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Old 10-05-2014, 09:59 AM   #61
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Just had an appropriate air density v speed test. Returning from the mancave last night, a section of a long downhill from 2750 feet to around 1300. Typically my pickup truck gets well past 75 in third gear foot off the gas pedal in temperatures of 65 to 80F.

The ambient temp was 37F at the top of the hill, and 43F at the house at 7 PM.

The truck attained max 67 MPH. Same body configuration, open bed, closed wndows, dirty as usual, tire pressure at 55 PSI, etc.. I typically brake around 65 and get it down to 55 several times, unless...... I am testing.

Edit add: This test excluded anything the engine does, strictly gravity, air density and speed. Was not a test for MPG.

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Old 10-05-2014, 10:11 AM   #62
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All else being equal (except *), we get our best fuel efficiency when it's moderate-warm outside (about 70F), like late Spring and early Fall. MPG falls when it gets really hot, but I am sure that's because AC * kicks in more and more. And as noted earlier, MPG falls off a cliff in winter, the lower the ambient air temps, the worse our fuel efficiency gets.

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Old 10-05-2014, 10:25 AM   #63
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Lotsa variables.

I'm pretty sure that an ICE is more efficient with colder air. Colder air is more dense, and therefore contains more oxygen. Engine like.

But in cold temperatures, an engine is harder to turn over, and takes longer to warm up and is less efficient until it is at normal operating temperatures. Plus tires have increased rolling resistance when cold (the rubber is stiffer), tire pressure will be lower, further increasing rolling resistance. All the lubricants are stiffer. The alternator has to make up the increased battery drain from harder starts. You might even be slipping and pushing through snow.

And... fuel mixtures change seasonally, and as mentioned, AC in summer versus heating.

My guess is that with extended highway driving, cold weather will give marginally better mpg. But the starts/stops that most of experience will mean lower overall mpg in winter.

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Old 10-05-2014, 10:35 AM   #64
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I don't drive much in wide-open spaces out West, but here in my region (Midwest) one can rarely drive a CONSISTENT 75+mph anymore. There are always clusters of vehicles driving slower, so real-world mpg penalty of trying to drive that fast is greater (from repeated accelerations vs steady speed) and real-world benefit is less.
On same highway trip between 2 big Midwest cities I've noticed no big net time savings trying to drive much over posted speed limits. The same cars you just passed a few miles back catch right up to you again as you reach the next rolling traffic jam (like big trucks matching speeds in both lanes ). So these days I just set the cruise at the posted speed limit & save (most) of the aggravation of fighting the slower traffic- and watching for speed traps.
BTW- on some length trips the better mpg's of driving slower & smoother can actually SAVE net time. On one of my mid-length (~350mi) routes I can drive ~65mph ave & easily make it straight through on one tank. Trying to drive 75+ knocks down the mpg's by ~20%(+) and forces a pit stop for gas. As any NASCAR fan knows, sometimes conserving fuel to avoid another pit stop can win you the checkered flag

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