Premium gas and mileage question.

General Questions
#1

I have a new ’06 Pathfinder with 2500 miles on it now. It’s about 2 months old.

I live at a very high altitude (11,200 feet) and 99.9 percent of my driving is done on the highway, over mountain passes. Using regular gas, a 100 mile trip from Denver to my house gives me a little better than 20 mpg. Pretty good for a 4700 lb car and a 6000 foot elevation gain.

Yesterday, on a trip back from Denver, I mistakenly put premium in it. I got just under 17 mpg.

The cars ‘specs’ call out ‘For best performance, use premium gas.’

That seemed a little odd. My understanding is that at altitude, premium does nothing for you. It appears though, it actually does worse. Is that a correct assumption?

Perhaps the premium squeaked out another horse power or two, but I sure couldn’t tell the difference, and the mileage difference is pretty significant. It’s much worse with premium.

Side note: I don’t plan on burning anything but regular in the car. I just screwed up and put premium in it. [sub]Damn gas stations [even at the same type] sometimes the premium is on the left, sometimes its on the right[/sub]

#2

whoa! I’m impressed.

Modern engine control computers will retard the timing if detonation (AKA knocking or pinging) is detected. If that doesn’t occur with regular gas, then there is no performance benifit to running premium. NONE. As a crude rule of thumb, you should be able to knock off 1 point of octane requirement for every 2000’ of elevation.

OTOH, the computer will only dial in so much advance. Lower octane gas actually burns faster, Giving more time for expansion, which is more efficient.

I believe that Denver still requires oxgenated fuel be sold at the pumps. Even if not, Ethanol addition is a cheap way to raise the octane. So it is possible that what you saw was due to running gasahol rather than premium.

Also keep in mind that aerodynamic drag is proportional to air density and speed squared. High altitude also reduces engine pumping losses for a given power level, and aids fuel vaporization. Thus you can expect improved fuel economy at higher altitude with a properly functioning closed-loop fuel injected vehicle. (carburators need not apply). I-70 into denver has most of the curves ironed out, so there is a good chance you were driving faster than you typically do when driving on “real” mountain roads.

As an aside, I’ve never lived below 5000’. The days before EFI were hell, with all auto manufacturers having a strict sea-level mentality. It got really bad in the 70’s when emission control started. Suddenly carbs were sealed so that they couldn’t be adjusted to run correctly at altitude, ironic, because the “improved” cars then had worse emissions than the old “bad” cars.

#3

Thanks for the response Kevbo.

That’s interesting. Never thought about the ethanol/ gasohol angle. When I make a trip down the ‘hill’ I usually buy gas in Denver. It’s about 18 cents a gallon cheaper.

But, if my mileage suffers that much, there is no point to it.

Not sure, but I don’t think ethanol is mixed into gas up at altitude.

Better fuel economy with less O[sup]2[/sup]? If that’s the case, I’m in the cat bird seat.

Yep, but I was talking about the same trip (on State Highway 285) wherein I got about 3mpg less by buying premium gas in Denver as apposed to regular in Denver (I’ve had to drive down to the big ‘D’ a lot lately to take care of my folks). I suspect that the premium has ethenol added while the regular in Denver doesn’t and it screwed up my milage.