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Old 09-03-2010, 05:53 AM   #1 (permalink)
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outside air temperature and mpg

I have been logging the fuel economy and outside air temperature for a month or so; and thought I'd try to make sense of it. There's a nice trend, a linear fit gives mpg = (0.4*temperature) +70. Nice mix of imperial mpg and degrees centigrade - that's just what we use in the UK!

I found some figures for the "dynamic viscosity" of air and so compared figures for 10 degrees C and 20 degrees C.
Air - Absolute and Kinematic Viscosity

The effect on economy is 6% (78.5 and 74mpg (imp) at 20C and 10C respectively) ... and the viscosity changes by 5.3% (3.44 and 3.5).

Convincing - provided I've chosen the right type of viscosity...

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Old 09-03-2010, 10:17 AM   #2 (permalink)
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The benefits are more than just aero. There's faster warmup times, warmer tires, hotter air charge, and less frequent rain.

It is not necessary to concern yourself with viscosity. Aerodynamic drag varies linearly with air density. Without a datalogger, it's difficult to know how much energy you spend overcoming aerodynamic drag, versus inertia and rolling resistance, but you can use the Tools -> Calculator link at the top of the page to concoct an estimate for steady-state driving (zero inertial loads).
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Old 09-03-2010, 11:05 AM   #3 (permalink)
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I concur that there is a linear relationship between temperature and mpg. Here are my results. This is 3 years of daily commuting, so it's as consistent as the real world can be. mpg is averaged at each point.


11-mile commute: 100 mpg - - - Tank: 90.2 mpg / 1191 miles
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Old 09-05-2010, 09:36 PM   #4 (permalink)
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...FWIW, I have 8-months worth of AM (to work) and PM (to home) mpg-vs-temperature data showing similar results.

...there's a definite trendline difference:

• AM is cooler with slightly less traffic (at 0600 AM).

• PM is hotter with more traffic (at 1700-1800 PM).

...overall altitude difference is 220-feet over 26.2 miles, all freeway at 65 mph.
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Old 09-06-2010, 04:12 AM   #5 (permalink)
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ecoxantia -

I agree that IAT is an important component of MPG. Here is what I found :

ElmScan + Customized Scantool Software = data logging!
Ok, I now have a workable formula, thanks to post #9 :

Calculating MPG from VSS and MAF from OBD2 - MP3Car.com
... For the record, the first "one-line" MPG formula above, taken from my Circuit Cellar article, is off by 100! The "4.54" should in fact be "454". The correct formula is:

MPG = (14.7 * 6.17 * 454 * VSS * 0.621371) / (3600 * MAF / 100)
MPG = 710.7 * VSS / MAF

Note that OBD-II VSS reading is in kilometers/hour and MAF reading is grams/sec times 100.

This formula works very well in a modern automobile because the engine computer spends almost 100% of its time managing the fuel-air-ratio to 14.7, which it can do very well because of the "closed loop" feedback from the O2 sensor(s).

In fact, the accuracy of this method has been proven in literally tens of thousands of gasoline-powered vehicles. Accuracy within a few percent is typical, often limited by the accuracy of the vehicle speed reading (i.e., VSS).

As for other ways of doing this, especially if you don't have a MAF sensor, by knowing the displacement of the engine, and after a simple "calibration" using fuel tank "fill-up" data to find the only unknown, namely the "volumetric efficiency" (VE) of the engine, MAF can be calculated from RPM, MAP and IAT. With VE, one can use the following formulas to calculate a synthetic "mass air-flow" (MAF) in grams per second, all without a MAF sensor, using the "Ideal Gas Law", as follows:

MAF = (IMAP/120)*(VE/100)*(ED)*(MM)/(R)

where manifold absolute pressure (MAP) is in kPa, intake air temp (IAT) is in degrees Kelvin, R is 8.314 J/°K/mole and the average molecular mass of air (MM) is 28.97 g/mole. Note that, in the above formula, the volumetric efficiency of the (4-cycle!) engine is measured in percent and the engine displacement (ED) is in liters.

The VE of my 1999 7.4L Chevy Suburban is about 65%. Smaller, higher performance engines can have VE's of 85% or higher.


What's your EPA MPG? Go Here and find out!
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Old 11-10-2012, 12:07 PM   #6 (permalink)
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Originally Posted by PaleMelanesian View Post
I concur that there is a linear relationship between temperature and mpg. Here are my results. This is 3 years of daily commuting, so it's as consistent as the real world can be. mpg is averaged at each point.

I know this was from a while ago, but would you be able to post the data for your graph, in a spreadsheet or equivalent?

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