Gas Mileage Calculator (Real-World MPG from a Fill-Up)

Fill the tank all the way up twice in a row. Enter the odometer reading at each fill-up and the amount of fuel you put in this time. The calculator finds your real-world fuel economy with the fill-up method.

Units

Current odometer (km)

Previous odometer (km)

Fuel added this time (L)

Gas price ($/L, optional)

The gas price can be left blank (if you enter it, the cost of the fill-up and the gas cost per km are also shown). If you reset your trip meter at the last fill-up, you can enter 0 as the previous reading and the trip meter reading as the current one.

Result

Enter the odometer readings and the gallons you put in on the left and press "Calculate". The result will appear here.

[ CLOSE ]

What you can do on this page
What is this calculation used for?
How to Use
Formula
Symbols and terms
- Symbols
- Terms
Good to know before you start
How to calculate it in Excel
How to calculate it in Google Sheets
How to calculate it in Python
How to write it in LaTeX and other math languages (copy and paste)
How to have ChatGPT do the calculation
DataChef Features
Related Features
NumberChef Calculators List

What you can do on this page

Enter the odometer readings from your last fill-up and this one, plus the gallons you put in, and you get your real-world mpg on the spot (the fill-up method)
The result is shown both in mpg (miles per gallon) and in gallons per 100 miles, the two numbers on the EPA fuel economy label. Switch "Units" to Metric to get km/L and L/100km instead
Enter the gas price ($ per gallon, optional), and you also get the cost of the fill-up and the gas cost per mile
A plain-language explanation of the formulas and copy-and-paste formulas for Excel, Google Sheets and Python are all on this page

This calculation works only when you filled the tank all the way up both last time and this time (the fill-up method). If you added gas in between, or one of the two was not a full tank, the gallons you put in do not match the gas you used, and the mpg will be off.

What is this calculation used for?

Knowing your car's real-world mpg to estimate running costs correctly

The EPA rating on the window sticker is measured under set test conditions, so your real-world fuel economy is often lower, depending on how and where you drive. Once you measure your own mpg with the fill-up method, you can estimate your monthly gas bill and trip costs with your car's true numbers.
All you do is write down the odometer reading and the gallons at each fill-up, so anyone can keep it up without special tools. Many fuel-tracking apps work the same way.

Checking the effect of fuel-saving driving with numbers

Avoiding hard starts and quick acceleration, keeping your tires at the right pressure, taking extra weight out of the car: if you track your mpg with the fill-up method, you can see the effect of these habits in numbers.
For example, if you drive 12,000 miles a year and raise your real-world fuel economy from 24 mpg to 26 mpg, the gas you use drops from 500 gallons to about 461.5 gallons, about 38.5 gallons less. At $3.20 a gallon, that saves about $123 a year.

Using mpg as an early sign that something is wrong with the car

If your car usually gets about 25 mpg and suddenly drops below 21 mpg without any change in how you drive, it may be a sign of low tire pressure or an engine problem.
A fuel economy record cannot tell you what is broken, but it is a simple health check that gives you a reason to take the car in for an inspection.

Knowing your gas cost per mile for commuting and errands (household budget)

At 25 mpg and $3.20 a gallon, each mile costs about $0.128 in gas (about 13 cents). A 20-mile commute each way is 40 miles a day, about $5.12, or about $102.40 for 20 workdays a month. Just multiply the miles by 12.8 cents.
Knowing "how much per mile" makes it much easier to compare driving with public transit and to budget for transportation.

Managing fuel costs for work vehicles (trucking and sales)

In trucking, delivery, sales and other jobs that depend on driving, companies track the fuel economy of each vehicle from fill-up records and use it as basic data for fuel budgets and cost calculations.
For example, a delivery van that is driven 3,000 miles a month at 15 mpg uses 200 gallons, about $640 a month at $3.20 a gallon. A fuel economy record helps not only with estimating expenses but also with deciding when to replace a vehicle.

