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Old 08-23-2014, 11:49 PM   #1 (permalink)
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Default Fill up often or run it dry

Lately I've noticed that my MPG is high for the first half tank of gas and then the last tank drags down the MPG quite a bit. I've tested this with several fill-ups. It has been hot lately in southern California so just wondering if an empty tank tends to have more gas vapors by volume on a hot day and maybe all that vapor is leaking out somehow??

I know a lot of people say carrying a full tank just adds more weight to the car which hurts the MPG, but my experience has been the opposite.

Anyone else noticing the same thing?
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Old 08-24-2014, 12:37 AM   #2 (permalink)
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its just the gauge. If you really watch it youll see a trend as to when it "moves". For me, my first light doesnt go off till about 150miles or more. By a 1/2 tank, i usually am at 400-500miles, then it falls fast. Each car is a tad diff on the amount/miles, but I think the same trend exists for all.
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Old 08-24-2014, 02:57 AM   #3 (permalink)
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Doesn't seem like OP's talking about the fuel gauge, but rather the actual mpg on the FCD... Not than I have anything to add about that...
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Old 08-24-2014, 07:17 AM   #4 (permalink)
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I log my mpg each day, to work, and home. I can see no difference full or empty tank.

I can get a lot more miles from full to half tank on the gauge, then a lot less miles from half tank to empty.
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Old 08-24-2014, 12:08 PM   #5 (permalink)
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It's more from calculating it during fill-up. If i fill up around half tank, the FCD numbers are pretty close to actual. But if I run it till dry, the FCD numbers are way off, anywhere from 5-10MPG.

It was either the last fill-up or the one before, but I noticed that I had about 3 bars left when I went to work. Parked it in the sun. Came back 6 hours later and it had 2 bars. Drove 8.6 miles home. Parked it in the sun. Drove to dinner that night (~12 miles) and it had 1 bar left when I got there. Empty gauge came on. mpg difference from FCD was off by like 8mpg.

I don't know if the tank is expelling gas vapor somewhere or if there's something wrong with my tank, but the mpg numbers seems to be further from actual on the hotter weeks.
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Old 08-24-2014, 11:14 PM   #6 (permalink)
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Could it be that the temperatures cause the gas and/or tank to expand some giving off faulty readings?
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Old 08-24-2014, 11:47 PM   #7 (permalink)
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Quote:
Originally Posted by Need4Speed View Post
I log my mpg each day, to work, and home. I can see no difference full or empty tank.

I can get a lot more miles from full to half tank on the gauge, then a lot less miles from half tank to empty.
same here. Very consistent.
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Old 08-25-2014, 03:12 AM   #8 (permalink)
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Quote:
Originally Posted by Insightlover2011 View Post
Could it be that the temperatures cause the gas and/or tank to expand some giving off faulty readings?
If this were possible it still wouldn't explain what OP's dealing with. He's basically recording fuel consumption using two methods - the FCD and calculations based on fill-ups. He says that when he fills up from half tank the two fuel consumption measurements coincide; when he fills up from empty, one or the other - I think the fill-up based reading - is 5-10 mpg lower than what's reported on the FCD...

Like others, I record fuel consumption with fill-ups and FCD and both of these usually are the same, like within 0-2 mpg difference... But I almost always fill-up from near empty...
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Old 08-25-2014, 03:19 AM   #9 (permalink)
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Wondering if OP is 'topping' the tank at fill up. I recall reading (and experienced myself) false high mpg on the fcd due to topping, allowing fuel into the evap canister. I no longer top off, and have very consistent measurements.
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Old 08-25-2014, 03:29 AM   #10 (permalink)
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That crossed my mind, but even if he were it wouldn't make the difference that he's reporting...
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