I use my trip odometer.

When I fill up, I divide the number of gallons bought into the reading
on the trip odometer.

Then I zero it for the next tank.

Simple and works well.

You have to do it over a long period of time (many tank fulls) and then
estimate the beginning fuel level the very first time. Any error in
estimating the beginning fuel level the first time will be minimized when
looking at 10 or more tanks of gas to calculate the mileage.

Fill the tank and record the mileage.

The next time you need gas, fill the tank and record the mileage.
Subtract the second mileage from the first mileage. Divide the result by
the amount of gas (gallons and tenths)  purchased the second time.

You can use the trip odometer if you have one. Reset when you fill up
the first time. When you refill, record the miles traveled, and divide
by the gallons purchased.

It's very simple.

Fill your tank at a given station to a given point (say, the second time
the nozzle clicks off). Reset your trip odometer or note the mileage
reading.

Drive the car. The longer you drive it, the more accurate will be your
mileage calculation.

Return to the same gas station -- use the same pump if possible. Fill the
tank once again to the same point as before (say, the second time the
nozzle clicks off). Note the number of litres or gallons, and the odometer
reading.

Divide the number of miles you travelled by the number of gallons you
purchased. That is your fuel mileage (in miles per gallon) for that tank
of fuel.

OR

Multiply the number of litres you bought by 100, then divide the
result by the number of kilometres you travelled. That is your fuel
mileage (in litres/100km) for that tank of fuel.

"newbie7" wrote:

Also, lacking a trip meter, record miles on odometer at fillup, take a
new reading at next fillup and subtract the old reading from the new
reading and then divide the result by the gallons used.

Amazing. Our educational system on display.

I agree wholeheartedly
davidj92

Fill the tank, record the mileage, run the tank down.  Fill it
again.  Subtract the mileage at the first fill from the mileage
at the second filling.  Divide that figure by the amount of fuel
consumed.  If that is too complicated ask your math professor for
additional help after class, WBMA

mike hunt

Newly wrote:

Fill tank, reset trip odometer or record actual mileage.

Drive.

Refill tank.  For most accurate results, use the same pump parked in
the same position.  If you routinely "top off" then do so by the same
number of "clicks".

Record number of gallons.

Read trip mileage, or record actual mileage.

Subtract old mileage reading from new mileage reading... should match
trip mileage.

Miles per gallon equals miles/gallons... miles divided by gallons, or,
for example, you went 340 miles on 19 gallons, then your MPG is
340/19 or 17.9mpg

http://personal.linkline.com/rlockyer/TundraGas.xls

 --- Rich
 http://richlockyer.tripod.com/

You need to mark the milage at each fillup.  Then take the gallons at
each fillup.  Take the milage since the last fillup (subtract previous
fillup milage from current milage.  Take gas you put in this fillup.
This is the gas you put in, even if there already was gas in tank, and
the milage you drove since then, so it doesn't matter how much was
previously in tank.

Then; miles PER gallon. PER means divide.  So divide miles by gallons.

Now, there are two trains of thought on how to handle statistics and
accuracy.

One thought is to take milage at beginning of several fillups.  Now keep
track of gas used in ten fillups, milage at last fillup.  Divide
cumulative milage by total of gas used.  This MAY eliminate some small
measurement errors.  But, it yields no statistics by which you can
measure accuray.

I prefer calculating milage each time, for ten times.  Average the ten
readings.  Now, compute variance.  This is done by taking each
individual milage calculation and subtracting average.  Square the
difference.  Some all the squares.  Now take square root of that sum.
Sounds like a lot of work, but with calculators it is a two or three
minute job.

Now, the average you compute is only accurate to within this square
root, with is the basic precision of your set of measurements.  For
instance, I tried ten tankfuls of regular, ten tankfulls of premium, to
see which gave better accuracy.  The value for regular was slightly
bette than the average for premium, but the amount of the difference was
only about half of the variance (that square root).  Thus I could not
conclude that regular actually gave me better milage- I have to conclude
that they are the same within the accuracy I can measure my gas milage.

The very best way is to ALWAYS fill the tank everytime you get gas, then
divide the distance you went, 250 miles, by the new load of fuel, 12.5
gallons. The result, 20, is your miles per gallon. Fill the tank and reset
the odometer (trip meter).

I consistantly get just a bit over 25 mpg, so in 300+ miles I put in 12
gallons of gas, for just over 325 miles, I put in 13 gallons, for 275 miles,
I get 11 gallons.

It doesn't matter how much gas is left when you fill up. It does matter if
you fill to the brim, or only fill to the first shut-off. I usually fill to
the first shut-off, then top off to the next even dollar. I used to top off
to the next even quarter, but gas is getting so expensive that topping to
the next dollar is less than a half gallon, and this won't change your
formula very much.

So, you go to the corner today and buy gas. If you filled the tank to the
top AND reset the odometer the last time you got gas, then you can divide
the distance by the new load of fuel and derive the mileage. If you did not
fill to the top and/or reset the odometer, then this tank will be your
baseline and you can make no calculations from it. Then, in a week or so
when you visit the filling station again, then you fill the tank and divide
the distance by the fuel load. The result will be your mileage. If you go
250 miles and put in 10 gallons, then you got 25 mpg. All you have to do is
always fill the tank, never just get $5.00, or whatever. If you do just get
$5.00, then you have to add the gallons to the next fill up and leave the
odometer counting as if you had not gotten the few gallons.

After you do this a few times, you will be able to quickly calculate in your
head to confirm that you are getting the same mileage that you always get. I
get 25 mpg in my normal driving routine.

Start with a full tank.
record mileage.
Drive.
At next fill, record mileage and the gallons to fill back up.

With my truck I just go to the first click.  Your mileage will vary
depending on the type of driving, the weather (-40 idling uses a lot of
gas) etc etc... so just keep tracking it.

I use a spreadsheet and track the average and the average for the last
10 tanks.  I found the last 10 tankfuls to be a good indicator of my
overall mileage.  I moved to a more rural area so I spent less time in
rush hour so the average mpg went up by a couple, but went back down in
the winter... and went up a couple mpg when I finally changed the plugs
and did a tune up.

My truck usually averages around 12-16mpg with the odd highway drive
above 20.

Why the average?  Because unless you drive the same road under the same
conditions at the same speed with the same brand of gas filled to the
same spot in the tank you're introducing all kinds of "error" into the
equation.  Averaging that over 10 tanks will give you a more "real
world" mileage.

If you have excel I can email you a copy of the spreadsheet...
Ray

One normally assumes that the tank becomes equally full right after each
fill-up.  Therefore, all you need to do is reset your trip-odometer after
each fill-up, but first, divide the number on it by the number on the
pump.  (In other words, the amount of gas you just put in equals the
amount you used between the previous fill-up and this fill-up.)

Ask your little sister, she'll help you.

It's a basic math exercise usually found in the form of word problems.
For the results to have any value you have to fill up every time you do
the calculation.

Number of miles driven divided by number of gallons equals the miles
per gallon.

A cumulative average can be computed by using the same numbers as above
but just keep adding them in and redividing.  After a while the number
becomes glacially slow in responding to change and a performance
problem may not be noticed for several tanks.

A moving average covering some moderate time period like five fillups
is probably the most useful because it retains some responsiveness to
short term changes while also reflecting performance over more than one
fillup.  Unfortunately it also requires the most work.