Is it true that you get better MPG on a Full to half full tank?

Question

It seems like I always getter better miles per gallon in my car when my gas tank is between full and half full. Does anyone know if this is true or not?

Answer 1

its not true. the reason you think you get better mileage with a full tank is because the gauge doesn't show the true amount of fuel in the tank until the level has come down a bit.

Answer 2

Yes it is true. Reason, engine uses fume from gas to burn it and as long as car moves produce with half tank more fumes than full tank and burns fumes instead of gas. u save money dude..

Answer 3

This is actually not true, in fact you MPG is worse when your tank is full to half full.

The simple reason for this is weight. A car with a full tank requires more force to move than a car with a near-empty tank because it is much heavier. More force means more power required from the engine. More power means more petrol/gas used.

Also when you fill your tank to the maximum you have a greater risk of 'overfilling'. This can mean some extra gas is lost in the cars vapour release system meaning lost money and wear. Fuel pumps are designed to cut off on a full tank, but the meter at the pump will still tick over as the hose collects the fuel vapours back - meaning your paying to give the petrol station petrol!

Its an idea to aim at filling up between quater to half a tank.

Running the car very low gives you the obvious risk of running out. Also it can damage the fuel system as the car has to burn the gunk at the bottom of the tank. Performance is also affected as not more than vapours in the tank causes bad fuel system pressure.

Try using your trip recorder to calculate the MPG difference between filling methods. Analogue gauges/dials are notoriously unreliable.

Answer 4

Gasoline evaporates into a gas vapor.  When your tank is half full or lower there is more air vs. fuel causing the gasoline to evaporate at a faster rate.  Also when there is more air in the tank this allows the gasoline to "slosh" around during driving increasing evaporation.  Try calculating your MPG at half tank vs. running your tank close to empty.  MPG will always be higher if you fill up at half.  

On a side note the gasoline vapor generated in your tank is collected in the car's EVAP system.  The system transfers the vapor to the engine where it is burned for emissions purposes.  It doen't actually power the vehicle.  

Answer 5

It couldn't be true.

P.S. That's right.  gas mileage is not calculated by inches of needle movement.

Answer 6

it is true... but not enough to bother with.  the reason is not fumes or anything else, but your car weighs less when the tank is half full, so in stop and go driving, or over hills, you can get better gas mileage with a half tank than a full one.

you can get the same affect by taking all the things you don't need out of your car so you don't have to carry extra junk.  losing weight helps too.

this applies so much that extreme hypermilers will remove headliners, extra seats, spare tires and anything else only nominally bolted down in a car to improve fuel economy.  

it's also why drag cars and short track cars rarely use a full tank... less weight to move.