I drive a 2011 Tundra 5.L 4X4 truck. why do I get better gas mileage while driving in the Rockies vs states like OK and TX? I get 2mpg better while driving in the Rockies. 17mpg vs 15mpg. I drive same speed, buy the same 87 octane gas & drive on Interstate highways. And it doesn’t matter whether I’m driving to or from. Is the gas quality different in the Rockies vs the south or plains states?
When I have been in states like Colorado, the least expensive gas has an octane rating lower than 87. 87 octane is a mid level gasoline in high altitude areas. My guess is that the computer on your Tundra is advancing the timing to take advantage of this and hence you are getting better mileage. When you are in OK and TX, test this theory by purchasing 89 or higher octane and see if your mileage increases.
It’s the elevation. Air is less dense up in the mountains and so there is less wind resistance, also, the lower atmospheric pressure means there is less back pressure at the exhaust that the engine has to fight, a reduction of pumping losses.
There’s a reason airliners climb to around 30,000 ft. before leveling off to cruise. At that altitude, the air is only about 1/3 as dense as sea level.
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Sometime I visit Colorado and play a game a golf. I constantly overshoot the green there. The ball flies considerably further due to lower air resistance. I’ve learned by trial and error if the shot would go 200 yards at sea level here in California, the same shot will go about 225 yards in Colorado.
Another vote for the combination of thinner air’s lower aerodynamic drag, and its effects on the engine management systems on your ride. (I.e. scaling back fuel inputs due to thinner air, lowered tendencies to knock leading to more advanced timing, etc.)
Also, the thinner air will lead to your engine (no turbo, right?) having a lower power output…around 30% lower at 10,000’. This makes the engine more efficient due to less pumping losses–essentially, the same reason a smaller engine tends to get better MPG.
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Just to illustrate how much thinner the air is in the mountains, this is what happens to an empty but sealed water bottle when you take it from Taos NM to Austin, TX.
The same thing happens to your tire pressure. Tires that are inflated to specs at sea level will be somewhat overinflated as you go up in elevation and vice versa.
@jasmith5:
I’m going to go against the grain here and say there is way too little information for anyone here to draw a conclusion.
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Can air resistance play a role? Yes, but prevailing wind speed and direction that you encounter while you’re driving will dominate that.
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Will the less dense air at higher altitudes lead to lower pumping losses? Yes. But engine efficiency drops much more dramatically with lower density mixtures in the combustion chamber. So if anything, that will tend to hurt your mileage at higher altitudes.
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At your lower OK and TX altitudes, your 87 octane is needed. At higher altitudes, your engine only needs 85 octane. That means when you’re using an 87 octane at higher altitudes, as @Triedaq noted, your engine will likely advance the ignition timing, and that can help.
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Do the gasolines that you’re using in your experiment contain the same levels of ethanol? If you’re buying ethanol-free gas at higher altitudes, that will make more of an mpg difference than any of the above.
yep. lots of variables…
Many gas stations sell 85 octane gas in Utah. My car does not like it. Owners manual says to use 87.
When your owners manual specifies 87 octane, that’s for below 4000’. In general, high altitudes reduce the demand for octane, allowing 85 octane to give you what 87 gives you at lower altitudes.
I have to agree with @JoeMario here because I used to live in the high desert of Arizona and used 85 octane all the time. My Jeep ran great but when I visited Tucson or Phoenix with 85 octane still in my tank…it ran a little rough. I’ve traveled all over the country and the many variables in altitude, terrain and fuel quality affect your fuel economy. It’s just a fact of life.
@JoeMario - Will the less dense air at higher altitudes lead to lower pumping losses? Yes. But engine efficiency drops much more dramatically with lower density mixtures in the combustion chamber. So if anything, that will tend to hurt your mileage at higher altitudes.
Does the engine really know the difference between a low density mixture due to high altitude and a low density mixture due to a more closed throttle?
