ThrottleJournalHow to Calculate and Cut Your Monthly Fuel Cost
By Chong Sem · Updated 7 November 2026Short answer: your monthly fuel cost is the kilometres you drive in a month, divided by your car's real mileage, multiplied by the fuel price. Drive 1,200 km a month in a car that really does 14 km/l, with petrol at ₹102.12 in Delhi, and you spend about ₹8,753 a month, or ₹7.29 for every kilometre.
That one formula is simple. Getting the inputs right is where most people go wrong, usually by trusting the brochure mileage. This guide shows how to calculate your monthly fuel cost properly, how much the same car costs to run in different cities, and the changes that genuinely cut your bill. You can plug in your own numbers with our fuel cost calculator as you go.
The fuel cost formula
Every fuel cost calculation comes down to three numbers:
Monthly fuel cost = (km driven per month ÷ mileage in km/l) × fuel price per litre
The first part tells you how many litres you burn. The second turns litres into rupees. For CNG, use km per kg and the price per kg instead.
Worked example
Say you drive 1,200 km a month, your car really returns 14 km/l, and you live in Delhi, where petrol costs ₹102.12 a litre (7 November 2026).
- Fuel used = 1,200 ÷ 14 = 85.7 litres
- Monthly cost = 85.7 × ₹102.12 = ₹8,753
- Yearly cost = ₹8,753 × 12 = ₹1,05,038
That's more than ₹1 lakh a year on fuel alone, for what is a fairly ordinary amount of driving. It's also why getting the inputs right matters: if any of the three numbers is off by 10%, so is your answer.
Find your real mileage (not the brochure's)
The mileage printed in a car's brochure is the ARAI-certified figure. It comes from a standard lab test that lets you compare cars fairly with each other, but it isn't how you drive. Traffic, AC use, short trips, load and driving style mean most owners see a noticeably lower figure on the road.
The difference matters a lot. Using the example above, if you budget with a claimed 20 km/l instead of a real 14 km/l, you'd expect to spend ₹6,127 a month. The real bill is ₹8,753. That's a ₹2,626 surprise every month.
The tank-to-tank method
This is the most reliable way to measure what your car actually does:
- Fill the tank until the nozzle clicks off. Don't top up beyond the click.
- Reset your trip meter to zero.
- Drive normally until the tank is at least half empty. A full tank gives a better average.
- Fill up again until it clicks off, at the same pump if you can. Note the litres added.
- Divide the kilometres on the trip meter by the litres you just added.
For example, if you drove 520 km and needed 38.5 litres to refill, your mileage is 520 ÷ 38.5 = 13.5 km/l. Repeat this over two or three tanks and average them, because one tank of mostly highway driving can flatter the figure. Our mileage calculator does the division and compares your result with the claimed figure.
Your car's trip computer also shows an average, and it's handy for spotting trends. But it's an estimate, so check it against the tank-to-tank figure at least once.
Work out how far you really drive
Most people guess their monthly kilometres, and most guesses are low. Two easy ways to get a real number:
- Read the odometer on the 1st of the month and again on the 1st of the next. Do this for two or three months, since festivals, holidays and trips move the number.
- Use your service records. Your service invoices show the odometer reading each time. Divide the kilometres between two services by the number of months between them.
As a rough guide, a 15 km commute each way over 22 working days is 660 km a month before a single weekend errand. Add shopping, school runs and weekend trips, and 1,000 to 1,500 km a month is common in Indian cities.
Fuel prices today and what they mean in each city
Because VAT and freight charges vary by state, the same car costs noticeably more to run in some cities than others. Here's the same car (14 km/l, 1,200 km a month) at the 7 November 2026 petrol price in each city:
| City | Petrol (₹/litre) | Diesel (₹/litre) | Monthly petrol cost | Yearly petrol cost |
|---|---|---|---|---|
| Lucknow | 101.89 | 95.36 | ₹8,733 | ₹1,04,801 |
| New Delhi | 102.12 | 95.20 | ₹8,753 | ₹1,05,038 |
| Chennai | 107.76 | 99.55 | ₹9,237 | ₹1,10,839 |
| Mumbai | 111.21 | 97.83 | ₹9,532 | ₹1,14,387 |
| Bengaluru | 111.68 | 99.56 | ₹9,573 | ₹1,14,871 |
| Jaipur | 113.19 | 98.25 | ₹9,702 | ₹1,16,424 |
| Patna | 113.37 | 99.36 | ₹9,717 | ₹1,16,609 |
| Kolkata | 113.51 | 99.82 | ₹9,729 | ₹1,16,753 |
| Thiruvananthapuram | 115.49 | 104.40 | ₹9,899 | ₹1,18,790 |
| Hyderabad | 116.15 | 104.23 | ₹9,956 | ₹1,19,469 |
Prices: Mint, 7 November 2026 (Hyderabad: same-day city price lists). Monthly and yearly costs are calculated by ThrottleJournal assuming 14 km/l and 1,200 km a month. Fuel prices change, so use today's price in your city.
