Why Is My Car's Fuel Consumption Suddenly Increasing in Sri Lanka?

Why Would Fuel Consumption Suddenly Increase?

If your car is drinking more petrol or diesel than usual, the most common causes are low tyre pressure, a dirty air filter, worn spark plugs, heavy traffic with the AC running, or rough driving habits. These are cheap and quick to check, and together they explain most sudden jumps in fuel bills. Only if you've ruled these out should you suspect a deeper engine fault.

Work through the checks below in order, from cheapest to most expensive, before you book your car in for diagnostics.

Key Facts

• Check tyre pressure against the sticker inside the driver's door jamb, not the number printed on the tyre itself.
• A clogged air filter restricts airflow into the engine, forcing it to burn more fuel for the same power.
• Worn spark plugs cause incomplete combustion, which wastes fuel and can trigger rough idling.
• Running AC in slow-moving traffic uses noticeably more fuel than driving at a steady speed with AC off.
• A Vehicle Emission Test (VET) certificate is still required, as of November 2026, before the Department of Motor Traffic's eRL system will let you renew your annual revenue licence (per the eRL portal requirements).
• The VET checks exhaust smoke, leaks and silencer condition, but it does not measure overall fuel economy, so passing it doesn't mean your car is fuel-efficient.
• Sudden acceleration and hard braking waste far more fuel than smooth, steady driving in traffic.
• A short-trip pattern (lots of cold starts, little highway driving) will always show worse fuel economy than long, steady runs.

Could Low Tyre Pressure Be the Cause?

Yes, this is the single most common and cheapest cause of rising fuel bills. Underinflated tyres create more rolling resistance, so the engine has to work harder to keep the car moving, and that extra effort shows up at the pump. Check pressure monthly, and always check it when the tyres are cold, before you've driven more than a kilometre or two. Use the pressure figure on the sticker inside the driver's door or in the owner's manual, not the maximum pressure printed on the tyre sidewall — those are different numbers for different purposes.

A quick top-up at any filling station takes two minutes and costs nothing. Do this first before anything else on this list.

Is a Dirty Air Filter to Blame?

It can be, especially if your car is overdue for a service or you drive a lot on dusty rural roads. The air filter stops dust and debris from entering the engine, but over time it clogs and restricts the air the engine needs to burn fuel efficiently. Pull the filter out (it's usually a simple clip-open box under the bonnet) and hold it up to a light. If you can't see light coming through, or it's visibly grey with dust, replace it. Most garages check this for free as part of a routine service, so ask next time you're in for an oil change.

Could Worn Spark Plugs Be Increasing Your Fuel Bill?

Worn or fouled spark plugs don't ignite the fuel-air mixture cleanly, so the engine burns more fuel to produce the same power, and you may also notice rough idling, hesitation, or the engine warning light coming on. Spark plug life varies by vehicle and plug type, so check your owner's manual or ask your mechanic for the recommended interval rather than guessing. If you've genuinely lost track of when they were last changed and the car is also idling unevenly, this is worth checking alongside the air filter.

Does Sri Lanka's Emission Test Catch These Problems?

Partly, but not fully. The VET looks at exhaust smoke, oil or water leaks, and silencer condition, and a vehicle with excessive smoke or an unusual noise can fail. But the test does not measure your actual fuel economy in litres per 100 km, so a car can pass its emission test and still be burning more fuel than it should due to tyre pressure, a dirty filter, or driving habits. Don't treat a VET pass as proof that your fuel consumption is normal — it tells you the exhaust is within limits, nothing more. Since the emission certificate is required before you can renew your revenue licence through the DMT's eRL system, it's worth getting any smoke or leak issues fixed anyway, as they usually go hand in hand with wasted fuel.

Is Your Driving Style or Colombo Traffic the Real Culprit?

Very often, yes. If nothing is mechanically wrong, the explanation may simply be more time spent in slow-moving traffic, more short trips with cold starts, or a change in how you drive. Sudden acceleration and hard braking burn noticeably more fuel than smooth, gradual driving, and idling in traffic for long periods (engine running while you're barely moving) adds up over a month. Running the AC at low speed in heavy traffic also increases the load on the engine. None of this means something is broken — it means your actual driving conditions have changed, which is common during the monsoon when trips take longer, or during school-term traffic in cities like Colombo, Kandy, and Gampaha.

Could It Be a Deeper Engine or Sensor Fault?

If you've checked tyres, air filter, and spark plugs, and your driving pattern hasn't changed, but consumption is still noticeably worse (and especially if the engine also feels rough, hesitates, or the check-engine light is on), it's time to see a mechanic. Possible causes include a faulty oxygen sensor, a leaking fuel injector, a clogged catalytic converter, or a wheel alignment problem that's dragging the car off-line. These need a proper diagnostic scan rather than guesswork, so don't keep driving and hoping it resolves on its own.

What Mistakes Do Sri Lankan Drivers Make?

• Checking tyre pressure against the number on the tyre sidewall instead of the manufacturer's sticker.
• Ignoring a rough idle or a faint smoke smell because the car still "runs fine".
• Assuming a recent emission test pass rules out any fuel-related fault.
• Not tracking fuel use consistently, so a real increase goes unnoticed for months.
• Blaming fuel quality first, when the far more common causes are tyres, filters, and driving habits.
• Continuing to drive for weeks with a check-engine light on instead of getting it scanned early, which can let a small fault turn into an expensive repair.

Quick Checklist to Diagnose High Fuel Consumption

1. Check and correct tyre pressure (cold tyres, use the door-sticker figure).
2. Inspect the air filter and replace it if clogged.
3. Check spark plug condition against your service schedule.
4. Note whether your recent trips have been shorter or more traffic-heavy than usual.
5. Reduce hard acceleration and braking; use AC sensibly in slow traffic.
6. If consumption is still high after these checks, get a diagnostic scan rather than guessing further.
7. Keep your service book updated — it helps you (or a buyer, if you later list the car on /vehicles) prove the car has been properly maintained.

If you're weighing up whether a thirsty old car is still worth keeping, or whether it's time to trade it in, you can check what similar vehicles are selling for on /price-check, browse replacements on /vehicles, or work out what a newer, more fuel-efficient car would cost monthly using the /leasing calculator. And if you're not sure whether what you're seeing is normal wear or a real fault, ask other owners of the same model in the Wahanaya forum — someone with the same car has probably seen the same problem before.

Sources

• Drivegreen - Required Documents for Vehicle Emission Test
• A Motorist's Guide to Improve Fuel Economy in Driving and Riding - Sri Lanka Sustainable Energy Authority
• Vehicle Registration Renewal Sri Lanka 2026 Guide