Formula

Distance driven (how far you went since the last fill-up)

Standard notation (the usual math form)

$d$ $=$ $b$ $-$ $a$

In words (symbols replaced with words)

③ $d$: distance driven (mi) $=$ ① $b$: current odometer (mi) $-$ ② $a$: previous odometer (mi)

The formula in words

① Take the $b$: current odometer (mi)

② subtract the $a$: previous odometer (mi)

③ and you get the $d$: distance driven (mi)

Quick example

If the odometer read 45,300 miles at this fill-up and 45,000 miles at the last one, the distance driven is

$d$: distance driven (mi) $=$ current odometer (45,300 mi) $-$ previous odometer (45,000 mi)

$45300 - 45000 = 300$

Key idea

The odometer shows the total distance the car has ever driven. Subtract the reading at the last fill-up from the reading now, and a single subtraction gives the distance you drove between the two fill-ups. If you reset the trip meter (the resettable distance counter) to 0 at the last fill-up, the trip meter reading now is the distance driven.

Real-world fuel economy (mpg) with the fill-up method

Standard notation (the usual math form)

$e$ $=$ $d$ $\div$ $F$

In words (symbols replaced with words)

③ $e$: fuel economy (mpg) $=$ ① $d$: distance driven (mi) $\div$ ② $F$: gallons added (gal)

The formula in words

① Take the $d$: distance driven (mi)

② divide it by the $F$: gallons added (gal)

③ and you get the $e$: fuel economy (mpg)

Quick example

If you drove 300 miles since the last fill-up and it took 12 gallons to fill the tank this time, your real-world fuel economy is

$e$: fuel economy (mpg) $=$ distance driven (300 mi) $\div$ gallons added (12 gal)

$300 \div 12 = 25$

Key idea

This is the heart of the fill-up method. You filled the tank last time, drove, and filled it again this time, so the tank is back to the same "full" state. That means the gas you put in this time is exactly the gas you used since the last fill-up. So dividing the miles you drove by the gallons you just put in gives your real-world fuel economy. This only works if you filled the tank all the way up both times. If you added gas in between, add those gallons to the total too. The point where the pump nozzle clicks off changes slightly each time, so a single fill-up has some error. Using several fill-ups together, "total miles ÷ total gallons", gives a more accurate number.

Converting mpg to gallons per 100 miles

Standard notation (the usual math form)

$e_{100}$ $=$ $100$ $\div$ $e$

In words (symbols replaced with words)

③ $e_{100}$: fuel economy (gal/100mi) $=$ ① 100 (to get "per 100 miles") $\div$ ② $e$: fuel economy (mpg)

The formula in words

① Take 100

② divide it by the $e$: fuel economy (mpg)

③ and you get the $e_{100}$: fuel economy (gal/100mi)

Quick example

A fuel economy of 25 mpg, written as gallons per 100 miles (the gas needed to drive 100 miles), is

$e_{100}$: fuel economy (gal/100mi) $=$ 100 $\div$ fuel economy (25 mpg)

$100 \div 25 = 4$

Key idea

Gallons per 100 miles is the amount of gas it takes to drive 100 miles. The EPA fuel economy label shows it next to mpg. If 1 gallon takes you $e$ miles, then 100 miles takes $100 \div e$ gallons, and that is the conversion. Unlike mpg, a smaller number of gallons per 100 miles means better fuel economy. It also shows savings more fairly: going from 10 to 20 mpg saves 5 gallons per 100 miles, but going from 30 to 40 mpg saves only about 0.83 gallons. In Europe, Canada and Australia, the same idea is written as L/100km.

Cost of the fill-up (when you enter the gas price)

Standard notation (the usual math form)

$C$ $=$ $F$ $\times$ $u$

In words (symbols replaced with words)

③ $C$: cost of the fill-up ($) $=$ ① $F$: gallons added (gal) $\times$ ② $u$: gas price ($/gal)

The formula in words

① Take the $F$: gallons added (gal)

② multiply it by the $u$: gas price ($/gal)

③ and you get the $C$: cost of the fill-up ($)

Quick example

With 12 gallons added at $3.20 a gallon, the cost of the fill-up is

$C$: cost of the fill-up ($) $=$ gallons added (12 gal) $\times$ gas price ($3.20/gal)

$12 \times 3.20 = 38.40$

Key idea

This is what you paid for this fill-up, and it should be almost the same as the amount on the pump receipt. Gas prices change from day to day and differ by state and by station, so use the price at the station where you filled up. On this calculator the gas price can be left blank (then the cost of the fill-up and the gas cost per mile are not shown).

