figuring gas mileage

[Lauri]

Does your tank have to be nearly empty and then topped off, to be able to figure mileage correctly?


I have been diligent recording mileage and gallons, but am coming up with a vast difference in the mpg.

[Karenrbw]

You don't have to drive it to almost empty before you refill - but it will give you a more accurate average.

[Wis Bang 2]

I keep a tablet in the car & write the gal & miles for each fillup. When the page is full I add up all the miles and all the gal and figure it again.

Then I average each page...

[oneokie]

As the previous poster stated, no. What you do need to take into consideration is the type of mileage you are driving between fillups. Stop and go city, longish trips with few stops, or a mix of both. Each has a large influence on mileage.

[geo in mi]

Not unless you are figuring it to the third or fourth decimal place. There's not enough volume in the filler neck from the point at which the hose clicks off to the actual top to make enough difference. And it won't matter at which point you start--it's the gallons used for the distance driven that gets figured. Keeping your tire pressure correct would make more difference in the long run, probably.

geo

[Fat Charlie]

You don't need it empty, but you do need it filled. When you fill it, reset the trip odometer. Next time you fill it up, your reciept will tell you exactly how much gas you burned and your trip odometer will tell you exactly how far it got you. Write the mileage on the reciept, reset the trip odometer (to measure your next tankful) and do the math once you're home. Keep the reciepts as a log, with a word or two on them as an explanation if you want (towing, highway, etc.)

It doesn't matter if you wait until empty to do it, you just have to fill it each time.

[watcher]

You can do it as long as you start with a fill up, keep tract of every bit you buy and end with a fill up and have your odometer reading at the begining and the end of the cycle.

Say you fill up today all you need is the odometer reading. Then every time you get gas write down the amount (go to at the tenth of gallon) you buy. When you are ready to check the MPG go to the station, fill the tank and note the odometer reading.

After that its all basic math. Subtract the first odometer reading from the second one and call it M (for miles). Add up all the gas you have bought between the first fill up and the last one and call it G (for gallons). Then you divide M by G (M / G) and you have the miles per gallon.

The longer time between your first fill up and your last one will give you a much more accurate MPG for your car and your driving.

[Ramblin Wreck]

Good advice above. I would add that, if possible, you fill up at the same pump at approximately the same time of day. If you fill up in the cool of the day, the tank holds a different amount than in the heat of the day...at least I think I remember reading that somewhere.

Also, if your car/truck has a digital readout of your estimated MPG, I've found that it is more accurate if you use a tank until it's almost empty.

[MichaelK!]

Hi Lauri
Good for you that you are at least trying to keep track of milage. A lot of people I talk to don't even know what it is.

Do you just fill the tank untill the automatic shut-off switch clicks off? That is one variable that will change from pump to pump. When I calculate milage, I always fill to a point where I can SEE the level of gasoline, not when the pump decides it's full. For my diesel truck, that might be 4-5 extra gallons after the auto shut-off has tripped. When I can see that the tank is filled to the same level, I get a very accurate reading for milage.

[fordy]

................A big part of the problem of accurately calculating fuel milage is that the fuel volume in the tank is going too vary ! During the summer heat gas vapors in the tank will expand and activitate the shutoff mechanism on the fill nozzle prematurely which will prevent the completely fillup of the tank . This , then , reduces the volume of fuel burned for a given number of miles and Falsely INcreases your calculated fuel milage . Another problem is when you park your vehicle on an angle shifting the prefillUP fuel volume too one side of the tank , this can either result in a premature shutoff or filling up with more volume than necessary . Depends on which way the vehicle is leaning . The only accurate way too calculate your correct milage is too beable too fill the tank too same total volume each fillup .
...............You can still calculate a relatively accurate number for your fuel consumption . , fordy

[watcher]

Originally Posted by MichaelK!

Hi Lauri
Good for you that you are at least trying to keep track of milage. A lot of people I talk to don't even know what it is.

Do you just fill the tank untill the automatic shut-off switch clicks off? That is one variable that will change from pump to pump. When I calculate milage, I always fill to a point where I can SEE the level of gasoline, not when the pump decides it's full. For my diesel truck, that might be 4-5 extra gallons after the auto shut-off has tripped. When I can see that the tank is filled to the same level, I get a very accurate reading for milage.

Big difference in filling a gasoline and a diesel tank; diesel foams, gas doesn't. As you have discovered with an oil burner you can add a lot more fuel after the automatic shuts off if you wait several seconds for the foam to break down and add more. With gas you might get another half gallon.