Fuel Economy Myths That Could Be Costing You Money at the Pump

Premium gasoline has a higher octane rating, which matters only if an engine is designed to use it. Higher octane resists premature ignition (called engine knock) in high-compression engines. In a standard family vehicle rated for regular 87-octane fuel, the engine's computer is already calibrated for that fuel. Pouring in 91-octane premium does not produce more power or better mileage. The EPA and most vehicle manufacturers confirm this. Check your owner's manual: if it says 'regular fuel recommended,' premium is money spent for no gain.

This habit made sense for carbureted engines common before the 1980s, which needed time to stabilize fuel delivery in cold air. Today's fuel-injected engines are managed by onboard computers that adjust the fuel mixture automatically. The U.S. Department of Energy has noted that idling a modern engine for more than 30 seconds before driving is generally unnecessary. An easy, low-speed start for the first minute or two warms the engine, transmission, and wheel bearings more effectively than sitting still, and uses less fuel in the process.

At low city speeds, this myth has more truth to it: A/C compressors do place a load on the engine, and at slow speeds, the aerodynamic penalty from open windows is small. However, as speed increases above roughly 45 to 50 mph, the drag created by open windows increases substantially. The EPA has acknowledged that the comparison is speed-dependent and that neither option is always the winner. A practical approach: use windows at low city speeds, and consider A/C on the highway, especially if your vehicle has a modern, more efficient compressor system.

There is a physical basis for this idea: a harder tire has less rolling resistance. However, the fuel economy difference between correctly inflated and slightly overinflated tires is marginal in practice, and the downsides are real. Overinflated tires wear faster in the center tread, reduce the tire's contact patch with the road, and can make handling less predictable, particularly during emergency maneuvers. The correct approach is to keep tires at the pressure shown on the door jamb sticker, check pressure monthly (tires lose roughly 1 PSI per month naturally), and replace valve caps to slow air loss.

Gasoline does expand slightly with heat, so the idea has a theoretical basis. But fuel is stored in underground tanks where ground temperature remains relatively stable regardless of the outside air temperature or time of day. The temperature difference between early morning and midday fuel at the pump is negligible, and any density difference translates to a fraction of a cent per gallon. Consumer Reports and the EPA have both addressed this myth. The time of day you fill up does not affect how much usable energy you get per gallon.

Drafting does reduce aerodynamic drag, a principle used in competitive cycling and racing. But achieving a meaningful draft effect requires a following distance that eliminates the reaction time needed to brake safely. At 65 mph, a typical passenger vehicle needs over 200 feet to stop in an emergency. Driving within that margin behind a loaded semi-truck is unsafe and illegal in many states. Effective and safe hypermiling practices do exist: smooth acceleration, coasting to stops, and maintaining a steady highway speed with cruise control all reduce fuel consumption without putting the vehicle in a hazardous position.