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Old 06-10-2011, 08:13 PM   #1
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MPG versus air temperature and Altitude

Correction to my previous comment re my 1993 Infiniti G20. A drop of about 40-50 degrees F causes an INCREASE in air density and hence drag of about 10% (air about 300 degrees Kelvin). This results in a DROP in MPG of about 10%. I noticed this on one unusually cold night trip from California to Phoenix. Anyone else have data on this? Also; anyone noticed better MPG at higher altitude due to lower air density/drag?
Mike J

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Old 06-12-2011, 06:21 AM   #2
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My daily commute is at elevations between 6500' and 7000'. I generally get better fuel economy commuting than I do on road trips to lower elevations.

I also get better fuel economy in warmer weather than in colder weather.

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Old 06-14-2011, 02:43 AM   #3
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The reason you lose 10% in winter is not from the cooler weather it's from the winterized fuel. It's a known fact that you lose HP and MPG on winterized fuel.
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Old 06-14-2011, 04:10 AM   #4
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Talley, I will most definitely disagree.

Here we do not have summer/winter blends. Here it is always 100% gas. And YES temperature does have a very distinct effect on your MPG. See my Caravan, you can tell there is a difference between winter and summer months. In fact it will affect it way more than a blend difference with ethanol/gas.

Everyone likes to think that the blend is what makes the difference, when in fact its the temperature your engine operates in. Takes longer for the engine to come up to proper operating temps, fuel density is higher, air density is higher, people drive their cars differently in the winter, tend to leave them running, or start them and let them warm up, etc.

Basically take that 10 percent, and about 1 percent may be due to blends in all truthfulness.

I'm willing to bet, that if you took two identical cars, drove them both the same way, drove one with only winter blend, and one with only summer blend (and drove them at say a neutral temperature) they would both be about the same MPG. The blend is more about making sure the mixture ratio stays constant (as the temperature changes the density of gas) than anything else.
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Old 06-15-2011, 12:07 AM   #5
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Absolutely not true. Temperature does make a difference, but the blend makes a much bigger difference depending on the actual blends used.

I don't remember where I left the actual numbers, but from Sept 2009 to April or May 2010 we never had a tank above 30mpg, averaging somewhere around 28.5. Then, one tank in spring, we hit 30mpg and never had a tank below 30mpg until the following September, averaging almost 32mpg for that period. The temperature was basically constant that year from March until late June (mid-60s and often rainy, summer did not start until the first week of July here). But the MPG change started at one tank and never went back -- and it was greater than a 10% jump. That is not explainable by the temperature, since the temperature did not change (in fact it was warmer in March 2010 than any point between April and June). Moreover, those dates corresponded (within a couple of weeks) to the legally mandated gas blend switch points.

I don't know exactly where you are DTMAce, but Michigan most certainly has summer and winter blends just like every other state. They just do not mandate the use of ethanol. They still have modifiers and additives that reduce the evaporative emissions.

10% increase in air density does NOT make a 10% increase in mileage. Air drag is not the only drag you are fighting.
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Old 06-19-2011, 07:04 AM   #6
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I'm going to take this from a different perspective, as a pilot I can tell you that at higher altitudes in gasoline powered reciprocting engine airplanes you definately get much better fuel economy at altitude than you do at sea level. Airplanes have a fuel mixture lever to control how rich or lean the mixture of gasoline is and a properly leaned mixture at altitude increases fuel economy dramatically. However, that has less to do with air density, which while a factor is one of many forms of drag involved in moving a solid body through a gas or liquid. What is the largest factor here is the thinner air in a non turbo or super charged engine is less dense and therefore the engine produces less horsepower, less horsepower production reduces fuel consumption and less fuel consumption at a similar speed equates to greater economy. In an airplane lower air density does work out to slightly higher speeds for the same horsepower output in straight and level flight so the airspeed stays fairly close inspite of the reduced fuel consumption, but there is some penalty in airspeed over full horsepower available altitudes. The greatest fuel economy versus airspeed altitude is theoretically at the highest altitude where the airplane still makes full cruise horsepower. Modern fuel injected automotive engines do this mixture adjustment automatically via computer controls which sample the density of the air charge entering the fuel injection pump and then meter the fuel delivery at an appropriate rate to ensure maximum consumption. What that means at altitude is that the injection pump delivers less fuel because the air density is lower and a sea level fuel delivery would not combust completely in the combustion chamber.

Like BDC pointed out there are numerous other drag factors that you're fighting here, primarily rolling resistance from the tires against the pavement and don't forget that drag increases with the square of speed so a 10 MPH increase in speed works out to a drag increase of 100.

Finally, you can go back into my history for my 2000 Acura Integra and see exactly what tank the summer blend started in Southern California. I suffered a 20% decrease in fuel economy prior to the MTBE ban in CA when ethanol would begin being added to the so called Summer Gas due to the lower BTU content of the oxygenated gasolines, alcohol just doesn't have the energy content of gasoline. Since MTBE was banned the difference isn't nearly as noticable since the gasoline is 10% ethanol year round now, but back in the day my commute mileage would drop from 34-35 MPG every tank down to 29-30 literally overnight.
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Old 06-21-2011, 05:16 AM   #7
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@bates Exponential increase in drag does not mean that

it increases 100-fold when the speed is increased 10 mph. It means that if the speed is doubled, the drag will quadruple. Going from 50mph to 60mph (20% increase) would increase the drag 44% (1.20^2 = 1.44). This also means that going 250mph results in about 25 times more drag as when going 50mph. No wonder cars need over 1000hp to go that fast.

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