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Discussion Starter #1
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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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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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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Discussion Starter #5
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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Hypermiler
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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.
same here. Very consistent.
 

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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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Hypermiler
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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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Discussion Starter #11
Sorry if my post was confusing guys. eq1, you do a much better job at explaining my problem than I am. :) You are exactly right.

I agree with Need4Speed that the FCD numbers stay pretty consistent with a full tank as well as an empty tank. But the FCD numbers won't show a change if you're losing fuel by some other means. Consider this: if you had a full tank and then burned 1 gallon of fuel and got 70 miles, your FCD would show 70mpg. But if somehow the remaining 9 gallons leaked out from the tank, your FCD would still show 70mpg, but in reality you've gone 70 miles in 10 gallons of fuel, bringing your mpg to 7. The FCD only calculates the mileage change vs the amount of fuel passing through the fuel line.

It's only when I compare it to the fill-up MPG that I notice the difference and I consider the fill-up calculations more accurate. The fill-up calculations are coming up much lower than FCD when I drive till empty. No 'topping' either. Once the dispenser clicks off, I remove it. No second or third extra squeezes on the handle.

I don't know what to make of it except maybe I have a very tiny leak somewhere in the tank or EVAP unit and with less fuel in the tank, there's more volume of gas vapors available to leak out. But if there was a leak, it would have thrown a CEL, no??

The weather is still scorching here so just going to do a base test and fill up my tank as soon as it gets around half. See if I can get a consistent match with the FCD numbers. Maybe my FCD is just miscalculating things.

If anyone else is noticing the same thing, please report back with what the issue was. Thanks
 

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When I refuel my I2, most of the time there is a hiss as I release the fuel cap. This indicates a difference between tank pressure and atmospheric pressure. If this doesn't happen it might indicate a leak, allowing the pressure to equalise.
 

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Hypermiler
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randomfire above makes interesting point: fuel loss somewhere. Gas smell in the garage after parking overnight?

Drove the 'glider' to work today. Have 2 bars left on the fuel gauge, 48 mile trip home, PLENTY of gas. :D
 

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Hypermiler
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I noticed that when I fill the tank for the first two or so miles I get really, really good mpg, 30-50 mpg more than normal. Only lasts for a mile or two. I believe it is burning vapors.
Some time ago there was discussion on this. 'Topping' the tank was found to be a contributor with fuel spilled into the evap canister, and the car burning the vapors. Many reported unusually high mpg for the 1st few miles. I saw the same effect. I no longer 'top'.

For those who don't top, I don't know if the answer was ever found.
 

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MAY cause gas to accumulate in motor oil if consistantly "topping"

I found with my 2000 Insight. I stopped letting it click more than once, and voila, no fuel smell in motor oil anymore...
Fuel in oil dilutes the oil shear strength, and other important properties the oil needs to protect our engines from wear, at a minimum!

Especially since we use 0w20 (very thin) oil for economy!

ANother issue I noticed (and has been reported by MANY forums for electric fuel pumped cars/vehicles) is that fuel is used as a COOLANT for the fuel pumps. That is why the pumps are immersed and mounted in the bottom of fuel tanks in almost every vehicle made with electric fuel pumps nowdays...

Motorcycles are known, (especially yamahas have reported this with their bikes when run to reserve or run dry, pumps don't last but a few thousand hours!
ADV forums especially the WR 250R. Yamaha had a upgrade and factory "fix" for this issue when it first was noticed. Eventually the issue was found to be due to lack of cooling of the pumps, which was "killing" them from the excess heat generated and unable to be dissapated w/o sufficient fuel to do so.

We could have the issue with our in tank pumps as well... possibly..
 
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