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Old 07-11-2015, 02:56 PM   #1
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Dash readout V's actual

I appreciate the average MPG dash readout is optimistic for almost every car out there, my car generally reads 2 to 3 MPG higher than the actual calculations. But I fuelled up a day or two ago and there was an almost 5 MPG difference. I checked all my figures and everything is correct, but I've never had such a big discrepancy like that. Any ideas why this is? Could the fuel pump have been over reading slightly? Any other ideas or suggestions?
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Old 07-11-2015, 10:31 PM   #2
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It's because you did not fill the tank to the same level on each consecutive fillup. Even if the dash were exactly 100.00% accurate, it would never match your calculation exactly because of the inherent inaccuracy in filling up to the same level each time. It's impossible to do.
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Old 07-12-2015, 12:38 AM   #3
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That's the thing, I do fill to exactly the same level. Even though it's not recommended by some, I carefully and slowly brim the tank until fuel is sitting about 2cm from the top of the cap. Ive always wanted as accurate figures as possible, so I try to be consistent. But this is the first time such a big difference has been noted.
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Old 07-12-2015, 01:02 AM   #4
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Very rarely is my car's calculation more than 1MPG over the actual. The few times it was actually higher I think it was due to a bad automatic shutoff on the pump making trigger happy. That was born out by the subsequent pump calculation.
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Old 07-12-2015, 07:15 AM   #5
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Quote:
Originally Posted by Draigflag View Post
I appreciate the average MPG dash readout is optimistic for almost every car out there, my car generally reads 2 to 3 MPG higher than the actual calculations. But I fuelled up a day or two ago and there was an almost 5 MPG difference. I checked all my figures and everything is correct, but I've never had such a big discrepancy like that. Any ideas why this is? Could the fuel pump have been over reading slightly? Any other ideas or suggestions?
Was the fill up with the high discrepency one of your better ones? If the gauge is off by a certain percent, better tanks will be off more.

Most of these gauges are monitoring injector pulse length for fuel consumed.
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Old 07-12-2015, 10:23 AM   #6
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No it was just an average tank. Funny though, its the second most amount of fuel I've put in apparently, which doesn't make sense as I filled up a day after the warning light came on, and I usually leave it much longer. Felt like I didn't let it run as low as I do most of the time, which leads me to believe the pump may have been over reading.
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Old 07-13-2015, 08:27 AM   #7
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Just filling to the brim isn't a guarantee of getting the same amount of fuel in. Wayne Gerdes of CleanMPG uses a jack to tilt the cars in order to make sure he lets all the air out of the tank. He got 10 gallons over the F150 rated tank size in.
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Old 07-13-2015, 09:29 AM   #8
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10 gallons, you mean 10 litres? I'm not going to that extreme ha! But I try and be consistent with my methods, I tend to drive with the warning light on as long as possible and when the gauge is as far down as it can go. Obviously garages are built on level ground so overall my method remains the same on every tank. That's why I'm a tad confused by the almost 10% difference. I guess it will average out, maybe on my next tank when I brim it, I might get less in and perhaps I'll understand what's going on.
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Old 07-14-2015, 07:22 AM   #9
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Nope, a full 10 gallons. 2015 Ford F-150 The 23 Gallon Tank That Holds 33+ Gallons - CleanMPG Forums
The engineers design fuel tanks with a head space to account for expansion of fuel, and it appears the difference between the standard gas tank on the F150 and the larger, extended range option is where the vent comes in. It enters at a lower level and you get a larger void in the filled tank.

Fuel station, and hopefully all, construction aims for level, but climate conditions at time of cement curing and settling afterwards means they won't be perfect in the real world. Then there is having inclines for rain drainage to consider.
So the only way to be sure of getting the same level of fuel, without using a jack, is to only fill at the same station and same pump.
Another thing to consider is the temperature of the fuel. Here, whole sale fuel deliveries take temperature into account to adjust the volume. If the station recently got a delivery, the fuel may not have completely cooled off in the underground tank yet.
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Old 07-14-2015, 09:34 AM   #10
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Jeez there's so many things that could affect it. I'm just thinking back, when fuelling up, I noticed the fuel seemed very frothy with lots of bubbles in it, maybe there was air in the fuel? I'm going to stop thinking about it now and juts accept it, I'll see what happens on my next fuel up next month.
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