Car Care Statistics With Sources, and What We Leave Out

A number goes on this page only if a public agency publishes it and we read it on that agency's page ourselves. We name the source next to each figure. Agencies update their pages, so if a figure here ever differs from the source, the source wins.

We do not publish industry-average percentages that nobody can trace, survey numbers from marketing reports, or shop numbers we have not measured. When a figure is not published, we say so plainly.

Every figure on this page appears in the table below with its source. Two agencies supply them: fueleconomy.gov, run by the U.S. Department of Energy and the EPA, and the National Highway Traffic Safety Administration (NHTSA). Read the sections that follow for what each figure means for your car.

According to fueleconomy.gov (opens in a new tab), keeping tires inflated to the proper pressure can improve gas mileage by about 0.6 percent on average, and up to 3 percent in some cases. The same source says underinflated tires can lower gas mileage by about 0.2 percent for every 1 psi drop in the average pressure of all tires.

Those numbers look small, but low pressure also flexes the sidewall more, wears the tire edges faster and makes the tire run hotter. On a long, hot drive up a grade, that heat adds up. Set pressure cold, to the door sticker number. Our tire pressure and care guide shows how.

Yes. NHTSA states that properly inflated tires can extend the average life of a tire by 4,700 miles. It also says only 19 percent of consumers properly inflate their tires, and that poor tire maintenance, such as low air and skipped rotations, can lead to a flat tire, a blowout or tread coming off the tire.

NHTSA adds that rotation, balance and alignment help tires last longer too. It does not attach a mileage figure to those, so neither do we. What we can show you is the wear itself: an inside or outside edge worn faster than the center points to low pressure or alignment, and we measure tread depth across the tire before recommending anything.

NHTSA's tire page lays out the method. The correct pressure is the one on the Tire and Loading Information label on the driver's door edge or post, or in the owner manual, not the number printed on the tire. A tire counts as cold when it has not been driven for at least three hours.

The steps, in order

1. Find the number on the door label or in the owner manual.
2. Measure cold, or allow for the extra pressure in warm tires.
3. Check all tires, including the spare.
4. Add the missing pounds to any tire that reads low.
5. Recheck later if you had to fill warm tires, since NHTSA calls a warm fill a temporary fix.

NHTSA also says it is safer to drive slightly under the cold number after a warm fill than to drive on a tire that is significantly low. If a tire keeps losing air, that is a repair question, not a gauge question. Come in so we can find the leak.

Fueleconomy.gov states that aggressive driving, meaning speeding, rapid acceleration and hard braking, can lower gas mileage by roughly 15 to 30 percent at highway speeds and 10 to 40 percent in stop-and-go traffic. It also says idling can use a quarter to a half gallon of fuel per hour, depending on engine size and AC use.

Hard braking also wears pads and rotors faster. We have not found a public figure that converts that into miles of pad life, because pad wear depends on the car, the pads, the load and the route. That is why we measure pads instead of estimating from mileage. See brake service.

Yes, according to fueleconomy.gov. It says an extra 100 pounds in a vehicle could reduce mileage by about 1 percent. A large, blunt roof-top cargo box can reduce fuel economy by around 2 to 8 percent in city driving, 6 to 17 percent on the highway, and 10 to 25 percent at Interstate speeds of 65 to 75 mph.

The same source says rear-mount cargo boxes or trays cost much less: about 1 or 2 percent in city driving and 1 to 5 percent on the highway.

Weight matters for tires and brakes too. A loaded car needs the pressure on the door label for its load, and brakes work harder on the downhill side of a grade. If a move is bigger than your trunk, a truck or trailer may be the better tool. Call (909) 608-7699 to reserve a U-Haul, and we check the hitch before you tow.

Fueleconomy.gov says using the manufacturer's recommended grade of motor oil can improve gas mileage by 1 to 2 percent. Its examples: using 10W-30 in an engine designed for 5W-30 can lower mileage by 1 to 2 percent, and using 5W-30 in an engine designed for 5W-20 can lower it by 1 to 1.5 percent. It suggests looking for "Energy Conserving" on the API performance symbol.

On air filters, the same page says replacing a clogged filter on modern fuel-injected, computer-controlled gasoline engines does not improve fuel economy, though it can improve acceleration. Older carbureted engines are the exception.

That is why we fill to the grade your owner manual lists and do not sell an engine air filter as a mileage fix. We show you the filter and let you decide if it can wait. See oil changes.

The National Highway Traffic Safety Administration publishes tire safety guidance at NHTSA tires (opens in a new tab). Points from that page:

• Tires get recalled too. NHTSA offers a tire recall lookup on its recalls page.
• Tires degrade over time. NHTSA recommends that a full-size spare not be used as a replacement for a worn tire except in emergencies, because spares age even when unused.
• Tires carry government ratings. The Uniform Tire Quality Grading System compares treadwear, traction and temperature resistance, and the ratings appear on the sidewall.
• TPMS comes in two types. A direct system uses sensors in the tires; an indirect system uses wheel speed and other vehicle sensors. Either one lights the dashboard symbol when pressure drops below the acceptable level.
• Tread wear matters. Tires have tread wear indicator bars in the grooves; when the tread is worn down flush with them, the tire needs replacing.

Tire age guidance varies by vehicle and tire maker, and we did not find a single federal age cut-off on NHTSA's page. Check your owner manual, read the date code guide, and have older tires inspected.

Heat speeds up the chemical wear inside a lead-acid battery, so batteries in hot inland climates tend to wear out sooner. We have not found a public, local figure for how much sooner, so we do not quote one. A load test tells you where your battery stands. See battery replacement.

Misalignment makes tires scrub sideways as they roll, which wears the tread unevenly and can shorten tire life. We do not publish a percentage because it depends on how far off the angles are and how far you drive. The alignment printout shows your actual numbers before and after.

Come in, or handle it yourself?

• Yourself: watch for a slow crank on hot afternoons, and look at the inner tire edges when you check pressure.
• Come in: for a battery load test before summer, a steering wheel that sits off-center, or a car that pulls on a flat road.

These are conditions, not statistics. Local drivers spend time on the I-10 and SR-210, where traffic can go from freeway speed to a crawl. Summer heat is long, rain is rare, and roads climb toward the foothills. Together, that means frequent braking, hot tires, hard-working AC systems and batteries that live in heat.

We use those conditions to shape advice, such as checking the battery and AC before summer and checking tread before the first rain of the season. We do not attach made-up numbers to them.

Some numbers readers ask for are not published here, and we would rather say that than guess.

• Prices and price ranges: not published until the service advisor signs them off. Your written estimate shows the exact number.
• Review ratings: the shop is building reviews under its current name. Ratings earned under the lot's former name are not shown as ours. See reviews.
• Shop volume, turnaround averages and comeback rates: not measured and published yet.
• Years in business and staff counts: not published on this page.
• Widely repeated figures we could not confirm: if we could not find a figure on the agency page itself, we left it off, even when other sites repeat it.

If one of these becomes available and checkable, we will add it with a date.