The most common causes of a sudden MPG drop are a faulty oxygen (O2) sensor, dirty MAF sensor, sticking brake caliper, low tyre pressures, or a clogged air filter. A 10–20% drop usually has a single fault behind it; a gradual long-term drop is often tyres + driving style + winter fuel blends.
A sudden 10–20% drop in fuel economy almost always points to a sensor or mechanical fault. Here's the order a UK mechanic checks them.
Will a service improve my MPG? — Yes — fresh air filter, oil and spark plugs typically restore 3–8% MPG on a tired engine.
Why does my car use more fuel in winter? — Cold engines run rich until warm, dense air needs more fuel, and winter diesel/petrol blends have slightly less energy. 10–15% drop is normal.
Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email [email protected].