Your car seldom announces a fuel economy problem with a light. You notice it in the arithmetic you do at the pump, if you do that arithmetic at all. When miles per gallon drifts down over several tanks while your driving stays the same, something is usually working harder than it should. That drift is worth catching early, because a small fault that wastes fuel can grow into a larger repair, and a contract will ask you to prove when you first saw it.
What a steady drop usually points to
Fuel economy falls for ordinary reasons as well as serious ones, so start with the cheap checks. Tire pressure is the most common culprit. Underinflated tires roll with more resistance, and the effect builds slowly enough that most drivers never feel it. Check pressure when the tires are cold, against the placard on the driver's door frame rather than the maximum printed on the tire sidewall.
Next, look at the air filter. A clogged filter restricts airflow and can make the engine run richer than it needs to. A dragging brake caliper or a sticking brake pad also adds drag that shows up as lower mileage, and you may notice a warm wheel or a faint smell after a drive. A thermostat stuck open keeps the engine cool when it should be warm, and the computer compensates with extra fuel. Worn spark plugs or a failing fuel injector can lower economy before any misfire is strong enough to light a warning.
Some causes sit inside sensors that the computer relies on to set the fuel mixture. A sensor that reads wrong can cost you mileage for months without a visible symptom. You cannot diagnose these from the driver's seat, which is why the next step matters more than any guess.
Keep a simple log
Write down the odometer reading and the gallons added at every fill-up, and keep the receipts in the same folder as your service records. Calculate miles per gallon the same way each time. A few tanks of consistent figures tell you far more than one bad week. The log also gives a shop something concrete to work from, and it shows a provider that you noticed a change in performance rather than inventing a complaint after the fact.
Save the tire pressure checks, filter changes, and any receipts for brake or plug work too. Many service contracts require proof of routine maintenance before they will consider a claim, and a gap in those records can slow a claim even when the failed part is plainly covered by the contract's terms.
What to check before you sign
Read whether the contract lists covered components by name, or excludes components by name. A fuel economy problem might trace to a sensor, a thermostat, an injector, or a plug, and each may be treated differently in the contract. Do not assume that because the engine is covered, every part that influences fuel use is covered too. The scoring method on this site explains how we read covered-component language, and the sample contracts linked from the sample contract list let you check the wording yourself before you pay anything.
Ask the provider what it requires before it authorizes diagnostic work. Some want the vehicle taken to an approved facility first, some want a written estimate, and some will not pay for diagnosis unless the claim is accepted. Ask whether a diagnostic fee is handled under the same approval as the repair, and get the answer in writing. Ask who decides whether the fault is covered, and what happens if you disagree with that decision.
When to stop watching and act
A single low tank proves nothing. A steady decline across several fill-ups, with tire pressure, the air filter, and your driving habits ruled out, is the point to have a shop read stored codes and test the sensors and temperature readings. Do not wait for a light to come on, because by then the fault may already have affected other parts.
Tell the shop what you logged, and ask them to note the readings in the work order. Keep a copy. That single document often does more for a later claim than any argument you make afterward.
The goal is not to treat every dip as a claim. It is to recognize a steady, unexplained loss early, document it, and get it examined before a small fault becomes a bigger one.