Driving the same car the same distance costs about ₹14,400 more a year in Hyderabad than in Delhi. If you're moving cities, it's worth factoring in.
Cost per km: the number worth knowing
Cost per kilometre is the single most useful fuel number, because it lets you price any journey in your head:
Cost per km = fuel price ÷ mileage
At Delhi's ₹102.12 a litre, here's what each kilometre costs at different mileages, along with the monthly bill for 1,200 km:
| Real mileage | Cost per km | Monthly cost (1,200 km) |
|---|---|---|
| 10 km/l | ₹10.21 | ₹12,254 |
| 12 km/l | ₹8.51 | ₹10,212 |
| 14 km/l | ₹7.29 | ₹8,753 |
| 16 km/l | ₹6.38 | ₹7,659 |
| 18 km/l | ₹5.67 | ₹6,808 |
| 20 km/l | ₹5.11 | ₹6,127 |
Two patterns stand out. First, the jump from 10 to 14 km/l saves far more (₹3,501 a month) than the jump from 16 to 20 km/l (₹1,532). Improving a thirsty car pays off most. Second, once you know your cost per km, a 30 km round trip to the mall is easy to price: at ₹7.29 a km, it's about ₹219 in fuel.
Petrol vs diesel vs CNG: running costs compared
The fuel type changes your cost per km more than almost anything else. Using Delhi prices on 7 November 2026, and assuming typical real-world figures for a compact car of 14 km/l on petrol, 18 km/l on diesel and 22 km/kg on CNG:
| Fuel | Price in Delhi | Assumed mileage | Cost per km | Monthly cost (1,200 km) |
|---|---|---|---|---|
| Petrol | ₹102.12/litre | 14 km/l | ₹7.29 | ₹8,753 |
| Diesel | ₹95.20/litre | 18 km/l | ₹5.29 | ₹6,347 |
| CNG | ₹86.98/kg | 22 km/kg | ₹3.95 | ₹4,744 |
CNG price: Times of India city CNG rate, 7 November 2026. Mileage figures are ThrottleJournal's assumptions for illustration; check real-world figures for the exact car you're considering.
On these assumptions, CNG costs roughly half as much per km as petrol. But the fuel bill isn't the whole story. Diesel and CNG versions usually cost more to buy, CNG takes up boot space and can mean queues at the pump, and CNG prices have been rising: Mumbai's distributor raised its price to ₹90 a kg from 1 November 2026, its sixth increase this year, according to the Hindustan Times.
The fair way to compare is to work out how long the fuel savings take to cover the higher purchase price. If a CNG version costs ₹90,000 more than the petrol and saves ₹4,009 a month on fuel (₹8,753 − ₹4,744), it pays back in about 22 months. Drive half as much and the payback doubles. Our EV vs petrol vs diesel calculator runs this comparison for your own mileage and prices.
How to cut your fuel bill
Every 1 km/l you gain is worth real money. For our 1,200 km-a-month driver in Delhi, going from 14 to 15 km/l saves ₹584 a month, or about ₹7,000 a year. A 10% improvement in mileage saves about ₹796 a month. These are the changes that actually move the number, roughly in order of impact.
1. Drive more smoothly
Hard acceleration and late braking waste fuel more than anything else in city driving. Accelerate gently, look further ahead so you can coast towards red lights instead of braking, and keep a steady speed on the highway. On highways, fuel use climbs quickly at higher speeds, so cruising a little slower is one of the easiest savings there is.
2. Shift up early and avoid idling
In a manual car, change up as soon as the engine is pulling comfortably rather than revving each gear. If you'll be stopped for more than a minute or so, at a long signal or railway crossing, switching off saves fuel. Many newer cars do this automatically with start-stop.
3. Keep your tyres at the right pressure
Under-inflated tyres increase rolling resistance, so the engine works harder for every kilometre. Check pressures once a fortnight when the tyres are cold, using the figures on the sticker inside the driver's door, not the maximum printed on the tyre.
4. Stay on top of servicing
A clogged air filter, old engine oil or worn spark plugs can all nudge mileage down. Following the service schedule in your owner's manual keeps the engine running as efficiently as it was designed to.
5. Cut the dead weight and drag
Extra weight in the boot and an empty roof carrier both cost fuel on every trip. Take out what you don't need and remove roof racks when you're not using them.
