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Monday, June 6, 2011

Gas Mileage Observation: Fill up at 1-4 gallons for best mileage

On the same trip (6/5, from Roanoke to DC Area), I noted a phenomenon that I've been noticing for some time now.  Any time I fill up and need four gallons or less, I get nearly phenomenol gas mileage.  E.g. I've managed 120 miles on less than 3 gallons in the past, for a gas mileage of 40 mpg.  But if I let the car use all the gas, I end up with an average closer to 30 mpg.  It's probably not worth the hassle to fill up every 100 miles, even though (at least for my car) that would save money and gas.

But let's do a quick calculation to check.  Let's say my numbers above are right.  If I travel 300 miles, filling up every 100 miles, and get 40 mpg by doing so, I would have used 7.5 gallons of gas.  If I filled up only once, at the end of the trip, I'd get 30 miles per gallon, and use 10 gallons.  So if my numbers are actually correct, that's a savings of 2.5 gallons of gas, which at today's prices would approach $10.

Hard data:  My figures for the latest commute are not quite that good.  At the exit 291 stop, I had gone 160 miles on a tank, and needed 4.4 gallons, for a mileage of about 37 mpg.  But I had been stuck in a half hour delay on the way, so I might have done better had the roads been clear.  The 48 miles I traveled from exit 291 on I-81 to exit 40 on I-66 cost me just over 1.2 gallons, which means I got just under 40 mpg for that leg.

But the more intriguing question is what is causing the disparity, and can cars be designed in such a way as to "simulate" a fuller tank for mileage purposes?  E.g. how is the pressure regulated in a gas tank?  Obviously, assuming the hole in the tank is in the bottom (it must be, right?), then there will be a greater pressure head on the downstream gas when the tank if full than when it is empty.  Does that suggest that there should be some way of artificially maintaining the pressure for the duration of the tank?  If it really saves 2.5 gallons out of every 10, that would be something worth investigating.

Note that if I am right, the gas mileage on the second half of the tank is truly pathetic.  If I am really getting 40 miles per gallon on the first half, then that would mean I'm only getting 20 mpg on the second half, to give me that average of 30.  I haven't checked the long-run average for a while, so that would probably be worth doing before going much farther down this line of thought.  Still, I know for a fact that I've never broken 35 mpg (and I don't recall ever hitting 35 mpg) when I've been filling an empty tank, and yet I routinely approach 40 mpg when I fill up early.

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