How to Calculate Car Mileage: Petrol, Diesel, CNG and EV Guide

Car mileage indicates the distance your car can travel on a specific amount of fuel/charge. The mileage of petrol/diesel cars is often calculated in KM/L. The mileage of CNG cars is calculated in KM/KG. The efficiency of an electric car is typically expressed as KM/KWH.

Knowing how to calculate mileage helps you understand your car’s actual fuel efficiency, running cost and overall condition. It also helps you compare claimed mileage with what your car delivers in daily use.

The simplest car mileage formula is:

Mileage = distance travelled ÷ fuel used

For example, if your car travels 480 km after using 30 litres of petrol, the mileage is:

480 ÷ 30 = 16 kmpl

This same reasoning applies to petrol, diesel or CNG cars, where your fuel usage will be based upon gallons of fuel consumed vs electric power used to charge an EV.

Car Mileage Formula by Fuel Type

Vehicle type	Formula	Unit
Petrol car	Distance travelled ÷ litres of petrol used	kmpl
Diesel car	Distance travelled ÷ litres of diesel used	kmpl
CNG car	Distance travelled ÷ kg of CNG filled	km/kg
Electric car	Distance travelled ÷ electricity consumed	km/kWh
Running cost	Fuel or charging cost ÷ distance travelled	₹/km

If you want the best possible results, use the tank-to-tank method when calculating fuel consumption for petrol, diesel or CNG vehicles. Do not rely solely upon your vehicle’s odometer reading for mileage.

How to Calculate Car Mileage Using the Tank-to-Tank Method

The tank-to-tank method is one of the most practical ways to calculate real-world car mileage. It compares the distance driven between two full refills.

Step	What to do
1	Fill the fuel tank completely
2	Reset the trip meter to zero
3	Drive normally for a few days
4	Refill the tank fully again
5	Note the litres or kg refilled
6	Divide distance travelled by fuel refilled

For example, if you drove 420 km and refilled 28 litres of petrol, your mileage is:

420 ÷ 28 = 15 kmpl

Effort should be made to use the same fuel station and preferably the same fuel pump when refuelling your vehicle. Doing so offers the most accurate calculation because of the slight variation between fuel shut-off levels at different pumps.

Also, avoid calculating mileage after very short drives. A longer driving cycle gives a better picture because it includes traffic, open roads, AC use and regular driving patterns.

Claimed Mileage vs Real-World Mileage

Claimed mileage does not equal real-world mileage. Real-world mileage is the type of driving you will do every day; claimed mileage is tested in controlled conditions.

Type of mileage	What it means
Claimed mileage	Company-listed mileage tested under standard conditions
Real-world mileage	Actual mileage in daily driving
MID mileage	Mileage estimate shown on the car’s display
Tank-to-tank mileage	Manually calculated mileage based on actual refill

A vehicle manufacturer’s specified mileage (in this case, the 20 kmpl figure) can vary from city to city and by driving style/conditions (road condition/AC use, load) from 12-16 kmpl. The mileages will not always indicate that a vehicle has an issue; they will simply indicate that actual driving does not equal test-driving.

Is the Dashboard Mileage Accurate?

Most modern cars show average fuel efficiency on the MID, or multi-information display. This is useful for quick reference, but it should not be treated as the final mileage figure.

Mileage found via the MID is calculated utilising information from sensors, and estimates are calculated on-board. Theoretically, the MID can be fairly accurate, but it may not actually reflect the actual fuel level at each fill-up. If casual tracking is satisfactory, then use the MID to track your mileage; however, to be fully assured of the accuracy of your mileage, use the tank-to-tank method.

A good approach is to use the MID to track driving behaviour and use manual calculation once in a while to check the actual fuel efficiency.

How to Calculate CNG Car Mileage

CNG mileage is calculated in kilometres per kg instead of kilometres per litre.

The formula is:

CNG mileage = distance travelled ÷ kg of CNG filled

For example, if your CNG car travels 240 km and you refill 10 kg of CNG, the mileage is:

240 ÷ 10 = 24 km/kg

Due to various factors such as refill pressure, the quality of CNG, traffic, and the load on the vehicle, the mileage from CNG can vary more than the mileage from petrol. The best way to establish reliable mileage for CNG is to drive through several fill-ups and then average the number of miles driven for each fill-up.

How to Calculate EV Efficiency

Electric cars do not use petrol, diesel or CNG, so their efficiency is measured differently. EV efficiency is usually calculated in kilometres per kWh.

