Gas mileage difference while driving in different states?

As a multi-thousand hour pilot, I know the answer to this one. The thing is, a modern vehicle’s engine management system leans out the mixture of the fuel to air ratio the higher the altitude. In a piston driven airplane engine this is accomplished by the pilot adjusting the mixture knob. If one is taking off from a high altitude airport, it is necessary to lean the mixture before the takeoff roll so maximum power will be available. The higher one goes, the farther out one pulls the mixture control. I recall flying back from Reno, Nevada several years ago at 11,500’. With the mixture adjusted properly, my Cessna 170B burned only 6-1/2 gallons an hour. Same thing happened a few years later coming home from Yellowstone. (Jackson Hole WY). At my normal cruise altitudes of 3500’ to 4500’ ASL around home it will burn about 9 GPH.

I have also experienced this same higher MPG phenomenon while DRIVING in mountainous areas throughout the Rockies in a modern fuel injected car with computerized fuel management. Years ago, in a carbureted car, the experience was more like having a poorly running car at higher altitudes. MPG suffered, a lot. New cars delivered to higher altitudes had smaller jets in their carburetors to make them run better. Today it’s all in the fuel management system.

I agree with MG McAnick. Back in the 80s before Subaru phased out EEC controlled carburetors they offered a “high altitude” model for dealers in mountain areas. My memory is fuzzy on this but the carburetion and ignition timing curve characteristics were different from what I can remember.

A non-turbocharged SAAB that ran fine in OK would become a real dog in the mountains of CO and the fuel mileage would suffer accordingly; and made even worse with an automatic transmission.

@B.L.E.

Yes - I agree with you about the ratio of oxygen and nitrogen remaining constant. And I’m sure we agree on lots more on this subject.

The point I was making was one can’t say the increased mpg at altitude is just from pumping losses. If it was pumping losses, then older carburetor-ed cars would have gotten better mpg at higher altitudes.

Carburetors tend to go rich at high altitude and this would make carbureted cars run badly and get lower mpg.
The reason is venturi physics.
If you double the air flow speed through a venturi, the suction that pulls fuel through the metering jets doesn’t double, it quadruples. Fortunately, quadrupling the suction causes twice as much fuel to flow through the jets and the mixture tracks the airflow nicely.

When you double airflow by doubling the density of air instead of flow speed, you merely double the suction that draws fuel. Doubling the suction causes the square root of two, not two times as much fuel to flow, so instead of two times the fuel flow, you only get 1.414 times the fuel flow and so carbs go lean at low altitudes where the air density is higher.

That’s also why throttles are downstream of the venturi. If they were upstream the venturi would get lower density air instead of slower air and the mixture would go rich at part throttle, as if the choke were closed.

B.L.E.
I agree with what you said in the ^^^ above reply.
But what is the point? Where is this going?

OK, now let’s discuss pumping losses.

Actually there is a difference between and engine that has only 7 psi intake manifold pressure due to being at a high altitude and an engine that has 7 psi intake manifold pressure at sea level because the throttle is not completely open.

The engine running at high altitude also has only 7 psi pressure at the tailpipe opening.
The engine running at sea level must pump air from the low pressure in the intake manifold to the 14.7psi pressure at the tailpipe. It takes power to do that and the power needed to pump that air from a low pressure to a high pressure is subtracted from the shaft horsepower.

That’s why the most efficient way to reduce the power of an airplane engine to 70% is to climb to an altitude of about 10,000 ft where the air density is about 70% of the sea level air density and thus the engine makes 70% of it’s rated power at full throttle.

. Back in the 80s before Subaru phased out EEC controlled carburetors they offered a "high altitude" model for dealers in mountain areas. My memory is fuzzy on this but the carburetion and ignition timing curve characteristics were different from what I can remember.

It’s not so much that a specified charge density needs a different ignition advance when high altitude and not a more closed throttle caused it. It’s more that old fashioned low tech vacuum advance fails to recognize that the charge density is low at high altitude and thus fails to advance the ignition sufficiently.
At sea level, when you have 6 psi absolute manifold pressure on one side of the ignition advance diaphragm and 14.7 psi atmospheric pressure on the other side, the diaphragm has a 8.7 psi differential between the two sides and gives the correct ignition advance for that 6psi absolute manifold pressure.
But if you drive with the same 6psi absolute manifold pressure at 8000 ft above sea level, the pressure on the other side of the diaphragm is only 10.4 psi instead of 14.7 for a pressure differential of only 4.4 psi instead of 8.7 psi, the result, the vacuum advance fails to give the distributor the advance needed for the 6psi absolute manifold pressure.

Modern EFI cars use either absolute manifold pressure sensors or the ECU calculates absolute manifold pressure from mass air flow and thus gives the correct ignition advance and fuel mixture at high altitude.

Like the original poster, who drove a EFI vehicle, many people notice a fuel mileage gain at high elevations, it’s not just lower speed limits and mountain roads. We have “mountain terrain” here in Texas also, it’s just at low elevations and there are flat interstates in the high elevation states also with high speed limits. They lay out the roads to go around the mountains, not over them.

