Why is my MPG all over the place on my 04FXT?! - Subaru Forester Owners Forum
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post #1 of 26 (permalink) Old 02-17-2012, 06:02 PM Thread Starter
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Exclamation Why is my MPG all over the place on my 04FXT?!

I calculate every time i fill the tank or top off, and the mpg varies from 16mpg all the way to 29mpg. What the hell is going on? It's not a city or highway issue either. I'm just looking for a little help. Haven't had it proffesionally tuned yet...will happen soon.

me: 04 FXT 5MT :icon_cool:
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post #2 of 26 (permalink) Old 02-18-2012, 03:59 AM
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One thing that you can do is eliminate the variables that result from topping off. Only do fill ups after some significant consumption or distance, like 10 gallons or 200 miles, and add no more after the pump shuts itself off.

The calculations on a top-off can vary greatly and do not give a true view of mileage. The few gallons added in a top-up are influenced little variations in how far the fuel splashed up in the filler neck to shut the pump off, or whether you added another click of fuel. Topping off can result in an under fill or an over fill.

However, the variations due to topping-off do cancel or average out. If you under fill the tank at one fill up, the tank will require extra gas at the next fill up. The good mileage of the first fill up will be balanced by poorer mileage of the second fill up.

Although your mileage may appear to be varying wildly on individual fill ups and top offs, the overall average of a half-dozen tanks will be more accurate and acceptable.
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post #3 of 26 (permalink) Old 02-18-2012, 04:19 PM Thread Starter
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thank you. very helpful advice.

me: 04 FXT 5MT :icon_cool:
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post #4 of 26 (permalink) Old 02-18-2012, 06:53 PM
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I have noticed that unlike my other cars on the Forester the pump shut off is very random and often very short of a "Full" tank. I have had to result to manually filling it to the very top to make sure I put the same amount in each time. I have noticed over a gallon difference sometimes when the pump automatically shuts off and when I manually shut it off.
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post #5 of 26 (permalink) Old 02-19-2012, 06:10 AM
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My owner's manual specifically cautions against adding fuel after the automatic stop mechanism on the pump activates. The reason given is that "temperature changes or other conditions may cause the fuel to overflow from the tank...". It seems to me that I have read an additional reason on this forum not to do so.. but I do not recall what it was.

We all know that you can usually add a considerable amount of fuel after the automatic cutoff activates. I just don't understand why one would feel compelled to do so.

2010 Forester Premium Paprika Red Pearl
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post #6 of 26 (permalink) Old 02-19-2012, 07:12 AM
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How much fuel you add is irrelevant to calculating mileage.

Are you always filling up with same fuel? I usually stick to Chevron only. 29MPG is borderline unheard of on an '04 XT, 16MPG is pretty normal.

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post #7 of 26 (permalink) Old 02-20-2012, 07:45 AM
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Quote:
Originally Posted by SierraHotel View Post
My owner's manual specifically cautions against adding fuel after the automatic stop mechanism on the pump activates. The reason given is that "temperature changes or other conditions may cause the fuel to overflow from the tank...". It seems to me that I have read an additional reason on this forum not to do so.. but I do not recall what it was.

We all know that you can usually add a considerable amount of fuel after the automatic cutoff activates. I just don't understand why one would feel compelled to do so.
^ This. also, if you over-fill the tank the extra fuel will just be routed to the charcoal canister and is basically wasted...

always stop filling up when the auto-shutoff occurs. if the auto-shutoff seems inconsistent you should check the vent line of the filler neck, it maybe restricted.
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post #8 of 26 (permalink) Old 02-20-2012, 01:04 PM
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Originally Posted by stan_t View Post
How much fuel you add is irrelevant to calculating mileage....
The amount of fuel put in is relevant to calculating mileage.

For any given miles, the more fuel you put in, the lower the mileage will calculate for that fill up.

And the less fuel you put in, the higher the mileage will calculate for that fill up.
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post #9 of 26 (permalink) Old 02-20-2012, 02:43 PM
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I didn't word this correctly (actually, I did, but you mis-understood) -- the total amount of fuel you put in this time vs. last time the car was filled up is irrelevant to calculating mileage. You can fill up with 9 gallons this time vs. 11 gallons last time -- you are still dividing the number of miles traveled on each "trip" by either 9 or 11. Clicking the pump multiple times to get every bit of fuel in there is pointless and will damage the system, that's what I was trying to get to.

Stan
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post #10 of 26 (permalink) Old 02-20-2012, 04:43 PM
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Originally Posted by stan_t View Post
I didn't word this correctly (actually, I did, but you mis-understood)...
I took the words for what they said, could not believe you in particular had said that, and figured everybody would have understood it the same.
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post #11 of 26 (permalink) Old 02-20-2012, 05:01 PM
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Bad wording on my part, sorry. Curious to hear how OP is calculating mileage, if OP is always using a specific amount of fuel for calculation (like, maximum tank capacity), that may explain the difference.

Stan
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post #12 of 26 (permalink) Old 02-21-2012, 03:35 PM
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I concur that adding fuel after the auto pump stop is normally pointless. I will check my vent line to see if there are any issue there.

My normal average is between 18-21. I have had as high as 26 and as low as 16. My 26 was when I put in too little gas and the 16 was after putting in too much.

I bet on a long cruise at 55 or below that 29mpg would be achieveable. However normal stop and go with highway, not so much.
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post #13 of 26 (permalink) Old 02-21-2012, 05:46 PM
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Originally Posted by InGroundEffect View Post
... I bet on a long cruise at 55 or below that 29mpg would be achievable...
That is achievable at a faster speed. I have a 2008 automatic On a round trip to Maine from Kentucky, not exceeding 65 mph, including 2-lanes across Vermont and New Hampshire, it went 2,563 miles in 2 weeks using 87.0 gallons for an average of 29.5 mpg. In August with AC all the time.
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post #14 of 26 (permalink) Old 02-21-2012, 05:57 PM
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Quote:
Originally Posted by InGroundEffect View Post
I have noticed that unlike my other cars on the Forester the pump shut off is very random and often very short of a "Full" tank. I have had to result to manually filling it to the very top to make sure I put the same amount in each time....
Odd. My 2007 Forester is very consistent about fuel pump shut off. I don't even think the car has any variable effect on when a pump shuts off. Any variations would be caused by the sensors of the different pumps.
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Originally Posted by InGroundEffect View Post
... I have noticed over a gallon difference sometimes when the pump automatically shuts off and when I manually shut it off.
Of course. And why would you want to do that. The extra gallon you force in comes way up in the filler neck and can go down the vent hose into the charcoal canister.
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post #15 of 26 (permalink) Old 02-21-2012, 06:16 PM
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I get like 16-24 on my 04. (city/ highway)

04 XT in silver (gone, sold but not forgotten)
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