How do you accurately calculate MPG?

[NacsMXer]

Alright, I know the basic scheme of how to calculate your vehicle's MPG. Distance traveled/Gallons used very simple.

What I am uncertain of, is how to accurately determine how many gallons your vehicle consumes between fill ups. You want to fill the tank full to start off, but how do you know what "full" is? Do you just go by the first "click" of the auto shut-off feature at the pump? How consistent are these auto shut-off mechanisms? I know I can put several gallons more into my tank after the first "click", so the tank isn't really "full".

I have thought about this, and the auto shut-off feature is the only way I can think of that will give you a consistent "full" reading to base your calculations on. If one pump varies from the next, or if you top off further, it will only skew your data.

How do you guys calculate it? Any tricks i'm missing out on?

Originally posted by NacsMXer
Alright, I know the basic scheme of how to calculate your vehicle's MPG. Distance traveled/Gallons used very simple.

What I am uncertain of, is how to accurately determine how many gallons your vehicle consumes between fill ups. You want to fill the tank full to start off, but how do you know what "full" is? Do you just go by the first "click" of the auto shut-off feature at the pump? How consistent are these auto shut-off mechanisms? I know I can put several gallons more into my tank after the first "click", so the tank isn't really "full".

I have thought about this, and the auto shut-off feature is the only way I can think of that will give you a consistent "full" reading to base your calculations on. If one pump varies from the next, or if you top off further, it will only skew your data.

How do you guys calculate it? Any tricks i'm missing out on?

My mom's cousin owns a dealership and told us that your tank is full after 2 clicks. You're filling up, it stops, you hold again until it clicks, hold again until it clicks and that's full.

That's what I do everytime and my mpg stays pretty much the same.

[greenblood01]

What I usually do if I'm going to check the mileage is let it go to the first click when I fill it up, then do the same the next I fill it up and check the mileage off of that. I almost always fill up at the same station to correct any errors from one stations automatic shut off being any different from another.

[tman12345]

fill it up, set your trip meter fill up later and see how many gallons it fills and the miles you went since yo first filled it. divide miles you went by the amount you needed to fill it again.

[NacsMXer]

Thanks for the tips guys. I was thinking along the same lines...i'll try to use the same gas station/pump and fill to the 2nd "click". That should keep the variables in check.

[wilkin250r]

Yeah, the first shutoff click generally doesn't do you any good, too many variables can affect that dramatically. Pump pressure, pump angle, ect ect.

I'm not even keen on the 2nd click, I like to use the 3rd or 4th. By then, you are indeed full and you're only filling the hose, so you'll only vary by about 1/8 of a gallon at most. If you used almost the entire tank, that 1/8 gallon uncertainty is only about 1%.


I don't think the "same gas station/pump" will give you the consistency that you think it will.

The short explanation is that the sensor is located at the tip of the gas pump, and it doesn't always have to be triggered by a full tank. It can be triggered by by foam or bubbles. This explains why "I know I can put several gallons more into my tank after the first "click", so the tank isn't really 'full'." Your tank was bubbling and foaming, and that foam reached the sensor and caused a false trigger.

Fuel pressure varies with the amount of fuel in the underground storage tanks, and fuel pressure and delivery will dramatically affect the amount of foam and bubbles produced. Since the pressure delivery varies widely, any given pump can produce different false "clicks" from day-to-day. It's not going to be consistent because the fuel pressure isn't consistent.

[<DRS>GPF]

i keep clicking until it wont take another drop, then do the math.

even then im sure its not truly consistant.(a cup or so difference..), but i cant imagine getting any closer using the means available.

[krt400ex]

look in the manual and see how many gallons your tank is, then put a 2 gallon jug in the back of your car and run the tank until it is empty...look at how many miles you have gone and divide it. the 2 gallons is so that when you go empty you have a little to throw in there to get you to a station.

[chris46250r]

Originally posted by krt400ex
look in the manual and see how many gallons your tank is, then put a 2 gallon jug in the back of your car and run the tank until it is empty...look at how many miles you have gone and divide it. the 2 gallons is so that when you go empty you have a little to throw in there to get you to a station.

That would give more of a false reading than anything else mentioned. The manual doesn't tell you where the reading is taken from, top of the tank, top of the filler hose or etc.

[krt400ex]

Originally posted by chris46250r
That would give more of a false reading than anything else mentioned. The manual doesn't tell you where the reading is taken from, top of the tank, top of the filler hose or etc.


it doesnt matter as long as you are consistently putting the same amout of gas in your car. if you only used ten gallons, but did that every time you can accurately and consistently figure your MPG's. the key is using the same amout of fuel

[chris46250r]

Originally posted by krt400ex
it doesnt matter as long as you are consistently putting the same amout of gas in your car. if you only used ten gallons, but did that every time you can accurately and consistently figure your MPG's. the key is using the same amout of fuel

I understand that but knowing what the manual says your tank holds is worthless.

[krt400ex]

Originally posted by chris46250r
I understand that but knowing what the manual says your tank holds is worthless.

unless you put the same amount in every time as according to the manual. thats all my point is to put a consistent amount of gas in every time

[chris46250r]

Originally posted by krt400ex
unless you put the same amount in every time as according to the manual. thats all my point is to put a consistent amount of gas in every time

You would have to be completely empty at the pump to put in exactly what the manual says. How you gonna do that, run it out and push the car in to the station?

[NacsMXer]

Originally posted by wilkin250r
Yeah, the first shutoff click generally doesn't do you any good, too many variables can affect that dramatically. Pump pressure, pump angle, ect ect.