Gas cost per mile (when you enter the gas price)

Standard notation (the usual math form)

$k$ $=$ $C$ $\div$ $d$

In words (symbols replaced with words)

③ $k$: gas cost per mile ($/mi) $=$ ① $C$: cost of the fill-up ($) $\div$ ② $d$: distance driven (mi)

The formula in words

① Take the $C$: cost of the fill-up ($)

② divide it by the $d$: distance driven (mi)

③ and you get the $k$: gas cost per mile ($/mi)

Quick example

If $38.40 of gas took you 300 miles, the gas cost per mile is

$k$: gas cost per mile ($/mi) $=$ cost of the fill-up ($38.40) $\div$ distance driven (300 mi)

$38.40 \div 300 = 0.128$

Key idea

Once you know how much each mile costs, you can quickly estimate the cost of any trip. For example, at $0.128 per mile (about 13 cents), a 20-mile commute each way (40 miles round trip) costs $0.128 \times 40 = 5.12$ dollars a day. This value is the same as "gas price ÷ real-world mpg" ($C \div d = (F \times u) \div d = u \div e$); here $3.20 \div 25 = 0.128$.

With the fill-up method, your real-world mpg is "(current odometer − previous odometer) ÷ gallons added". This assumes you filled the tank all the way up both last time and this time. To get gallons per 100 miles, divide 100 by the mpg.

Symbols and terms

Symbols

$b$	The current odometer reading (miles), shown when you filled up this time.
$a$	The previous odometer reading (miles), shown when you filled up last time.
$d$	Distance driven (miles). How far you drove from the last fill-up to this one, found with $b - a$. It is the first letter of "distance".
$F$	Gallons added this time. With the fill-up method, this equals the gas you used since the last fill-up. It is the first letter of "fuel".
$e$	Real-world fuel economy (mpg). How many miles you actually drove on 1 gallon of gas. It is the first letter of "efficiency".
$e_{100}$	Fuel economy in gallons per 100 miles, the gas needed to drive 100 miles. It is found with $100 \div e$.
$u$	Gas price. The price of 1 gallon of gas ($ per gallon). It is the first letter of "unit price".
$C$	Cost of the fill-up ($). What you paid this time, found with gallons × price. It is the first letter of "cost".
$k$	Gas cost per mile ($ per mile). What each mile of driving cost you in gas.

Terms

odometer	The meter that shows the total distance a car has ever driven. It is on the instrument panel and cannot be reset. The mileage it shows is also used when a used car is valued.
trip meter	A distance counter you can reset to 0 at any time with a button (also called a trip odometer). If you reset it at the last fill-up, its reading now is the distance driven since then.
fill-up method	A way to measure real-world mpg by dividing the miles driven between two full fill-ups by the gallons it took to fill the tank the second time (also called the tank-to-tank method). It needs no special tools, only your fill-up records.
fuel economy	How efficiently a car uses fuel, also called gas mileage. In the US it is usually written as miles per gallon (mpg); the larger the number, the farther the car goes on less gas.
real-world fuel economy	The fuel economy you actually get. It changes with road conditions, driving style, air conditioning, passengers and so on, and is often lower than the EPA rating. You can measure it yourself with the fill-up method on this page.
EPA rating	The official fuel economy on a new car's window sticker (the Fuel Economy and Environment Label). It comes from standard EPA tests and is shown as city, highway and combined mpg. Real driving usually differs from it.
mpg (miles per gallon)	The unit of fuel economy that tells how many miles a car goes on 1 gallon of gas. It is the number used on US window stickers and in everyday talk. The larger the number, the better the fuel economy. (Japan uses km/L the same way.)
gal/100mi (gallons per 100 miles)	The unit of fuel economy that tells how many gallons it takes to drive 100 miles. It appears on the EPA label next to mpg. Unlike mpg, a smaller number means better fuel economy. Europe, Canada and Australia use the metric version, L/100km.

Good to know before you start

Here is what helps you use the calculation on this page with real understanding, not just by pressing the button.
If you get stuck, going back to these topics is the quickest way forward.

