“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Tuesday, May 16, 2006


I'm having trouble now deciding if I'm in the midst of an experiment or just trying to save all the gas possible. If it's the former there are several things I'd like to try, but I'm loathe to see the "average mileage" display creep down as I test hypotheses.

What hypotheses? The least likely to reduce my mileage has to do with fuel in the tank. I've always heard gas mileage is better during the first half of a full tank. The fuel gauges of all the vehicles I've owned act this way, but I suspect that that behavior is an artifact of the gauging mechanism. After all, half a tank weighs less than a full tank, and though the air doesn't know how much the car weighs so drag won't change, it does take more fuel to accelerate a heavier car. Further, increased weight adds to the tire loading, thus increasing road loads. I suppose it's possible that a larger hydraulic head in a full tank could somehow improve pumping efficiency but it seems far-fetched.

I'm a pilot, and I know from that avocation that fuel weighs about 6 pounds/gallon, so a full tank (21.5 gallons) in my car weighs about 129 pounds, the weight of an extra adolescent or perhaps female passenger. So it stands to reason that mileage would improve as fuel is used.

Finding out would take quite a while, filling and emptying to half a tank is not as exact as topping off to measure fuel usage. I'd fill to full, drive until I get as close as possible to, say, 5/8 full and refuel. I'd do this maybe 15 times, and calculate the mileage at each fill up. Then I'd drive down to 1/8 tank, add fuel to bring it to 1/2 full, drive down to 1/8, repeat, etc. Again, I'd do it maybe 15 times and compare. It wouldn't be exact because of the difficulty of filling and reading to exact level using the gauge but after sufficient trials, a conclusion should be possible.

Or perhaps the best way to measure this, since full tank is the easiest level to which to accurately fill, would be to run a series of trials, always filling the tank but alternating between using about 1/2 of a tank and close to a full tank. This way the accuracy of the miles per gallon achieved would be maximized, but the difference in the results would be minimized. Well, if I do the experiment I'll put a lot more thought into its design.

What else? I've achieved what I regard as a drastic reduction in fuel consumption by taking many different steps, as detailed elsewhere in this blog. Which of the measures are most effective and which are minor? Is it the slow acceleration that contributes the most, or perhaps the reduced freeway speed? The way to find out is to eliminate each measure I've taken, one at a time, and measure the result. Unfortunately, at least one or two of the experiments would likely lead to my having to watch the "average mileage" indicator show significantly deteriorating results, and I just hate to give back my hard won tenths of a mile per gallon.

Maybe I AM eccentric.


Anonymous said...

Well presented.. I think you should run your vehicle completely out of gas and then watch the fuel pump to refill with precisley the amount of fuel to test.

King of the Road said...

I guess I could do that. It would certainly give me more precise result, but I'd have to drive around with a 5 gallon jerry can or something. I'd also need to be very careful about where I ran out of gas. Further, I suspect the very VERY last drops of fuel would lead to anomalous data.