Planning a road trip or trying to budget your monthly commute? The cost difference between petrol, diesel, CNG, and electric vehicles in India is far bigger than most people realise — sometimes 5× per kilometre. This guide breaks down exactly how to calculate fuel costs accurately, why your real mileage is lower than the showroom claim, and which fuel type actually saves you the most money.
Whether you're planning a road trip or budgeting your daily commute, the underlying formula for fuel cost is always the same — just adapt the fuel price and mileage to your vehicle type:
For petrol and diesel, "unit" is litres. For CNG, it's kilograms. For electric vehicles, it's kWh (units of electricity). The formula structure stays identical — only the units and prices change.
This is the number that actually matters for budgeting and comparison — cost per kilometre normalises across all fuel types regardless of unit differences:
The EV advantage: An electric vehicle costs roughly 5× less per km to run than petrol, and 2.4× less than CNG. Over 15,000 km/year of driving, that's a difference of ₹80,000+ in annual running cost between petrol and EV — though purchase price and charging infrastructure remain real considerations.
Every car's brochure proudly displays an ARAI-certified mileage figure — and almost no one achieves it in daily driving. Understanding why prevents wildly inaccurate cost calculations.
ARAI (Automotive Research Association of India) tests mileage in a controlled laboratory using a chassis dynamometer — constant speed, no AC, no traffic, ideal tyre pressure, and a standardised driving cycle. Real roads have none of these conditions.
| Condition | Typical Mileage Drop vs ARAI |
|---|---|
| City traffic (stop-and-go) | 20-30% lower |
| AC running continuously | 10-20% lower |
| Highway cruising at steady speed | 5-10% lower (closest to ARAI) |
| Under-inflated tyres | 5-10% additional drop |
| Aggressive acceleration/braking | 15-25% additional drop |
| Fully loaded vehicle (5 passengers + luggage) | 8-15% additional drop |
Practical rule of thumb: For city driving, expect 70-80% of the ARAI figure. For highway driving, expect 85-95% of the ARAI figure. If your car is rated 18 km/l ARAI, budget for 13-14 km/l in city traffic and 15-17 km/l on highways for realistic cost calculations.
Fuel prices vary meaningfully by state due to differing VAT rates layered on top of the base price set by oil marketing companies. Here is an indicative comparison for 2026:
| City | Petrol (₹/litre) | Diesel (₹/litre) | Note |
|---|---|---|---|
| Delhi | ~₹94.72 | ~₹87.62 | Among the lowest due to lower VAT |
| Mumbai | ~₹103.44 | ~₹89.97 | Higher VAT in Maharashtra |
| Bengaluru | ~₹101.94 | ~₹87.89 | Karnataka VAT structure |
| Chennai | ~₹100.75 | ~₹92.34 | Tamil Nadu pricing |
| Kolkata | ~₹103.94 | ~₹90.76 | West Bengal pricing |
Prices change frequently. Fuel prices are revised based on global crude oil rates and can change daily in some cities. Always check current rates via the IOCL website, your fuel station's app, or ToolLoom's calculator before finalising trip budgets — the figures above are indicative.
| Vehicle Type | Mileage | Fuel Price | Total Fuel Cost |
|---|---|---|---|
| Petrol car | 15 km/l | ₹100/litre | ₹6,667 |
| Diesel car | 20 km/l | ₹90/litre | ₹4,500 |
| CNG car | 25 km/kg | ₹80/kg | ₹3,200 |
| Electric vehicle | 6 km/kWh | ₹8/unit | ₹1,333 |
Note: highway mileage is typically 15-20% better than city mileage for the same vehicle — so a long-distance trip may cost slightly less per km than the same vehicle's city commute would suggest.
Example: a daily commute of 25 km each way (50 km/day) over 22 working days = 1,100 km/month. For a petrol car at 15 km/l city mileage and ₹100/litre: Monthly Cost = (1,100 ÷ 15) × 100 = ₹7,333/month.
Use your odometer or a trip tracking app over 2-3 weeks to get a realistic average, including weekend driving.
Fill the tank completely, reset trip meter, drive until empty, refill and note litres used. Distance ÷ litres = your actual mileage.
Use ToolLoom's Fuel Cost Calculator to get an instant monthly budget figure based on your real numbers.
| Mistake | Why It's Wrong | Fix |
|---|---|---|
| Using ARAI mileage for budgeting | Real-world mileage is typically 20-30% lower, leading to under-budgeted trips | Calculate your own real mileage from actual fuel-ups, or use 70-85% of ARAI as an estimate |
| Ignoring AC and load impact | A fully loaded car with AC running can use 25-35% more fuel than the empty, AC-off test condition | Factor in a buffer for AC-heavy summer trips or fully loaded family travel |
| Comparing fuel types only on per-litre price | Diesel looks "cheaper" per litre than petrol, but mileage differences change the real per-km comparison | Always compare on cost-per-km, not cost-per-unit-of-fuel |
| Forgetting toll and parking costs in trip budgets | Fuel is only one part of total trip cost — tolls on a Delhi-Mumbai highway can add ₹2,000+ | Add toll calculator estimates and parking costs separately to get total trip cost |
| Using stale fuel prices | Prices change with crude oil fluctuations; using last month's price can skew budgets by 5-10% | Check current prices via IOCL or your fuel app before finalising any cost estimate |
ToolLoom builds free financial and utility tools for Indian students, professionals, and creators. Fuel price data is indicative and updated periodically — always verify current rates before major trip planning. Found an error? Email contact@toolloom.in