Dash readout V's actual

[Draigflag]

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?

[BDC]

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.

[Draigflag]

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.

[gundamit]

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.

[trollbait]

Originally Posted by Draigflag

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.

[Draigflag]

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.

[trollbait]

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.

[Jay2TheRescue]

Originally Posted by trollbait

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.

I've been with Wayne on drives. Being 6'4" and 240 lbs I always seem to be bouncing on the back bumpers to get more fuel in.

[luckypants]

The computer MPG on my car has been 7.5% optimistic for my entire ownership (5500 miles so far) but I have seen some 'blips' as high as 12% out. I put this down to gremlins in the recording system. My MPG via fuelly is pretty consistent, so I'm happy its not more serious than a gremlin. I'm keeping tabs on the average discrepancy as it is possible to enter a correction factor into my computer, to make it read closer to real measured figures.

[Draigflag]

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.