I'm not even keen on the 2nd click, I like to use the 3rd or 4th. By then, you are indeed full and you're only filling the hose, so you'll only vary by about 1/8 of a gallon at most. If you used almost the entire tank, that 1/8 gallon uncertainty is only about 1%.


I don't think the "same gas station/pump" will give you the consistency that you think it will.

The short explanation is that the sensor is located at the tip of the gas pump, and it doesn't always have to be triggered by a full tank. It can be triggered by by foam or bubbles. This explains why "I know I can put several gallons more into my tank after the first "click", so the tank isn't really 'full'." Your tank was bubbling and foaming, and that foam reached the sensor and caused a false trigger.

Fuel pressure varies with the amount of fuel in the underground storage tanks, and fuel pressure and delivery will dramatically affect the amount of foam and bubbles produced. Since the pressure delivery varies widely, any given pump can produce different false "clicks" from day-to-day. It's not going to be consistent because the fuel pressure isn't consistent.

You bring up a bunch of good points, some of which I was thinking about before I posted this thread. With the auto shut-off you are absolutely right. Pump the fuel at full blast, and it foams up quick into the filler tube falsly triggering the mechanism. Pump it real slow and you don't get this result. I think the only time i've actually gotten my tank "full" was a long time ago when I was pumping the gas real slow trying to top off as much as possible. Of course it ended up in gas overflowing out of my filler tube and all over the ground, but with today's gas prices I don't think i'll be trying that again LOL.

So far it seems like the best I can do is get the "most" accurate number with some inherent error unavoidable involved every time. Doesn't really matter I guess, I just want to see what my truck is getting with different driving habits...i've never actually calculated its MPG before.

[krt400ex]

Originally posted by chris46250r
You would have to be completely empty at the pump to put in exactly what the manual says. How you gonna do that, run it out and push the car in to the station?

i have a truck...so that is what i was thinking...if you read my first post in this thread, you would have known. take a gas can wit you. ive done it b4

[chris46250r]

Originally posted by krt400ex
look in the manual and see how many gallons your tank is, then put a 2 gallon jug in the back of your car and run the tank until it is empty...look at how many miles you have gone and divide it. the 2 gallons is so that when you go empty you have a little to throw in there to get you to a station.

This is your first post, it says car. What I'm trying to get you to tell me is if your manual says you have a 20 gallon tank, how do you know you have 20 gallons in it?

[krt400ex]

my manual is accurate on mine. i drove until i had a little under an 1/8 of a tank and i put 23.5 gallons in it. my manual says i have a 25 gallon tank. that seems like it would be pretty accurate. you are correct, my first post says car, but i was thinking truck because that is what i have and therefore you could put a gas can in the back if you ran it empty so that you wouldnt be stranded on the road

[bulkdriverlp]

heres what i did, i ran my truck till the low fuel light came on the dash. jammed the tank till it was spilling out of the neck. waited for the foam to settle and everything. hit the trip odometer. wrote down how many gallons i put in. when the low fuel light came on again i noted the milage. did the math. my explorer gets 13.7 mpg city. there probobly was only a half gallon left in the tank before i filled it. the "click" thing is inaccurate. jmo

[Anvil]

seams like people over complicate everything.

Fill your tank up ( I fill until I can see fuel )

0 your trip meter

drive 100 miles ( this gives you a mix of driving)

fill tank back up until you can see it

do the math------------and thats it.

[chris46250r]

Originally posted by Anvil
seams like people over complicate everything.

Fill your tank up ( I fill until I can see fuel )

0 your trip meter

drive 100 miles ( this gives you a mix of driving)

fill tank back up until you can see it

do the math------------and thats it.

Bingo! No extra gas in the trunk required, doesn't matter what size tank you got, no counting clicks at the pump...

[NacsMXer]

Originally posted by Anvil
seams like people over complicate everything.

Fill your tank up ( I fill until I can see fuel )

0 your trip meter

drive 100 miles ( this gives you a mix of driving)

fill tank back up until you can see it

do the math------------and thats it.

What, do you push down the little flapper door with your finger and look down the filler tube? I've never tried that before, i'll bust out a flashlight and try that next time.

[NacsMXer]

Figured out my truck's MPG for the first time today. I knew it was bad, but not as bad as I thought it would be.

Stop and go traffic around town, stoplights everywhere, driving it as easy as I possibly could with just a couple part throttle romps she got 16.7 MPG. It's not great mileage, but not bad either considering it's a 5000 lb 4wd supercharged 5.4 V8 truck with bigger fuel injectors, fuel pumps, all that good go fast stuff. I figure i'll get around 9-10 mpg around town if i'm boosting it everywhere lol good thing I try not to :devil:

I'm curious what it will get on the highway with the cruise on...if I get closer to 20 I think that's even better. The tall 3:55 final drive really helps out there.

[Lowcountrydave]

Something else to remember is idling. I mention this because I once drove my truck for a certain amount of miles. I knew I could get that amount of miles without running out of fuel. I also carried a can in the back of my truck for several months, "trying" to see the most mileage per tank. So one day I decided to take the can out the back of my truck. Knowing I had at least another 30 miles until I needed to refuel.


Would you believe I ran out of fuel on a bridge over pass. So you have to think how long do I let my truck idle in a parking lot or warming up in the winter in your drive. Sitting at a light. There are just to many varables to consider. It would be interesting to set it up on a dino that provided wind resistants and a load on the drive train.

Dave