Subtracting large numbers (Grades 2–3)	Being able to subtract large numbers, such as $45300 - 45000$
Division and decimals (Grades 4–5)	Being able to give a decimal answer when the division does not come out even, as in $300 \div 8 = 37.5$ Being able to divide by a decimal, as in $100 \div 12.5$
Unit rates (Grade 6)	Understanding "the amount per 1", such as how many miles a car goes per gallon (that is exactly what mpg is) Understanding that "total amount ÷ how many units = amount per 1 unit"
Reading units (elementary to middle school)	Being comfortable reading "B per A" units such as mpg and gallons per 100 miles Knowing that the gallon is a unit of volume and that gas is sold at a price per gallon

How to calculate it in Excel

Copy the whole table below and paste it into cell A1 in Excel. It works as is.

Table to find the distance driven

Current odometer (mi)	45300
Previous odometer (mi)	45000
Distance driven (mi)	=B1-B2

Table to find the fuel economy (mpg)

Distance driven (mi)	300
Gallons added (gal)	12
Fuel economy (mpg)	=B1/B2

Table to convert to gallons per 100 miles

Fuel economy (mpg)	25
Fuel economy (gal/100mi)	=100/B1

Table to find the cost of the fill-up

Gallons added (gal)	12
Gas price ($/gal)	3.20
Cost of the fill-up ($)	=B1*B2

Table to find the gas cost per mile

Cost of the fill-up ($)	38.40
Distance driven (mi)	300
Gas cost per mile ($/mi)	=B1/B2

After pasting, B1 and B2 are your inputs and the last row is calculated automatically.
The first table subtracts the odometer readings, and B3 shows 300 (miles). The second table is the fill-up method itself, and B3 shows 25 (mpg).
The third table converts to gallons per 100 miles, and B2 shows 4. The fourth and fifth tables use the gas price, and B3 shows 38.4 ($38.40) and 0.128 ($ per mile). Just replace B1 and B2 with your own numbers.

How to calculate it in Google Sheets

Copy the whole table below and paste it into cell A1 in Google Sheets. It works as is.

Table to find the distance driven

Current odometer (mi)	45300
Previous odometer (mi)	45000
Distance driven (mi)	=B1-B2

Table to find the fuel economy (mpg)

Distance driven (mi)	300
Gallons added (gal)	12
Fuel economy (mpg)	=B1/B2

Table to convert to gallons per 100 miles

Fuel economy (mpg)	25
Fuel economy (gal/100mi)	=100/B1

Table to find the cost of the fill-up

Gallons added (gal)	12
Gas price ($/gal)	3.20
Cost of the fill-up ($)	=B1*B2

Table to find the gas cost per mile

Cost of the fill-up ($)	38.40
Distance driven (mi)	300
Gas cost per mile ($/mi)	=B1/B2

These formulas use only subtraction, multiplication and division, so the same formulas as in Excel work as is. Copy the whole table, paste it into cell A1, and replace B1 and B2 with your own numbers.

How to calculate it in Python

current_odometer_mi = 45300    # current odometer (miles)
previous_odometer_mi = 45000   # previous odometer (miles)
fuel_gal = 12                  # gallons added this time
price_per_gal = 3.20           # gas price ($ per gallon)

# distance driven (miles) = current odometer − previous odometer
distance_mi = current_odometer_mi - previous_odometer_mi
# fuel economy (mpg) = distance ÷ gallons added (fill-up method)
mpg = distance_mi / fuel_gal
# gallons per 100 miles = 100 ÷ mpg
gal_per_100mi = 100 / mpg
# cost of the fill-up ($) and gas cost per mile ($/mi)
cost = fuel_gal * price_per_gal
cost_per_mile = cost / distance_mi

print(f"Distance driven: {distance_mi} mi")
print(f"Fuel economy: {mpg:.2f} mpg ({gal_per_100mi:.2f} gal/100mi)")
print(f"Cost of the fill-up: ${cost:.2f}")
print(f"Gas cost per mile: ${cost_per_mile:.3f}/mi")

Runs with the standard library only. Replace the first four values (the two odometer readings, gallons added and price) with your own and run it. It assumes you filled the tank all the way up both last time and this time.

