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Old 01-21-2012, 05:04 AM   #1 (permalink)
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A new idea for better FE at the pump - literally

This won't be for everyone but I suspect it will be effective for me. It's really a simple way to decrease weight by carrying less fuel but still have a way to accurately compare tank to tank data. When I first started hypermiling I was so excited to see where I was at, I kept filling it up when I reached a half tank to see the results. The first time I drained the tank down to E and filled up I noted an increase in FE and I attributed this to driving around with less fuel. Since then I've hypothesized that you could get better FE by only filling the tank half full - but the problem with that is that you can't get accurate mileage results tank to tank because without a fill constant you wouldn't know how many gallons you actually used.
I theorized that you could somehow modify the fuel tank to make it smaller but I saw two problems with this: One, I don't know how to do that, and 2 it would limit your range which would be lame for road trips (which I take semi-regularly).

It's funny but this has been one of those brain teasers for me, I really thought there would be a way to do it so I never forgot about it but i never really came up with anything either. UNTIL RECENTLY!

On a road trip return leg I was in Phx with about 40 miles left on the tank. I know that if I get gas in Gila Bend I will have just enough gas to get back to San Diego and it means on the very strenuous hill climb (which is a lot of work for a 3 cylinder car full of 3.3 people and luggage) I will be on my last gallon of gas and therefore save about 56 gallons (assuming gas weighs 8 pounds/gallon like water does). The problem was that Gila Bend is 50 miles from Phx. If I got gas in Phx I would have to fill up before the hill climb and carry an extra 50 lbs. That's when I finally figured it out ! In Phx I stopped to get gas and I put in EXACTLY 1.000 gallons and did not do a fillup in the SG and did NOT reset the trip odometer. This got me safely to Gila Bend, where I filled the tank to full as usual but when I calculated MPGs I simply did odometer miles divided by Gallons in Gila Bend + Gallons in Phx (1.000).

What I realized is that any time I want I could fill the tank partially full, note the actual gallons I put in the tank, and so long as I don't reset the odometer or calculate mileage until the next proper fillup (all the way) then make sure when I calculate MPGs I add the actual number of gallons used in both the partial fill and the top-off fill: My FE will be real world accurate and I will effectively carry less weight.

This wont' be for everyone. You'll have more limited range, have to stop for gas more often, and get less chances to log your fuel (which, let's be honest is pretty addictive)... but It should be very effective on cars like mine where every pound seems to matter. If you wanted to be really crazy you could basically try to keep your tank super low by only adding a gallon for a few refuels in a row, then maybe monthly filling it all the way to check your FE. The more partial fills the more effective but also the more annoying. I've decided to try it for a while adding only 4 gallons when E then doing a proper fillup on the next tank.

What do you guys think? Anything I'm not seeing that makes this a terrible idea?

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Old 01-21-2012, 05:16 AM   #2 (permalink)
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Math is not my strong suit, does the following sound correct more or less?
Let's say my range is 400 miles on my 8 gallon tank (which it often is).
Let's say gas weighs roughly 8# per gallon.
If I do 400 miles I would imagine I carry on average 4 gallons or 32# of fuel.
Now lets say I do a half fillup.
This would allow me another 200 miles carrying an average of 2 gallons or 16# of fuel.
For the combined trip I would have been carrying 32#s 66% of the time and 16#s 34% of the time.
By my calculations that's 600 miles carrying an average of 26.5#s of fuel.
32-26.5=5.5#s saved. Not the most impressive ever, but think about bigger tanks, or if you did this for an extended period of time with multiple one or two gallon fills. Food for thought...
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Old 01-21-2012, 05:36 AM   #3 (permalink)
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Instead of guessing how much gas weighs, why don't you google it?

In addition, I know I've posted about "gas tank games" before... are you figuring in the extra stops, extra un-cappings of the tank, extra engine starts, extra accelerations and brakings, and general extra farting around by stopping for gas in small quantities?
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Old 01-21-2012, 06:49 AM   #4 (permalink)
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I had a similar idea, but with jerry cans.

U'd fill them up when you filled your car at the pump, then u'd take them out of the car when you got home. Now you could keep filling your car the minimum without making extra stops.

But yeah..
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Old 01-21-2012, 07:42 PM   #5 (permalink)
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jakobnev - that's a great idea. This would eliminate the fuel waste of pulling into gas stations more often (though I usually engine-off coast in anyway). I don't think I would actually do it because of storage issues, but it is a really good solution to the mind puzzle of what would be the best way to commute with the least weight in fuel and the least inconvenience.
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Old 01-22-2012, 01:17 PM   #6 (permalink)
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Well, i suppose if you could be bothered, you could make a sealed transfer system, and eliminate evap and dripping losses too.
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Old 01-22-2012, 10:00 PM   #7 (permalink)
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Isn't this why you buy a scanguage or mpguino?

I'm all for trying to monitor mileage, but, when it results in wasting time and fuel, it gets kind of silly. Reminds me of the TDI guys and their ridiculous (IMO) ventectomies. never could figure out why someone trying to get better mileage would want to mod their vehicle to be heavier and risk fuel spillage.
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Old 01-23-2012, 03:15 PM   #8 (permalink)
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You could also drive until the reserve light comes on solidly, then always fill up ASAP with the same volume of fuel.

You won't have a tank average, but you can monitor your miles-to-reserve-on-X-gallons and try to better that number.

You can make it more accurate of keeping track of how many miles you do before filling up, and adding them to the next miles-to-reserve.


That said, the weight savings of half a tank won't matter very much when there's 3 people and their luggage in the car.
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Old 01-25-2012, 04:56 AM   #9 (permalink)
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For venctonomy. Extra 10 liters or 8.4 kg in the tank increases fuel consumption about 0.3% to 0.6%. Difference in diesel price between various stations can be easily over 10% at least in Finland. Calculate which saves more if you can drive over 200 km more and the cheapest station happens to be on your way... So you are able to get to the cheapest station more often than with "smaller" tank.

If the cheapest station is next to you it does not help so much, but even then you can do longer trips without the need to fill in at those more expensive pumps.

And yes Geo Metropolis I think your theory works just fine.
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Old 01-25-2012, 01:20 PM   #10 (permalink)
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Quote:
Originally Posted by euromodder View Post
You could also drive until the reserve light comes on solidly, then always fill up ASAP with the same volume of fuel.

You won't have a tank average, but you can monitor your miles-to-reserve-on-X-gallons and try to better that number.

You can make it more accurate of keeping track of how many miles you do before filling up, and adding them to the next miles-to-reserve.


That said, the weight savings of half a tank won't matter very much when there's 3 people and their luggage in the car.
Of the 4 cars I've driven regularly enough to have an idea of when the low/reserve gas light comes on, none of them have been anywhere near reliable. one of them (admittedly an old lug of a car) has a 2+ gallon variation in terms of when it comes on. It's also heavily influenced by the angle the car is on. Sort of the same flaws as logging by analog gas gauge, really.

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