Hi, i've only had my mini since last Oct. and so far so good. But 2 weeks ago I filled up and then watched my little bars and I swear I drove a whole week and only one bar went dark. It had never done that before. And I noticed the number of miles I had left had hardly gone down. Well I had started driving a tad slower to save gas, but not that much slower. After driving for 7 days I filled up again and noted the gauge said something around 567 miles to drive with a full tank. On the way home I watched those numbers. Not only did they hardly go down, they would go up and down. 567, 568, 567, 566, 567, 568, 569, up and down, up and down.

Is there a sensor messed up - is there anything I can do? I hate to have to take it in - it's a long ways to the dealer just for this.

I am keeping track of my mileage so I don't get too low on gas so I don't run out. Thanks...

 

IIRC, the miles-to-go indicator is based on your average mileage over your most recent 18 miles, and is not altered if you reset your average mileage value.

There is at least one sensor involved -- the fuel tank float -- which is supposed to float on the surface of your fuel. If it gets stuck in the "up" position while fuel is consumed beneath it ... then your tank would continue reporting "full" even while fuel is consumed.

Another sensor that must be involved is fuel-flow, from which, in combination with distance measurement provides instant and average mileage. Distance might be from odometer (historically read from outlet of transmission) or from axle.

 

Of the two indicators, your fuel gage should produce the most linear indication of actual consumption. Miles to go is a calculation based on multiple sensors as jggimi outlined. Although the feeders to the algorithm are unknown, you do have the singular report of the fuel gauge.

Question: When you filled up after driving for a week, how many gallons did you add to the the tank?

 

i didn't actually look at the gallons but with the price and I know it was $29 I figured up 8 gallons.

I appreciate your responses, butI don't understand anything of what either of you has said - all that lingo.

 

OK, I'll try and explain more completely.

Your fuel gauge on the dash gets it's information from a sensor installed in your fuel tank. That sensor has two parts: there is a vertical stick that runs from the top to the bottom of the tank. Attached to the stick is a float -- something that rides up and down on the fuel as the fuel goes up and down. When the float is at the bottom of the tank, the sensor tells the gauge you are "empty", when the float is at the top of the tank, the sensor tells the gauge you are "full". The different values from the sensor are based on where along the stick the float currently sits.

With me so far?

When you fill your tank, you insert the fuel pump hose into the inlet on the side of your car. That inlet, mounted on the side of your car, is not an opening directly into your tank. There is an internal hose that actually leads down to your tank, underneath the rear seats and the boot. When you fill your car until the hose shuts off, you have filled the tank as well as this inlet hose between the tank and the side of the car.

That's why your tank reports "full" for a good long time before the gauge begins to register less-than-full. The tank is actually a bit-more-than-full, and the float stays up at the top of the tank until enough fuel is burned so that the float begins to lower.

Your car's on-board computer can report mileage in many ways to you. The tach's display can be set to show "instant" mileage or "average" mileage. Both instant and average are calculated the same way: the car looks at how much fuel it is pumping into the engine, and how far it's gone. It gets "how much" information from a sensor that measures fuel flow into the engine, and it gets "how far" information from a sensor that measures distance, either from the transmission, drive shaft, or axle, that measures rotations and converts that to feet or meters. After that, it simply divides "how much" by "how far" and reports that number on the display on your tach, or on the display on your speedometer.

I do not know how long the "instant" time is, I would guess on the order of one or two seconds. The "average" time is based on all the time the engine has been on since you last reset the average.

As far as I know, the miles-to-go-on-this-tank uses it's own "average" value that it does not report to you on the dash directly. I have read that this is an average mileage calulated from the last 18.75 miles (30km) of distance and the amount of fuel remaining in the tank as reported by the tank fuel sensor I discussed above.

If this did not explain things clearly enough, please feel free to ask a specific question.

 

Thanks I understand now. Appreciate the help