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

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.

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.

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

smug is much thicker than smog fyi

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.

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.

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... :D

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.

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

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.

I noticed a sizeable increase in mileage by using Regular Unleaded that usually is 85.5 Octane at pumps sold at higher altitude. If I understand correctly, lower octane fuel actually burns hotter and the computer probably adjusts for it by reducing the amount of fuel sent to the cylinders.

I know Honda recommends fuel 86 Octane or higher, but .5 shouldn't hurt much.

My experience here should be tempered by the fact that I changed my air filter (quite dirty!) at 14,500 miles during this period and that could have contributed to something.

Tom

If I understand correctly, lower octane fuel actually burns hotter and the computer probably adjusts for it by reducing the amount of fuel sent to the cylinders.
In a way it burns hotter, but actually it burns faster. The flame front from the spark plug moves across the combustion chamber faster than a higher octane fuel. This is the cause of detonation, or pinging. Here's a definition of detonation - "....when we ignite the fuel/air mixture with a spark, the flame front starts moving across the cylinder, burning the mixture of air and gasoline vapour as it goes. This increases the temperature (and therefore the pressure) of the remaining fuel/air mixture rapidly. The remaining fuel starts to decompose before the flame front reaches it. If this decomposition produces 'auto-ignition' compounds (those which will start burning without a spark), you end up with an uncontrolled over-rapid burning of the remaining fuel/air mixture, which sets up an opposing pressure wave in the cylinder. This uncontrolled burning and the opposing pressure wave produces the characteristic clicking/pinging sound of detonation, and results in the piston getting a 'hammer blow' instead of a steady push."
So if your engine computer is doing anything, it is probably retarding the timing to prevent pinging if it is occuring.
I would suspect the changing of your air filter had a lot more to do with it.

Webwader,

You're right, the air/fuel RATIO doesn't change with altitude, but the AMOUNT of air and fuel required to achieve that ratio DOES change.

Let me try to explain it this way. In the old days before electronic fuel injection there were carburetors. If the carb was adjusted to acheive the 14.7:1 air/fuel mixture at sea level and was then driven to, say, Denver at 5,000 feet, the amount of air entering the engine would be less, but the amount of fuel would be the same, thus creating a rich mixture (meaning something LESS than 14.7:1).

Today, with the computerized engine management systems, the engines always operate at the proper 14.7:1 air/fuel mix regardless of altitude. The reason the MPG increases is because the air is thinner and the ONLY way to maintain the proper ratio is to LEAN the mixture. By leaning the mixture, less fuel is admitted to mix with the air. Less fuel in means less fuel burned means more MPG.

Like I said, it is the SAME thing pilots of piston engine aircraft do to INCREASE RANGE after climbing to a cruise altitude; but it isn't automatic. The pilots have to do it manually with the mixture control.

Hope this explains it.

It's not a matter of explaining it, Hennie. It's a matter of coming to the correct conclusion. Pilots lean out the mixture for the same reason automotive computers do. When they start out at low altitude, they set the throttle for the correct fuel/air mixture at that altitude. When they reach a higher altitude, the mixture is too rich because the air is less dense, so they lean it out to the correct fuel/air mixture. It has nothing to do with range, just to make the engine run correctly.
You already understand that less air and less fuel mean less POWER, or at least you say you do. What you don't seem to grasp is that it takes the same amount of power (minus a minute amount for the less dense air) to maintain the same speed you were doing at a lower altitude. So if you want to maintain the same speed at altitude, you have to open the throttle wider to admit more of the less dense air and consequently more fuel. Just like the pilot, who after leaning out the mixture, has to increase the throttle if he wants to maintain the same cruising speed. The laws of physics are not suspended because you are driving at a higher altitude.
However, if you want to believe you are correct, that is entirely your prerogative.

