My fuel economy dropped. What changed?

At your next visit

General guidance for this symptom. A warning light that is flashing or an overheating engine: stop driving and call.

Likely causes

• Low tire pressure

  Tires lose pressure as the weather cools, roughly a pound for every 10 F drop, and a Reno October that goes from 80 F afternoons to 30 F mornings takes several pounds out of them. Fueleconomy.gov puts the cost of underinflation at about 0.2 percent per pound below spec and says proper inflation can improve mileage by up to 3 percent. We set pressures to the door-jamb label at every service.

  Multi-point maintenance check
• A clogged engine air filter

  On older carbureted engines a dirty filter cost real mileage; on modern fuel-injected engines fueleconomy.gov says it mostly costs acceleration, not economy. It still matters for how the car feels and for emissions, and in the high desert it clogs fast. We replace it when daylight will not pass through.

  Engine air filter
• The wrong oil grade

  Fueleconomy.gov says using the manufacturer's recommended grade of motor oil can improve mileage by 1 to 2 percent, and that a heavier oil than specified can cost about that much. If the last service center put 10W-30 in a 0W-20 engine, the next oil change with the right grade is the fix.

  Full-service oil change
• Deposits in the fuel system

  Injectors that dribble instead of mist and a carboned intake burn fuel badly. A fuel system cleaning clears deposits and is worth doing before anyone chases sensors. On a car that hesitates as well as drinking, it is the first thing we suggest.

  Fuel system cleaning
• A failing oxygen sensor or a dragging brake

  Fueleconomy.gov notes that fixing a serious maintenance problem such as a faulty oxygen sensor can improve mileage by as much as 40 percent. A brake caliper that does not release costs mileage and makes a wheel hot. Both are repair-shop items. We can tell you if a wheel is hot after a drive and whether the check engine light has a code behind it.

  Multi-point maintenance check

Nobody notices a mile per gallon. People notice when the tank that used to get them from Gardnerville to Reno and back twice now does it once and a half. Fuel economy is one of the most honest readouts a car has, because it adds up everything: tires, air, oil, fuel delivery, temperature, and how the car is driven. The federal fueleconomy.gov site publishes what each maintenance item is worth, and the numbers are useful because they tell you what to fix first.

First, rule out the things that are not the car

• Winter blend gasoline, sold from roughly October to spring, carries less energy per gallon; fueleconomy.gov says cold weather alone can cut short-trip mileage sharply
• A roof box for ski season: fueleconomy.gov puts a large roof cargo box at up to a 25 percent penalty at highway speed
• A/C in July, which is a real load on a small engine at Reno's 90 F afternoons
• A new commute with more stop lights, or a new driver in the household
• Carrying the snowboards, the tools and the dog bed all year

If none of those changed and the mileage still fell, the car is telling you something, and the bay check is where to start.

What we check and set

Tire pressure to the placard on the door jamb, cold. Engine air filter, held up to the light. Oil grade on the cap against what the last sticker says went in, and the oil's condition on the stick. Fuel cap seal. A look at the fuel system history, and a fuel system cleaning if the car has never had one or is hesitating. All of that happens during a full-service oil change at Mae Anne Avenue, Longley Lane, College Parkway or US 395, and the invoice lists what we found.

Oxygen sensors, thermostats stuck open (an engine that never warms up burns more fuel), dragging brakes and transmission problems go to a repair shop. We tell you which of those we suspect and why.

Altitude and the number on the window sticker

The EPA mileage on the window sticker was measured at sea level. At 4,500 feet the engine makes less power per stroke, the computer adjusts, and on most modern engines the economy is close to the sticker on flat roads and worse on grades. The Mount Rose Highway from Reno climbs almost 4,000 feet, and a Subaru that does 30 mpg on I-80 does nowhere near that on the way up. That is not a problem to fix; it is the hill. Track your mileage on the same route for a fair comparison.

Keeping the number up

The cheapest fuel economy in Northern Nevada is maintenance on the manual's schedule: the right oil at the manual's interval, a filter that gets looked at every oil change because of the dust, tire pressure checked monthly through the fall temperature drop, and a fuel system cleaning every so often on a car that lives on short trips. Our between-oil-changes checklist turns that into a monthly habit.