Altitude vs Gas mileage

[Bacpacr]

I am back from my trip to New Mexico with the Boy Scouts from South Carolina. The Ridge was a absolute dream to drive cross country. One curious observation made at each fill-up was my mileage was getting better as the trip progressed.May be this thing is finally breaking in(16,000 mi). I was getting 21.5 when I filled up in NM 6000ft+. I checked it on the way home and the mileage slowly went back down to the old 18.5. In retrospect I wish I would have been more scientific about the readings and recorded each fill-up. SIGH

[MikeT]

There are so many variables to consider when doing milage comparisons. The quality of the fuel, wind direction, tire pressure, atmospheric pressure, spousal pressure, ect.

The best milage I ever achieved was in Colorado at over 8000 feet altitude so you may be on to something.

[Webwader]

On our trip back east, I was surprised that mountain driving mpg was not as negatively affected as I thought it would be. I actually got my best mileage between Rock Springs, WY and Sutherland, NE at 25.66 mpg, but it was affected by two factors. First, although we had to cross the Continental Divide, the majority of the leg was a net altitude loss. Second, a large portion of the downhill leg was assisted by a nice tail wind. What does affect mileage in the mountains is the grade. I80 thru the South Pass is mostly gentle grade, so the RL rarely got out of 5th gear. Our worst tank was coming back through eastern Washington, where the mountains are not nearly as high but are constantly up and down with fairly steep grades. A lot of 3rd and 4th gear climbing.
But on the whole, our mpg was fairly consistent whether we were in the mountains or on the plains.

[steppy]

.......could be due to the increased smug levels on the east coast?

smug is much thicker than smog fyi

[xridgelinex]

Best was on our recent return trip from cape cod... 27+... I attribute it to being at sea level, flat roads and having to stay around 60 mph.

[Hennie]

The higher one drives, ie, Colorado, the thinner the air. The engine computer then adjusts the fuel flow to compensate for the thinner air to maintain the proper 14.7:1 air/fuel mixture. The result is better MPG.

[Teezer]

Originally Posted by Hennie

The higher one drives, ie, Colorado, the thinner the air. The engine computer then adjusts the fuel flow to compensate for the thinner air to maintain the proper 14.7:1 air/fuel mixture. The result is better MPG.

Less air to push aside up here, too...

[Webwader]

The higher one drives, ie, Colorado, the thinner the air. The engine computer then adjusts the fuel flow to compensate for the thinner air to maintain the proper 14.7:1 air/fuel mixture. The result is better MPG.

Nice try, but no cigar. If the computer cuts back on the fuel flow, a loss of power will result. So if you need to maintain enough power for the conditions you are trying to drive at, you would have to open the trottle farther than you would at low atltitudes to ingest enough air to maintain the fuel burn required. At some point in elevation rise, the engine can no longer compensate, and a loss of engine power results.

[Hennie]

Webwader, it was actually a fantastic cigar!!!

There is definately a loss of POWER at altitude, that's why piston engine aircraft are limited to 10,000 feet MSL (mean sea level) if they don't have a super/turbocharger installed. What the engine computer is actually doing is leaning the mixture automatically to compensate for the less dense atmosphere. It's exactly what piston engine pilots do manually with something called a mixture control.

Hennie

[Webwader]

We may agree that there is a loss of POWER at altitude, but a loss of POWER does not equate to better MPG, merely loss of POWER. If it takes X amount of HP to maintain 60 mph, you can no longer maintain 60 mph at the same throttle setting when you have a loss of POWER (less air=less fuel to maintain air/fuel ratio=less power. In an AUTOMOBILE, to compensate for that loss of POWER, you have to open the trottle wider to try to bring in more of the less dense air, thus more fuel to reattain X HP.
The air/fuel ratio doesn't change just because the air is less dense.