Why does my gas gage read more gas than I have in the tank?

Question

During the day when I start my car the gas gage indicates one level of gas... but by the time I get to my destination it indicates more gas that I started with. For example it starts at 3/4 th tank at start then by the end of the travel it reads full. I travel only 15 miles to work... but in 100 degree weather. Is it heat and pressure ???? How does a fuel gage work and measure GAS? Should I have the fuel system cleaned so I get a more accurate reading start to finish?

Answer 1

Your tank volume is read by a sensor in the tank.  It's possible that the sensor reacts slowly and isn't reading full until you've been in the car a while.  My sister's old Corolla took a good 30 minutes to register a fill-up.

Sometimes, if you overfill your tank, meaning once the pump shuts off, trying to get that extra gallon in, you can cause the sensor to lose calibration.  It's like bending the float valve in your toilet to get more water in the tank.  Once you bend it, it will always overfill the tank.  If this is the case, you may not be getting accurate readings anyway.

Try filling up the tank and driving a set distance before refueling, say 200 miles.  Most cars on the road today are capable of getting 300+ miles per tank, so you should be safe.  Fill the tank until the pump clicks off and don't put another drop in.  Let your car sit with the switch on ( don't need to have the engine running) and wait 30 minutes.  Check your fuel gauge and see what it registers for a known full tank.

Drive the 200 miles, doesn't have to be all at once.  Note the reading on the tank gauge and your miles.  Refill your tank.  You now know how many gallons it takes to drive 200 miles and you should be able to estimate the tank reading.  Let's say you get 20 mpg.  You should have used about 10 gallons of gas.  If your tank is 15 gallons, your tank reading should have been about 1/3.  If it's very far off, say less than 1/4 or over 1/2, I'd take it to a repair shop and have them check the sensor.  A mechanic at a dealer would probably be familiar with any issues related to your car's fuel sensor.

Heat and pressure won't change your volume so much that you'd be able to notice it on the gauge and cleaning the fuel system won't have any effect on the tank sensor.

Answer 2

My guess is that you park your car on a downslope, which will cause a higher reading on the gas guage.

Answer 3

First query --Do you park on an incline?

Second - how old is the car?

Third - does the gauge change more dramatically when near full, or near empty?

The guage works by a float in the gas tank.  As the level goes up/down, so does the float, which relays that to your gauge.

First safe bet... know how much gas you can put in the car...know your mpg...and use your trip meter to ensure you don't run out of gas.