Saving Money on Gas: Fill the Tank or Half Full?

German electric car, 1904, with the chauffeur ...
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Let’s start by saying that there are evil,evil people who are speculating on gas prices to drive up all of our costs.  And where’s my electric car already? 

Whew, now I feel a little better.  How do I maximize my fuel efficiency?  AAA has a nice booklet which has a bunch of tips, including maintaining tire air pressure.  But I had a specific question:  should we be filling our cars to full or just to half full tanks

I’ve heard that half-full gasoline tanks can have vapor leaving constantly, making your gas mileage worse.  But everyone also says to lighten your load (sorry uncle Frank, no more joyriding for you until you hit the gym) so if I fill my tank all the way aren’t I giving myself the equivalent of six to twelve gallons of liquid to tote around?  Wouldn’t it be more fuel efficient to almost be running on empty? 

So I started checking around for gas tips like this one, or the 29 tips guy.  29 tips guy says that I shouldn’t fill up until I’m on empty, but he doesn’t say anything about filling only half full to save with my drag coefficients.  This is an oversight, especially since he recommends “drafting” or following closely behind other vehicles (and here I just thought they were being incredibly impatient and rude.  Next time I’ll just smile and wave.  Maybe I’ll even return the favor when they pass;)

Evidently, we are all better off roasting, because both the AC and open windows destroy gas mileage.

I looked forever for a definitive answer, but no expert seems willing to go out on a limb on this one because cars are all different, etc.  I did read a nice point that you’ll probably eat up whatever you save by having to go refill more often. 

I finally found the answer I liked, on a Lexus forum of all places.  Here it is and a big congratulations to KT22CLIFF for taking the plunge:  “No need for a math genius. Gallon of gasoline weigh basically 6 lbs. So if you were to drive with 10 gallon less gasoline than a full tank – that’s less than 1/2 tank – you will save 60 lbs. About 1.67% in a 3600 lbs car”  So that weight gives less than the 2% saved by keeping your tires inflated (where’s my pressure reader?) and WAY less than the 15% saved by not using your AC.  (One more reason why Maine rocks!  Shhh!  We’re not talking about the heater.  Lalala, I can’t hear you.)

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