Petrol, Diesel, CNG and LPG: How to Compare Running Costs Per Kilometre

Pakistan's drivers can choose between several fuels, and every one of them is sold in a different unit. Petrol and diesel are sold per litre, CNG per kilogram and LPG per kilogram or per litre. A price in one unit cannot be compared with a price in another, and the fuel with the lowest price tag is not necessarily the cheapest to run. The right comparison is the cost of covering one kilometre, and this guide shows how to find it.

Step one: find the cost per kilometre

For any fuel, the formula is the same.

Cost per kilometre = price per unit divided by kilometres per unit.

The unit is whatever the fuel is sold in. If petrol costs Rs 400 per litre and your car travels 12 km on a litre, then the cost is 400 / 12 = Rs 33.3 per kilometre. If CNG costs Rs 250 per kilogram and the same car travels 14 km on a kilogram, it costs 250 / 14 = Rs 17.9 per kilometre. The numbers in this article are illustrations only, so use current prices and your own mileage.

Where to get the mileage figure

Use a figure you have measured yourself, which is more reliable than a brochure. Fill the tank, note the odometer, drive normally, then fill again and divide kilometres by units of fuel. If you have never tracked it, our guide to keeping a fuel expense log shows how. Remember that mileage on CNG is often a little lower in terms of power and that results vary with the vehicle and its tuning.

A worked comparison

Take a small car that covers 1,200 km a month. The figures below are only an example.

• Petrol: Rs 400 per litre at 12 km per litre gives Rs 33.3 per km, so a month costs about Rs 40,000.
• CNG: Rs 250 per kg at 14 km per kg gives Rs 17.9 per km, so a month costs about Rs 21,400.

In this example the difference is about Rs 18,600 a month. But the comparison must also include the cost of getting the cheaper fuel, which we turn to next.

The costs people forget

• Conversion cost. A CNG or LPG kit costs money to buy and install. Divide the kit price by the monthly saving to see how many months it takes to pay for itself. A kit costing Rs 150,000 that saves Rs 18,600 a month pays back in about eight months, but a lower-mileage driver may wait several years.
• Cylinder weight and space. Cylinders take up boot space and add weight.
• Maintenance. Gas systems need regular inspection, and the cylinder has a legal test and validity period.
• Availability. A cheap fuel is worth little if the nearest station is far away, or if supply is cut for some periods. Consider your regular routes.
• Resale value. Some buyers prefer a car with the original fuel system only.

Diesel is different

Diesel cars and light trucks are usually bought as diesel from the start and do not run on the other fuels. Diesel engines normally travel further per litre than petrol engines of similar size, which narrows the gap in running costs even when the price per litre is higher. For commercial vehicles that carry loads over long distances, the more useful figure is the cost per tonne-kilometre or per trip, because that is what the business earns against. Use the same formula, but with your own vehicle's real figures.

Safety is not optional

Gas systems are safe when they are installed and maintained properly, and dangerous when they are not. Use only certified fitters and approved cylinders and fittings, keep to the inspection and testing schedule, and never use a domestic LPG cylinder as a vehicle fuel tank. Do not try to modify or refill a cylinder yourself. If you smell gas, move away from the vehicle, avoid any spark and have it checked. Check the rules in your area about the use of each fuel in vehicles, because they have changed over the years.

A quick decision checklist

1. Measure your real monthly distance.
2. Measure your real mileage on each fuel you can use.
3. Calculate the cost per kilometre for each, using today's prices.
4. Multiply by your monthly distance to get a monthly cost.
5. Add the one-off conversion cost and divide by the monthly saving to get the payback time.
6. Consider availability, safety and resale value before deciding.

Prices change, so repeat the sums

Fuel prices move often and not always together, so the cheapest option can change. Check the current prices of petrol and diesel in our price table, and use our fuel calculator for the petrol or diesel side of the calculation. Then repeat the comparison when prices change noticeably.

Frequently asked questions

Does a CNG car lose power? Many cars give somewhat less power on gas than on petrol, and the difference depends on the engine and the quality of the conversion. Test a car on both fuels before deciding.

Can I switch between fuels? Bi-fuel systems let a car run on either petrol or gas, which gives flexibility if supply is interrupted. Check that the system is certified and approved.

Does diesel always beat petrol? Diesel vehicles often use less fuel per kilometre, but the purchase price and maintenance costs can be higher, so work through the whole comparison.

If you are comparing fuels because of high running costs, remember that efficiency applies to every fuel. Smooth driving, correct tyre pressure and regular servicing lower the units you need whatever you burn. See our guide to fuel-efficient driving habits and the wider guide to fuel prices in Pakistan. If you are also thinking about a two-wheeler, our article on electric versus petrol motorcycle running costs uses the same cost-per-kilometre method.