FE Mileage Calculator: How to Use It

The FE mileage calculator works out your fuel economy in miles per gallon (MPG) or litres per 100 km from the distance you drove and the fuel you used. You enter trip distance and fuel consumed, and it returns your average fuel economy and fuel cost per mile (if you add a fuel price). This page explains the formula, how to get accurate readings, and common mistakes to avoid.

Formula

The core formula is simple:

MPG = Distance (miles) ÷ Fuel used (gallons)

For metric fuel economy, the standard is litres per 100 kilometres:

L/100km = 235.215 ÷ MPG

Where:

• Distance – the total miles (or kilometres) driven on that tank or trip.
• Fuel used – the gallons (or litres) consumed during that distance.
• MPG – miles per gallon, the US customary unit for fuel economy.
• L/100km – litres per 100 kilometres, the metric unit. Lower is better.
• 235.215 – conversion factor between US MPG and L/100km (exact: 235.214583).

If you measure in kilometres and litres, the formula is: L/100km = (Fuel litres ÷ Distance km) × 100. To get MPG from those, divide 235.215 by the L/100km value.

Worked Example

Using the calculator’s defaults: you drove 300 miles and used 12 gallons of fuel.

1. Divide distance by fuel: 300 ÷ 12 = 25.0 MPG.
2. Convert to L/100km: 235.215 ÷ 25 = 9.4 L/100km.
3. Cost per mile at $3.50/gallon: $3.50 ÷ 25 = $0.140 per mile.

These match the calculator’s outputs exactly.

Why Use a Fuel Economy Calculator?

Knowing your real-world MPG helps you:

• Spot sudden drops in efficiency that might indicate a maintenance issue (e.g., tire pressure, air filter, or engine problems).
• Compare your actual fuel use to the official EPA or WLTP figures, which are often optimistic.
• Estimate trip costs or monthly fuel budgets.
• Decide whether a more fuel-efficient car is worth the upgrade.

For fleets or long commutes, even a 1 MPG difference can add up to hundreds of dollars a year.

How to Get an Accurate Reading

To measure your vehicle’s fuel economy reliably, follow the “full tank to full tank� method:

1. Fill the tank completely and note the odometer reading (or reset the trip meter).
2. Drive normally until you need fuel again.
3. Fill the tank completely again. The amount of fuel pumped is the fuel used.
4. Note the new odometer reading; subtract the first reading to get distance.
5. Enter those numbers into the calculator.

For best accuracy, use the same pump and the same “click� level each time, and average over several tanks to smooth out variations from traffic, weather, and driving style.

Common Mistakes and Misconceptions

Several errors can ruin your calculation:

• Mixing units – If you enter kilometres with gallons (or miles with litres), the result is wrong. Convert first: 1 mile = 1.60934 km; 1 US gallon = 3.78541 litres; 1 imperial gallon = 4.54609 litres.
• Using the car’s trip computer – These are often 5–10% optimistic. Manual calculation is more trustworthy.
• Not filling the tank completely – If you don’t fill to the same level, the fuel used won’t match the distance.
• Short trips – A 5-mile trip includes cold-start penalties; it won’t represent your typical highway MPG.
• Confusing US and imperial gallons – A US gallon is smaller than an imperial gallon. 1 US MPG = 1.20095 imperial MPG.

Real-World vs. Official Ratings

Official EPA ratings (US) and WLTP ratings (Europe) are done on standardized tests. Your real MPG will usually be lower because of:

• Highway speeds above 50 mph (80 km/h) – aerodynamic drag rises sharply.
• Rapid acceleration and braking.
• Carrying extra weight or roof racks.
• Air conditioning and electrical loads.
• Cold weather – engines run less efficiently until warm.
• Stop-and-go traffic versus steady cruising.

That’s why a calculator using your actual driving data is more useful than the sticker.

How to Improve Fuel Economy

Simple changes can improve your MPG by 10–30%:

• Maintain proper tire pressure (under-inflated tires increase rolling resistance).
• Remove unnecessary weight (e.g., roof racks, heavy items in the trunk).
• Drive smoothly – avoid hard acceleration and heavy braking.
• Use cruise control on highways.
• Keep up with regular maintenance (air filter, spark plugs, oil changes).
• Combine short trips to avoid multiple cold starts.
• Avoid idling for long periods – modern engines use fuel even when stationary.

Limitations of the Formula

The simple distance/fuel formula gives an average over the trip. It doesn’t tell you:

• Instantaneous fuel use (you’d need a real-time gauge).
• How driving conditions (hills, wind) affected the result.
• Whether the engine was running efficiently (e.g., cold starts).
• The difference between city and highway driving – you’d need separate measurements.

Also, fuel pump accuracy varies; a small error in the fill level leads to a larger error in MPG when the tank is small.

Frequently Asked Questions

How do I calculate MPG from litres and kilometres?

First convert litres to gallons (divide by 3.78541) and kilometres to miles (divide by 1.60934), then use MPG = miles ÷ gallons. Alternatively, compute L/100km = (litres ÷ km) × 100, then convert to MPG by dividing 235.215 by that L/100km value.

What is a good MPG?

It depends on the vehicle. A small sedan might get 30–40 MPG, an SUV 20–30, a pickup 15–25. Hybrids can exceed 50 MPG. Compare your number to the EPA rating for your specific car model to see if it’s normal.

Why is my calculated MPG lower than the sticker?

Official ratings are from laboratory tests with idealized conditions. Real driving includes traffic, weather, terrain, and your driving style, which all reduce efficiency. Also, the sticker may reflect a combined city/highway number that’s higher than your typical city driving.

Does the calculator work for electric vehicles?

No, this calculator is for fuel (gasoline/diesel). EVs use kilowatt-hours per 100 miles (or km). You’d enter electricity used in kWh instead of fuel, but the units differ – the result would be kWh/100km, not MPG.

How many tanks should I average for a reliable number?

At least three full tanks, ideally covering a mix of driving conditions. The more data you collect, the more representative the average. One tank can be skewed by a long highway trip or a week of short city commutes.

Is it better to use imperial or US gallons?

Use the gallon type that matches your fuel pump. In the US, pumps dispense US gallons. In the UK and some other countries, they use imperial gallons. Mixing them will give a result that’s about 20% off.

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in d | Distance travelled | miles | 300 | 0
in f | Fuel used | gallons | 12 | 0
in p | Fuel price per gallon | $ | 3.50 | 0
out mpg | Fuel economy (MPG) | mpg | d / f | 1
out l100 | Fuel economy (L/100km) | L/100km | 235.215 / (d / f) | 1
out cpm | Cost per mile | $/mile | p / (d / f) | 3