Hey Webwader,

You're right again. The HP needed to go 60 MPH at sea level here in New Orleans and the HP needed to go 60 MPH at 5,000 feet in Denver is exactly the same. However, the engine can produce that HP at any infinite air/fuel ratios. It DOES NOT need to be the correct 14.7:1. It can produce that HP at 8.0:1 (very rich) or 18.0:1 (very learn). What happens with EFI is that when you step on the gas to accelerate to 60 MPH the air/fuel mixture is very rich in order to provide the power needed and it stays rich until after reaching 60 MPH, the engine settles down and the computer THEN sets the mixture to 14.7:1.

Now, some areas where you might not want to believe that you are wrong:

First of all, leaning the mixture has EVERYTHING to do with range at altitude. An aircraft will fly at, say, 8,000 feet at full rich mixture just as well as at full lean, but it will not fly as far. Like you said, simple physics.

Second, a pilot does not use the throttle to set the mixture, he uses the mixture control.

Third, any pilot who first leans the mixture and then advances the throttle won't be a pilot for much longer. What is done is to climb with the mixture set at full rich, then when the desired altitude is reached the throttle is retarded to the RPM needed to maintain altitude, THEN the mixture control is slowly pulled out until the engine just begins to sputter, then the control is pushed in for one click and the mixture is set, FOR MAXIMUM RANGE AT THAT ALTITUDE. Any change, up or down, first requires the mixture control to be pushed in to full rich, before moving the the throttle. Sound familiar? See paragraph one.

But let's say I'm a complete idiot. So, how 'bout explaining to me why, in 1972 my '71 Corvette (with carb) got 14 MPG in New Orleans but only 10 MPG in New Mexico and a mechanic told me I would have to reset the carb jets to get MPG back to what I was used to. And why my 2003 Honda Element (with EFI) gets 25 MPG in New Orleans, but 28 in New Mexico?

Thanks,

Hennie

Second, a pilot does not use the throttle to set the mixture, he uses the mixture control.
I didn't say that. Try rereading the post/
Like they say at Burger King, have it your way.

Hey,

I just re-read your earlier post. Allow me to quote what you wrote. "When they start out at low altitude, they set the throttle for the correct fuel/air mixture at that altitude."

That was the fourth sentence and note the part following the comma.

Still patiently waiting for you to explain the real reason that I get better MPG in New Mexico than in New Orleans.

Hennie

Wait on, Hennie. Do you really think that I can explain that without knowing any of the facts, only that you say you get better mileage. First, I don't even know you arrived at that figure. Second, there are so many variables that affect mileage beside altitude that it would be next to impossible to explain even if your figures were arrived at accurately and over a long period of time in both locations.
But let's say I'm a complete idiot. So, how 'bout explaining to me why, in 1972 my '71 Corvette (with carb) got 14 MPG in New Orleans but only 10 MPG in New Mexico and a mechanic told me I would have to reset the carb jets to get MPG back to what I was used to.
If you think you understand this so well, why did it take a mechanic to tell you what was wrong with your Corvette? Pretty basic stuff here. The air/fuel ratio changed due to altitude change and carbs don't compensate for altitude. I may have to agree with you about the idiot part.

BYW, I also said "Just like the pilot, who after leaning out the mixture, has to increase the throttle if he wants to maintain the same cruising speed." but you ignored that.

Don't expect any more replys. Like I said, have it your way.

Webwader,

You're getting too emotional here. In 1972, I was 21 and didn't know squat about engines and that's why I asked a mechanic why my MPG dropped so significantly.

Yes, I read your comment about the pilot setting the mixture, then increasing the throttle and replied that any pilot who did that wouldn't be a pilot much longer. The throttle is increased first, then the mixture is set, not the other way around.

Maybe you should go to the local general aviation airport and talk to a CFI (certified flight instructor) about the relationship between mixture and range.

If that's not possible the local library probably has a book on Charles Lindbergh. If the book is detailed at all, it will have a part on how he taught P-38 and F4U pilots in the Pacific how to drastically increase their range by leaning their engines way beyond what the official flight manuals said was possible.

Finally, there are people today who, DESPITE THE FACTS, still deny that the holocaust ever happened.

How does altitude effect fuel economy? The simplified answer is that it doesn't. There are many variables to the equation, but if you do an apples to apples comparison the results will be nearly identical.