6. Use the AC sensibly
The AC compressor draws power from the engine, so it does use fuel. In heavy city traffic, using it at a moderate setting is fine. At highway speeds, keep the windows up and use the AC rather than driving with windows open, which adds drag.
7. Combine trips and plan routes
A cold engine is at its least efficient, so several short trips use more fuel than one longer trip covering the same errands. Avoiding the worst traffic, even if the route is a little longer, often saves fuel too.
What doesn't help much
Premium petrol only makes sense if your car's manufacturer recommends it, typically for high-performance engines. In a regular commuter car, it costs more without a meaningful mileage gain. Fuel-saving gadgets and additives sold online are best avoided.
Why did my fuel bill suddenly go up?
If you're spending more than usual, work through these in order. Most have simple fixes, and checking them costs nothing.
- Did the price change, or did you drive more? Compare this month's litres, not rupees, with a normal month. If the litres are the same, it's the price. If the kilometres went up, it's your driving pattern, not the car.
- Has your mileage dropped? Run a tank-to-tank check. A drop of more than about 10% from your usual figure, with similar driving, is worth investigating.
- Check tyre pressures. This is the most common and cheapest cause.
- Think about the season and the route. More AC in summer, more short trips during the monsoon, or a new route with heavier traffic can all lower mileage without anything being wrong with the car.
- Is a service overdue? Old oil and a dirty air filter add up over time.
- Listen and feel for warning signs. A burning smell after driving, a car that pulls to one side, or a clutch that bites very high can point to dragging brakes or a slipping clutch. These waste fuel and need a mechanic. A check-engine light on the dashboard should always be looked at.
Keeping a simple log of fill-ups, with the date, odometer reading, litres and price, turns this from guesswork into a two-minute check. A notes app or spreadsheet is enough.
Calculating fuel cost for a road trip
The same formula works for any trip. Use your highway mileage, which is usually better than your city figure. For a 280 km drive, such as Delhi to Jaipur, in a car that does 16 km/l on the highway:
- Fuel needed = 280 ÷ 16 = 17.5 litres
- Cost one way = 17.5 × ₹102.12 = ₹1,787
- Return trip = ₹3,574
Add tolls, which you can estimate from FASTag deductions on previous trips, and parking. If you'll fill up along the way, price the fuel at the rate in the state you're filling in, since prices differ by state. Split between four passengers, the fuel for that return trip comes to under ₹900 each.
Building fuel into your car budget
Fuel is usually the biggest running cost after the EMI, and unlike the EMI, it never ends. Over five years, our example driver in Delhi would spend more than ₹5 lakh on petrol at today's price. Three habits make the number manageable:
- Budget with your real mileage and real kilometres, not the brochure and a guess. Add 10% as a buffer for price rises and longer months.
- Track it. Note every fill-up for a month or two. It's the fastest way to spot a mileage drop that could signal a tyre or maintenance problem.
- Think in cost per km when choosing between a car, a cab or public transport for a trip.
When you're choosing a car, fuel should sit alongside the EMI, insurance and servicing. Our total cost of ownership calculator adds them all up, and our guide to car loans covers the EMI side.
Frequently asked questions
How do I calculate my monthly fuel cost?
Divide the kilometres you drive in a month by your car's real mileage to get litres, then multiply by the fuel price. At 1,200 km, 14 km/l and ₹102.12 a litre, that's 85.7 litres and ₹8,753.
Why is my car's mileage lower than the company claims?
The claimed figure comes from a standard lab test. Real driving, with traffic, AC, short trips and passengers, almost always uses more fuel. Measure your real figure with the tank-to-tank method and budget with that.
What is a good cost per km for a petrol car?
At Delhi's November 2026 petrol price, a car doing 14 km/l costs ₹7.29 a km, and one doing 18 km/l costs ₹5.67. Anything that gets you below about ₹7 a km on petrol is economical in today's prices.
Is CNG still cheaper than petrol after the price hikes?
Per kilometre, yes, on current prices. In Delhi, a CNG car at 22 km/kg costs about ₹3.95 a km against ₹7.29 for a petrol car at 14 km/l. Whether it saves you money overall depends on the extra purchase price and how much you drive.
How much fuel does a 100 km trip use?
Divide 100 by your mileage. At 15 km/l, it's 6.7 litres, which costs about ₹681 at ₹102.12 a litre.
Last updated: 7 November 2026. Fuel prices are those published for each city on the dates shown and change often. Mileage figures marked as assumptions are for illustration. All costs are calculated by ThrottleJournal. This guide is for information only.