How to write it in LaTeX and other math languages (copy and paste)

Distance driven (how far you went since the last fill-up)

d = b − a

d = b - a

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>d</mi>
    <mo>=</mo>
    <mi>b</mi>
    <mo>&#x2212;</mo>
    <mi>a</mi>
  </mrow>
</math>

d = b - a

distance = currentOdometer - previousOdometer

d := b - a;

d = b - a;

d = b - a

Real-world fuel economy (mpg) with the fill-up method

e = d ÷ F

e = \frac{d}{F}

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>e</mi>
    <mo>=</mo>
    <mfrac><mi>d</mi><mi>F</mi></mfrac>
  </mrow>
</math>

e = d / F

efficiency = distance/fuel

e := d/F;

e = d/F;

e = d/F

Converting mpg to gallons per 100 miles

e₁₀₀ = 100 ÷ e

e_{100} = \frac{100}{e}

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <msub><mi>e</mi><mn>100</mn></msub>
    <mo>=</mo>
    <mfrac><mn>100</mn><mi>e</mi></mfrac>
  </mrow>
</math>

e_100 = 100 / e

efficiencyLPer100km = 100/efficiency

e100 := 100/e;

e100 = 100/e;

e_100 = 100/e

Cost of the fill-up (when you enter the gas price)

C = F × u

C = F \times u

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>C</mi>
    <mo>=</mo>
    <mi>F</mi>
    <mo>&#xD7;</mo>
    <mi>u</mi>
  </mrow>
</math>

C = F * u

cost = fuel*price

C := F*u;

C = F*u;

C = F×u

Gas cost per mile (when you enter the gas price)

k = C ÷ d

k = \frac{C}{d}

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>k</mi>
    <mo>=</mo>
    <mfrac><mi>C</mi><mi>d</mi></mfrac>
  </mrow>
</math>

k = C / d

costPerKm = cost/distance

k := C/d;

k = C/d;

k = C/d

How to have ChatGPT do the calculation

You are a fuel economy calculation assistant. Do the following calculation by actually running Python code, and base your answer only on the numbers from the execution result (do not answer by mental math or guessing).

At the last fill-up, the odometer read 45,000 miles. At this fill-up, it read 45,300 miles. It took 12 gallons to fill the tank this time (the tank was filled all the way up both times). The gas price is $3.20 per gallon.
Find each of the following:
1. The distance driven since the last fill-up (miles)
2. The real-world fuel economy by the fill-up method (mpg), and the same value in gallons per 100 miles
3. The cost of this fill-up ($), and the gas cost per mile ($ per mile)

Show the formulas you used and the numbers from the execution result.

How to Use

1

Enter your numbers

Type the numbers you want to calculate with into the input fields
2

Calculate

Press the "Calculate" button
3

Check the result

The result appears on the spot. The same page also explains the idea behind the calculation and the formula

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\(b\)	The current odometer reading (miles), shown when you filled up this time.
\(a\)	The previous odometer reading (miles), shown when you filled up last time.
\(d\)	Distance driven (miles). How far you drove from the last fill-up to this one, found with \(b - a\). It is the first letter of "distance".
\(F\)	Gallons added this time. With the fill-up method, this equals the gas you used since the last fill-up. It is the first letter of "fuel".
\(e\)	Real-world fuel economy (mpg). How many miles you actually drove on 1 gallon of gas. It is the first letter of "efficiency".
\(e_{100}\)	Fuel economy in gallons per 100 miles, the gas needed to drive 100 miles. It is found with \(100 \div e\).
\(u\)	Gas price. The price of 1 gallon of gas ($ per gallon). It is the first letter of "unit price".
\(C\)	Cost of the fill-up ($). What you paid this time, found with gallons × price. It is the first letter of "cost".
\(k\)	Gas cost per mile ($ per mile). What each mile of driving cost you in gas.

Gas Mileage Calculator (Real-World MPG from a Fill-Up)

Table of Contents

What you can do on this page

What is this calculation used for?

Formula

Symbols and terms

Symbols

Terms

Good to know before you start

How to calculate it in Excel

How to calculate it in Google Sheets

How to calculate it in Python

How to write it in LaTeX and other math languages (copy and paste)

How to have ChatGPT do the calculation

How to Use

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