10.3 Tires, Equipment, and Fuel Economy

10.3 Tires, Equipment, and Fuel Economy

Quick Answer: Tires can lose up to 1 psi every month. Check all tires, including the spare, once a month or before a long trip. Use a trusted pressure gauge. Recommended psi is on the sticker inside the driver's door jamb or in the owner's manual — do not compare to the psi on the tire sidewall. Check cold (before you have driven, or at least three hours after). If the reading is above the number, let air out; if below, add air. Penny test monthly or before a long trip: hold Lincoln's body; put Lincoln's head into a groove at the lowest tread. If any part of Lincoln's head is covered, you have the legal and safe amount of tread. If tread is below that (approximately 1/16 inch), grip in adverse conditions is greatly reduced. Fuel consumption increases steadily above 45 mph. Passenger cars and light trucks use about 50 percent more fuel at 75 mph than at 55 mph. Drive sensibly, observe the speed limit, avoid roof cargo, remove excess weight, avoid rapid acceleration and sudden stops, maintain the engine, inflate tires, use the recommended oil, and plan and combine trips. A cold engine uses more fuel; several short cold-start trips can use twice as much fuel as one longer trip covering the same distance.

Tire pressure, tread depth, and fuel economy sit together in Indiana Driver's Manual Chapter 7, just before uncertain weather. The knowledge exam will ask where the correct psi lives (door jamb, not sidewall), when you measure (cold), whether the spare counts, and what the penny test actually proves. Fuel questions cluster on 45 mph, the 50 percent jump from 55 to 75, and twice the fuel for stacked cold-start errands. Occupant-protection equipment (seat belts and child restraints) is the next chapter — do not mix those rules into this one.

Tire pressure: monthly, including the spare

Tires have been known to lose up to 1 pound per square inch (psi) every month, so check all tires, including your spare, once a month or before a long trip.

A slow leak of 1 psi per month is easy to ignore until a rainstorm or a winter morning. Underinflation builds heat, wears the outer tread, and is the bald-tire cousin of the hydroplaning problem in the weather section. The spare is part of the monthly job. A doughnut that has sat flat for two years is not equipment; it is a hope.

How to check, in the manual's order:

1. Purchase a trusted pressure gauge. The gas-station hose gauge is not the standard.
2. Open the car door. On the inside jamb there should be a sticker with your vehicle's recommended psi.
3. Check the tires "cold" — before you have driven, or at least three hours after you have driven.
4. Insert the pressure gauge into the valve stem.
5. The gauge will "pop" out and show a measured number.
6. Compare that number to the psi on the sticker inside the driver's door or in the owner's manual. DO NOT compare to the psi on your tire's sidewall.
7. If your psi is above the number, let air out until it matches. If below, add air (or have a retailer help you) until it reaches the proper number.

The sidewall number is a maximum the tire can hold, not the pressure your vehicle needs. Using it overinflates the tire for that car. The door-jamb sticker (or the owner's manual) is the vehicle specification. "Cold" means not heat-soaked from driving. Checking immediately after a highway run will read high; you then let air out and leave the tire low once it cools. Wait at least three hours, or check before the first trip of the day.

Tread depth: the penny test

Once every month, or before a long road trip, check your tires for wear and damage. One easy way to check for wear is the penny test.

1. Take a penny and hold Abraham Lincoln's body between your thumb and forefinger.
2. Select a point on your tire where the tread appears the lowest and place Lincoln's head into one of the grooves.

If any part of Lincoln's head is covered by the tread, you are driving with the legal and safe amount of tread. If your tread gets below that (approximately 1/16 of an inch), your car's ability to grip the road in adverse conditions is greatly reduced.

Lincoln's head goes in, not the memorial building, and not the whole coin flat on the rubber. You test the lowest groove, not a healthy block you picked because it looks fine. Any part of the head covered is a pass. If the entire head is visible, you are at or under about 1/16 inch, and rain, slush, and snow grip collapse — the same conditions the weather section warned you not to meet on bald or badly worn tires.

