but you get great gas mileage...
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I agree that maximum power is reduced at altitude due to less oxygen. In practice, the driver is likely to just push down on the throttle more to compensate, so I don't see why this alone would change fuel consumption. If anything, I would think that the fuel map would be less optimized, and fuel consumption would therefore increase at altitude. A quick search on the web for a source suggested that EGR mapping will be off as well, thus reducing efficiency.
I used to drive an Expedition with a 4.6 litre V8 from sea level to 15,000 feet and back to visit a customer, and it was a dog beyond 12,000 feet, even unloaded. Mileage was reduced, likely because it couldn't pull the usual gears.
I would put the observed mileage variables down to driving practices, roads, tailwinds, stop/start cycles vs steady cruise, and so on. Also, who says that the fuel purchased is really what it says on the pump? The pump label is a minimum, not an actual. Then there are the variable effects of ethanol %, ambient temperature changes, and ambient humidity changes.
All that said, the best fuel economy I ever got on my X5 3.0 was on 89 octane, measured over a full tank and not on the OBD.