We aren’t running our engines anywhere close to wide open when we cruise at 70-75 mph, unless of course, you happen to be riding a 250cc motorcycle.
Willys Jeeps used to be available with “high compression heads” for use at high altitude. The US Forest Service and other agencies as well as ski resorts used these vehicles a lot.
The higher compression allowed the flat head engines to develop similar power output as those at lower altitudes.
My guess is that it’s either more a function of the higher octane in gasoline or you are getting your mileage figures wrong. In the mountains the lower air density means fewer horses and increased fuel consumption along with climbing grades will also bump up fuel consumption. Less air resistance is a factor but negligible at lower speeds and more then offset by loss of power. There will be lots more downshift and your motor will generally turn over at higher RPMs resulting in increased fuel consumption. I can’t imagine the decrease in air density and air resistance can more then make up for these factors.
this is the second poster to note this phenomena…
Being a Westerner I am familiar with the high altitude 85 octane regular although I have never checked for any difference in fuel mileage. Some of the Western States tend to be anti Federal Government so I would suspect the 0% ethanol theory may be the answer. I live at 220 feet above sea level with mandated 10% ethanol so it matters not to me.
Some of the best gas mileage our Honda Element ever got was in the mountains around Taos NM. One summer, we spent a week there and drove all over the Enchanted Circle region visiting Red River, Angel Fire, That bridge over the Rio Grand that was featured in the movie “Wild Hogs”, and other places. We got over 30 mpg in a vehicle that usually gets around 22-24 mpg in Texas.
Less air resistance is a factor but negligible at lower speeds and more then offset by loss of power.
Less power does not necessarily mean less efficiency. Engines tend to have their minimum specific fuel consumptions at around 1/2 to 3/4 open throttle and the reduced power at high altitude allows the engine to be used at that throttle opening more. The hills also have the engine running at those throttle openings more than flat roads do. Yes you use a lot of gas climbing hills but hills go down also so after climbing that hill, you go downhill using almost zero gas, it can actually average out to better gas mileage than what the same car would get on a level road.
Does the engine really know the difference between a low density mixture due to high
altitude and a low density mixture due to a more closed throttle?
It does care because the low density mixture at high altitude has less oxygen.
I’m not arguing that you or the OP aren’t getting better mileage at high altitudes.
What I disagree with is writing it off to lower pumping losses. There are too many other variables, (ECM controls, driving conditions, fuel) involved to make it sound that simple.
I would argue that there are lot’s of other contributing factors. The biggest being speed. I drive much more slowly and less aggressively in the mountains then I do as a flatlander.that has to count for something. Mountains have many more corners and driving slowly and more efficiently compared to tooling along at 70 plus mph makes a big difference.
IMHO, it isn’t just one factor but a combination of factors and there would be many who did worse in the mountains. It’s one thing to compare miles age in a light car or on a motor cycle but if you tow or are heavily loaded, you will be downshifting going down and downshifting going up then mountainous driving could have the reverse effect. It is vehicle, driving situation and load dependent and I would make no generalizations that cars or trucks do better in the mountains.
It does care because the low density mixture at high altitude has less oxygen.
The ratio of oxygen to nitrogen is almost exactly the same at 30,000 ft as it is at sea level. Your engine doesn’t know the difference between an absolute manifold pressure of 6.15 psi due to being 20,000 ft above sea level and an absolute manifold pressure of 6.15 psi due to the throttle being partially closed at sea level. They both fill your cylinders with air that’s 21% oxygen at 6.15 psi.
People experience hypoxia at high altitude not because the air has less oxygen, but because the air is less dense. Above 40,000 ft, people will experience hypoxia even if they are breathing pure oxygen. Astronauts in space suits breath pure oxygen at 4.7 psi. By using pure oxygen, the space suit doesn’t have to be built strong enough to withstand 14.7 psi pressure and thus can be lighter and more mobile.