The formula is:

EV efficiency = distance travelled ÷ electricity consumed

For example, if an electric car travels 300 km after using 40 kWh of electricity, the efficiency is:

300 ÷ 40 = 7.5 km/kWh

To calculate the charging cost, multiply the battery capacity or electricity consumed by the electricity tariff.

For example, if your EV uses 30 kWh and electricity costs ₹8 per kWh, the charging cost is:

30 × ₹8 = ₹240

If the car travels 240 km on that charge, the running cost is:

₹240 ÷ 240 = ₹1/km

How to Calculate Running Cost Per Kilometre

Mileage tells you fuel efficiency. Running cost tells you how much you spend for every kilometre driven.

Fuel type	Formula
Petrol	Petrol price per litre ÷ mileage in kmpl
Diesel	Diesel price per litre ÷ mileage in kmpl
CNG	CNG price per kg ÷ mileage in km/kg
EV	Electricity price per kWh ÷ efficiency in km/kWh

For example, if petrol costs ₹105 per litre and your car gives 15 kmpl:

₹105 ÷ 15 = ₹7 per km

If CNG costs ₹90 per kg and your car gives 25 km/kg:

₹90 ÷ 25 = ₹3.60 per km

This is the easiest way to compare petrol, diesel, CNG and EV running costs.

Does E20 Petrol Affect Mileage?

In India, E20 petrol is widely available now, and car owners often ask how E20 petrol will impact their mileage (or how it can affect their fuel economy). There can be some variation in how each car’s engine, condition and tuning may affect each driver’s fuel economy with E20.

However, fuel blend is only one part of the mileage story. Traffic, tyre pressure, wheel alignment, servicing, AC use and driving style can make a bigger difference in daily use.

When finding out whether your older petrol vehicle can use E20 fuel, refer to the vehicle owner’s manual or advice from service providers. Manufacturers of more recent vehicles usually build them with ethanol-blended fuel in mind.

Do Flex-Fuel Cars Impact Car Mileage?

Flex-fuel cars can run on different blends of petrol and ethanol. For example, a flex-fuel car may support blends from E20 to E85 or even E100, depending on the vehicle.

The mileage calculation method remains the same:

Distance travelled ÷ fuel used

Each fuel blend should be compared on its own. The mileage from a car using E20 is likely different from a car using E85 or E100. Each fuel blend has a different energy content, which will affect the miles per gallon that can be achieved.

Factors That Affect Car Mileage

Many things can change your car’s real-world mileage.

Factor	How it affects mileage
Traffic	Stop-go driving reduces mileage
Driving style	Hard acceleration and braking use more fuel
Tyre pressure	Low pressure increases rolling resistance
AC use	Adds load to the engine
Vehicle load	Extra weight reduces efficiency
Maintenance	Dirty filters, old oil and poor alignment reduce mileage
Route type	Highways usually give better mileage than city roads
Idling	Wastes fuel without adding distance

Vehicle mileage isn’t solely dependent on the vehicle itself; how and where you drive the same vehicle will give different mileage results.

Tips to Improve Car Mileage

You can improve mileage by making small changes to your driving and maintenance habits.

• Accelerate smoothly instead of pressing the throttle hard
• Maintain correct tyre pressure
• Avoid unnecessary idling
• Service the car on time
• Remove extra luggage from the boot
• Use AC sensibly
• Maintain steady speeds on highways
• Avoid sudden braking where possible
• Use the right gear in manual cars
• Avoid very short trips when possible

While these behaviours will not turn a less fuel-efficient vehicle into a much more fuel-efficient one, they will help you get closer to the ideal real-world fuel mileage for your vehicle.

Final Verdict

The best way to calculate car mileage is to use the tank-to-tank method. Fill the tank fully, reset the trip meter, drive normally, refill the tank and divide the distance travelled by the fuel refilled.

The mileage of petrol and diesel vehicles is measured in kilometres per litre (kmpl), while the mileage measurement system for CNG vehicles is in kilometres per kg (km/kg) and in kilometres per kilowatt hour (km/kWh) for electric vehicles. To determine the running cost, use the cost of either fuel or charging divided by the distance travelled.

Dashboard mileage is useful, but manual calculation gives a clearer picture. With changing fuel types like E20 and flex-fuel options entering the market, tracking real-world mileage becomes even more useful for understanding actual ownership costs.

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