What effect does altitude cause? Less power.

Altitude has the effect of raising gasoline octane ratings.

If you do two ten mile "flat and level" driving tests (same vehicle, temps, humidity, etc.), the first at sea level and the second at say 5,000 ft. above sea level you will have very similar fuel economy results.

If you change the above to a quarter mile race the results would be quite a bit different. The vehicle at sea level will love that denser air and perform much better. Same results if you try and pull a trailer or heavy load.

Not taking one side or the other here, but keep in mind that the fuel mixture on a modern automobile is not variable in the sense that piston aircraft use. An automobile will meter the volume of intake air and inject fuel to match that volume of air. In a small aircraft you can adjust the mixture from rich to lean for any given volume of intake air.

The fuel / air mixture on a modern automobile is managed with low tailpipe emissions in mind. The mixture for maximum power, or for maximum fuel economy are not the determining factor (overall).

I live in sea-level (san diego) and My dad lives in Flagstaff Arizona (7300 ft) I just spent 2 weeks up there, and put a good 600 miles up in that area. My Ridge gets a Significant difference in fuel mileage. Might be something to do with Cali vs Arizona Gas.

But I usually get around 310-320 miles out of 19 gallons in san diego,
I got around 400-410miles out of 19 gallons out there.
I have never gotten even close to that down here.

Sea Level is 14.5psi is our atmospheric pressure, it is about 12psi at 7000ft.

The oxygen content is accually the same, however its the pressure levels that we breath in. Your body has to work harder to breath in the same oxygen content as at sea level.

The Ridgeline motor, cant "work" harder to take in more air like the human body can, so it results in a loss of power. Quite alot accually, however with the atmospheric pressure change, the engine compensates by reducing the duty cycle of the injectors to maintain a 14.7:1 air-fuel ratio. Your fuel economy is increased. Power is still lost, no real way to make that up unless it was a turbocharged vehicle. What people are saying 60mph at sea-level uses the same power as 60mph at 7000ft is accually wrong. There is less pressure at that altitude less pressure means less resistance on the vehicle. Thats why people do see altitude gas mileage difference. But there are alot of factors involved with it.

But in my experiance, I have seen results in fuel mileage. We have a fleet of trucks there, and even some of the Ford F-150's result in 20mpg up there..

Maybe ford should do there EPA ratings at pikes peak or something? ;)

-Jay
06 Silver Ridgeline RTL w/ Nav

Great discussion. I know in Drag racing, fuel mixture,altitude,humidity, and may other variables are considered to get the best ET. That's for just a quarter mile distance! I do enjoy learning on this Forum.

I'm at 4500 - 5100 ft local conditions, hilly and usually windy

I get about 18.5 mpg, mostly highway use cruising 77 mph and some city driving, I hammer on it a lot. IMO the cruise contro,l sucks up here, it doesn't downshift agressively and you lose too much speed uphill before it kicks in

I get better mileage at lower altitudes

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.

Not sure which is correct as I have been ranting too much but this I know. The 14.7:1 relates to weight of air to weight of fuel. Therefore the thinner the air then the computer will adjust less fuel but stays consistant it really depends if your going up the mountain or down it. But then again we do have four oxygen sensors in the ridge and that's a lot of fine tuning.

oops.

San Diego is sea level? Maybe right at the base or the airport, but anything inland and you start to climb. Of course my opinion of sea level is different from everybody else's. I grew up at sea level (costal Florida) and even driving to the hills of LA (Lower Alabama) you could tell the difference.

I got better mileage going from the mountains of NC to the coast of VA than I did on the way back, of course I'm not so naive to think altitude had more to play on it that just change of elevation.

Keep in mind that altitude is only one component. What you really need to know is the Density Altitude.

Density Altitude takes into account not only the distance above sea level, but the barometric pressure, temperature and humidity of the air.

So we can come up with a scenario where you're at sea level with high ambient temperature and humidity that results in worse performance than an identical vehicle at 2500 ft. elevation in cool dry weather.