Which is less wrong- gas gauge or gallons consumed?

[StoBro2]

I spent a few hours in my C-Max today doing a bunch of suburb-to-suburb driving. I had a lot of time sitting at traffic lights, so I punched up various screens in the left side display. I saw that my Trip 1 screen, which I reset at every fill-up, stated I had consumed 4 gallons. Meanwhile, my gas gauge was showing the tank exactly 3/4 full. Unless someone snuck into my garage and swapped my gas tank for one that holds 16 gallons, one or the other or maybe both of those displays is wrong.

 

So my question is for those who pay closer attention to these things than I have:

 

Which is more trustworthy?

 

I want to believe that the gallons consumed figure is more accurate, mainly because the same Trip 1 screen is showing that I am running at 53MPG so far on this tank. On the other hand, if I really only consumed about 3 gallons like the gas gauge suggests, my MPG would be around 75!! I guess I don't know what I want to believe.....

[Ryan McEachern]

Gas gauges aren't accurate like that. The drop downwards is not linear, and different parts of the gauge drop faster or slower, depending on the resistance in the float switch, and the shape of the tank. All my cars have done this. The worst one I have is my 1969 chevy truck, which drops all the way down to 1/4 tank left in the first half of the fuel used, then slowly drops the remaining quarter tank over the next half of fuel used.

[StoBro2]

I've lived with non-linear gas gauges before, but I've never been able to directly compare two different measurements coming from the vehicle at the same time. If the tank truly holds 13.5 gallons, I'll be sure to make another comparison when I get to 6.75 gallons consumed to see how far from 1/2 tank the gas gauge shows.

Edited July 6, 2013 by StoBro2

[ScubaDadMiami]

It seems that the last quarter of the tank goes down pretty quickly on the C-MAX, compared to the top part of the gauge.

[homestead]

It seems that the last quarter of the tank goes down pretty quickly on the C-MAX, compared to the top part of the gauge.

 

All my Fords have done that.

[GaryM]

I have to burn close to 2 gallons before the gauge falls below the full line, but it does have about 2 bars above full. I see about the same, 4 gallons burned is down to the 3/4 full line. I have been running to 11 gallons burned and it is then at about 1/8 tank on the gauge. The gallons burned on mine is about 7% off. On each fill it has taken a bit more gas than the computer said I used. My last tank which I topped up a bit early, said I only used 10 gallons, but it took 10.7 to top up. The trip 1 screen said I had gotten 45 mpg using 10 gallons to go 450 miles, but since I really used 10.7 gallons, the real MPG came out to just over 42 MPG which is still quite good. I am very curious how this next tank is going to come out. I am averaging 48 MPG so far with 2 round trips to work and a few small side trips. It was showing as high as 53 MPG in the first 1/4 tank. Using my same 7% error, I figure my real world MPG on this tank should be 44.8

[salsaguy]

you need to develop ONE filling technique and stick with it to be accurate

either stop at first click off at the pump, or stop and then add a little more to stop at second click off.

don't change your methods from tank to tank and always use the hand calculated value to put into fuelly.com (miles divided by gallons) not what the dash shows. you can put the dash info as a note in your Fuelly data entry.

Edited July 17, 2013 by salsaguy

[ScubaDadMiami]

I think a lot of it is the pumps at the gas stations.  Costco seems to maintain their pumps well, and I noticed that the fill, per the pump and per the fuel used according to the C-MAX, were virtually identical.  The gas gauge does not track perfectly.  I definitely notice that the last part of the tank drains quickly.

Edited July 17, 2013 by ScubaDadMiami