Gas Mileage Myths That Cost Drivers Real Money

Premium gasoline has a higher octane rating, which resists engine knock in high-compression engines. Most passenger vehicles on the road are designed for regular 87-octane fuel. The EPA has found no evidence that premium gasoline improves mileage or performance in engines that don't require it. Check your owner's manual: if it says "recommended" rather than "required," regular fuel is the cost-effective choice — and the performance difference is negligible. Paying 20–50 cents more per gallon for no gain adds up quickly.

This advice made sense for carbureted engines common before the mid-1980s. Today's engines use electronic fuel injection and computer-managed systems that reach operating temperature far more effectively under light load than at idle. The U.S. Department of Energy notes that idling gets zero miles per gallon and can waste a significant amount of fuel over a winter season. In most conditions, 30 seconds is sufficient before gentle driving. Extended idling also deposits fuel residue in the engine — the opposite of warming it up properly.

There's a kernel of truth here: underinflated tires hurt mileage by increasing rolling resistance. But inflating beyond the vehicle manufacturer's recommended PSI is a different matter. Overinflated tires wear unevenly in the center of the tread, reduce the contact patch with the road, and make the vehicle more susceptible to impact damage. The tire sidewall maximum is not a target — it's an engineering ceiling. The correct PSI for your vehicle is found on the driver's door jamb sticker. Proper inflation, not over-inflation, is where the real efficiency gain lies.

At low speeds — city driving, for instance — switching off AC and opening windows does reduce fuel consumption measurably. However, at highway speeds above roughly 50 mph, open windows increase aerodynamic drag to a degree that erodes or eliminates those savings. The AC system does create an engine load, but modern systems are more efficient than in previous decades. The practical guidance from engineering research suggests using windows in city traffic and AC at highway speeds — but the difference is relatively modest either way, and obsessing over it distracts from higher-impact behaviors like driving smoothly.

A small engine working hard under heavy load can consume more fuel than a larger engine operating efficiently at a lower percentage of its capacity. Turbocharged small-displacement engines, for example, can be thirsty when pushed. EPA fuel economy ratings reflect standardized test cycles, and real-world results vary with driving behavior, load, and terrain. A large SUV with a modern 8-speed transmission may outperform an older small-engine vehicle on a highway trip. The EPA's fuel economy label for each vehicle is a more reliable guide than displacement alone.