Filling Up Full Tank Adds Unnecessary Weight Thus Reduces Mpg. - Fuel Economy, Hypermiling, EcoModding News and Forum

dsq · 05-22-2008, 04:09 PM

GAS weighs 8lbs a gallon.
Weight reduces gas mileage.
So i would think it is best to drive with as little gas in the tank as possible.
LEts say you have a 12 gallon tank and fill up to full tank.That fuel weighs 96lbs...If you fill up with only 4 gallons youve just reduced your weight by 64lbs.
Thoughts?

ihatejoefitz · 05-22-2008, 04:22 PM

There is a point of diminishing returns because of the increased trips to the gas station.

MetroMPG · 05-22-2008, 04:52 PM

You could fill up, then siphon most of it out and keep it in jerry cans, adding it back to the tank as needed.

Sure, weight savings are good. The impact of removing 64 lbs will be very, very small (too small to detect in your fuel consumption log), but it's real.

Also, I thought gas was closer to 6 lbs per US gallon...

PaleMelanesian · 05-22-2008, 05:06 PM

Some of you have seen this before, or an earlier copy of it.

This is my daily commute, over the last 1.5 years. 500+ data points.

125% (above full on the gauge) and 1% (empty / low fuel light) both have 1/3 as many data points as the others.

My car stumbles / sputters a bit when it gets low, but still has fuel. I haven't run it dry yet. I may need to fix something there.

I have an 11 gallon tank.

Q3 is the 75th percentile, or halfway from average to max.
Q1 is the 25th, or halfway from average to min.

The linear portion (25%-125%) of the mean chart calculates out to about 0.5% decrease per gallon added.

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dsq · 05-22-2008, 05:10 PM

yes you are right...it does weigh aprox 6lbs.
I thought it was the same as water.

http://www.santacruzpl.org/readyref/...gasoline.shtml

igo · 05-22-2008, 05:25 PM

That is a really interesting graph. You average over a 2 mpg increase riding with a 1/4 tank vs. a full tank. It makes sense but, it is cool to see a graph of the mpg.

Just seeing that graph makes me want to do some weight reduction.

ihatejoefitz · 05-22-2008, 06:03 PM

What can account for the drop from 25% -> 1% ?

PaleMelanesian · 05-22-2008, 06:15 PM

Updated the post above, but:
My car sputters and really doesn't want to start/restart when it's low. I do a lot more engine-on coasting and leave it running at stops instead of shutting down. There's still fuel available, but there's something it doesn't like. It's probably an issue with my car.

MetroMPG · 05-22-2008, 10:17 PM

Quote:

Originally Posted by PaleMelanesian

Some of you have seen this before, or an earlier copy of it.

I hadn't - thanks for reposting.

Quote:

The linear portion (25%-125%) of the mean chart calculates out to about 0.5% decrease per gallon added.

Any theory on why you're seeing such a large gain for such a small weight difference?

PaleMelanesian · 05-23-2008, 12:06 AM

I don't know. It shouldn't be that large a difference, but there it is. My strategy now is to run it low, but not quite to empty.

I did notice after filling up this morning, my glide points were off - rolling farther from the same starting speed. I do LOTS of P&G, so maybe the weight difference is more relevant there.

I like your idea of siphoning out gallons and then adding them back later.