Do the penny test on each tire, and look for damage (cuts, bulges, uneven wear) while you are there. Uneven wear is often the monthly pressure job you skipped. This section is equipment: pressure, tread, and the spare. It is not a second weather lecture, but bald tread is why hydroplaning gets worse.

Fuel economy: speed, load, and a cold engine

Fuel consumption increases steadily above 45 miles per hour, with passenger cars and light trucks using approximately 50 percent more fuel traveling at 75 miles per hour than at 55 miles per hour.

45 mph is the speed where the climb starts in earnest. 75 versus 55 is the exam pair: about half again as much fuel, not twice, not 10 percent. A rural interstate posted 70 is already well above 45. Observing the speed limit is both a Chapter 7 speed rule and a fuel rule.

How to maximize fuel economy, from the manual:

• Drive more efficiently by driving sensibly, observing the speed limit, avoiding hauling cargo on your roof, and removing excess weight from your vehicle.
• Avoid rapid acceleration or sudden stops.
• Keep your vehicle in shape by properly maintaining your engine, keeping your tires properly inflated, and using the recommended grade of motor oil.
• Plan and combine trips. Your fuel economy is worse when your engine is cold than when it is warmed up. Several short trips taken from a cold start can use twice as much fuel as a longer, multipurpose trip covering the same distance.

Roof cargo is a wind brake. Extra weight is extra fuel on every hill. Jackrabbit starts and panic stops dump fuel that a smooth drive would have kept. Soft tires (the monthly 1 psi leak) raise rolling resistance. The recommended oil grade is part of "keep the vehicle in shape," not an optional brand preference.

The cold-engine rule is the one people miss. Three separate five-mile cold starts — home to store, home to school, home to gym — can burn twice the fuel of one 15-mile loop after the engine is warm. Plan and combine trips. That is equipment-and-fuel, not a lifestyle lecture.

Indiana scenarios

Saturday in the driveway. It has been a month. You check every tire plus the spare with your own gauge, before you drive, against the driver's-door sticker. The sidewall says a higher number; you ignore the sidewall. One tire is low: you add air. One is high from yesterday's heat? You wait until it is cold (or three hours after driving), then decide.

Penny at the lowest groove. Lincoln's head goes in. Part of the head is covered — legal, safe tread. On the next tire the whole head shows — you are at or under about 1/16 inch, and rain grip is greatly reduced. That tire does not belong on I-65 in a storm.

I-65 at 75. You are using about 50 percent more fuel than you would at 55, and you are well above 45, where consumption increases steadily. Observing the posted limit is still required; this chart explains why 75 is expensive, not why you may pick a speed.

Three cold errands versus one loop. Home to the pharmacy, home to school pickup, home to the grocery — three cold starts — can use twice the fuel of one combined trip covering the same miles. You plan the loop.

Roof bin and a trunk full of unused tools. You avoid hauling cargo on the roof when you can, and you remove excess weight. Soft tires from a skipped monthly check waste still more fuel.

Exam traps

• Setting pressure from the sidewall instead of the door-jamb sticker or owner's manual.
• Checking tires hot and bleeding air, or skipping the spare.
• Forgetting the 1 psi per month leak and the monthly / before a long trip schedule.
• Putting Lincoln's body into the groove, or testing the best tread instead of the lowest.
• Thinking the penny test fails unless the entire head is covered; any part of the head covered is legal and safe.
• Treating 1/16 inch as "still fine in rain." Below that, adverse-condition grip is greatly reduced.
• Memorizing twice as much fuel at 75 versus 55 (the 75/55 fact is 50 percent more; twice is the cold short-trip fact).
• Ignoring 45 mph as the speed where consumption increases steadily.
• Adding seat belts or child-restraint rules to this chapter — those are next.

Door sticker, cold tires, spare included, Lincoln's head partly covered, 45 mph, 50 percent more at 75 than 55, twice the fuel for stacked cold starts.