The difference between an engine operating at MAP = 15"Hg at sea level pressures…and one doing the same at 18,000’ MSL, is the S/L one is pushing that air “uphill”: it takes air in at 15" and exhausts it to 30". The 18,000’ engine exhausts air at the same 15" it took it in at.


SOME of the S/L engine’s power is used up by functioning as a “glorified air compressor.”

Also keep in mind that gasoline formulations may vary from region to region and even over the course of the year in areas with serious air pollution. Summer gas in California gives worse mileage than winter gas. They add various chemicals (including ethanol) to summer gas so it burns cleaner. Other western states also have pollution problems and it wouldn’t surprise me if they also had special formulations

In Texas, it seems to be the opposite, I get better mpg during the summer than winter. In winter, the refineries blend butane into the gasoline. It’s cheap, it has a high octane rating, and it makes cars start when it’s minus 20 outside, but it has a low density and lowers the specific gravity of the fuel meaning fewer BTU’s per gallon.
They can’t add it during the summer months because its high vapor pressure causes too much evaporative hydrocarbon pollution and makes older carbureted cars vapor lock.
It seems my summer gas mileage comes back right about the time of the year that gas prices jump up for summer which lead me to believe it’s fuel related. I don’t get summer gas mileage during a winter Indian summer.

California tends to have much worse smog problems in the summer, especially days when the winds are blowing from the deserts and inland valleys towards the coast (the winds known as Santa Anas and several other local names.) So California gasoline is formulated to have a lower vapor pressure during the summer, which means it has fewer cheap additives than winter gas, costs more, and often gets slightly worse mileage.

Does California even have winters?

Well, it has skiing, so…

I ended up in Southern California from January 1974 to November 1976 for employment reasons. There were no jobs or gas in Oregon at the time. Once you crossed the border both were available. My buddy and I had guaranteed jobs so it was not a leap of faith. As a 21 year old I had a pretty good time. The coldest temperature I remember was 59%F. There was one time it was actually snowing on the roofs of the downtown LA skyscrapers. Of course it melted immediately. At 59% the natives were wearing their ski parkas! If it rained the driving skills which normally involved exceeding posted speeds by 20% and was considered normal was not adjusted. Many fender benders. Nowdays they seem to have “Winter” temperatures in the 40s. How do they survive?

Well, yea, like California is the third largest state in the union and reaches all the way to Oregon, but most of us who don’t live there think of L.A. when someone mentions California, just like when New York is mentioned, everybody imagines New York City and not dairy farms, forests, mountain ranges, ski resorts, and lakes.
Did you know that Hawaii also gets snow on top of some of its mountains?

I was on top of Mauna Ken a few winters ago with two feet of fresh snow on the ground. It was about 20 degrees at sunset. Brrr. And California has a wide range of climates, too. Except for the very tops of mountains nowhere is extremely cold, but there is lots of snow lower down in the mountains. Not far from there you have Death Valley, one of the world’s hottest, driest places.

The summer gas is only required in the counties with serious smog problems. Some of those are urban, but many are rural counties on the eastern side of the San Joaquin Valley. Fertilizers, pesticides, diesel exhaust from tractors, trucks, and irrigation pumps, even fine dust rising from fields all contribute to truly awful air that can’t blow away because of the Sierra Nevada directly to the east. At least they now regulate the burning of rice stubble carefully. When I lived in Sacramento in the eighties there were fall days when a noxious cloud of smoke blew into the city and stayed for days. Now they can only burn when the wind will carry the smoke away. Where I live in San Francisco smog is rarely a problem, though it can be in other parts of the Bay Area.

just like when New York is mentioned, everybody imagines New York City and not dairy farms, forests, mountain ranges, ski resorts, and lakes.

People think of NY state as one big city. Most don’t know the large agriculture industry in NY state. Second largest wine producer in the country, second largest dairy producer in the country, second largest apple producer in the country and second largest Maple Syrup in the country.

SO…
Test your theory and head ‘‘out west’’ this weekend and next week.
The Albuquerque ( 5000 ft ) International Balloon Fiesta begins this weekend.
A sight to see.

And if you like hot air ballons,
come to Gallup NM ( 6500 ft ) December 4,5,6 for the Red Rock Balloon Rally.
less balloons than Abq but you can get up close and personal here easier.

SO.... Test your theory and head ''out west'' this weekend and next week. The Albuquerque ( 5000 ft ) International Balloon Fiesta begins this weekend. A sight to see.

I’d love to but that’s a really long 2-day drive, even though New Mexico is the next state. The real Texas ain’t like Hollywood Texas where outlaws can break out of jail in Abilene, steal a couple of horses, and be in Mexico by sundown.

Speaking of Hollywood Texas, I was amazed when in California how many areas looked like Texas. And Maine and Pennsylvania and… So, no way you can put your finger on mileage in California.