Top 10 Fuel Economy Myths That Are Costing You Money

Myth 1: Warm Up Your Engine Before Driving

False for modern fuel-injected vehicles. The fastest way to warm up a cold engine is gentle driving — not stationary idling. Idling gets zero MPG. Drive gently for the first mile and warm-up is complete in less than a typical idle warm-up period.

Myth 2: Small Engines Are Always More Efficient

Not if they're working harder. A turbocharged small engine under heavy load runs with high boost, high fuel consumption, and poor thermal efficiency. A naturally aspirated mid-size engine at light cruise may actually be more efficient than a small turbo working hard.

Myth 3: You Should Always Fill Up in the Morning

The idea is that cold morning fuel is denser. True, but underground storage tanks maintain nearly constant temperature year-round. The density difference between morning and afternoon fuel is negligible (less than 0.1%).

Myth 4: Replacing Air Filter Dramatically Improves MPG

Only on carbureted vehicles (pre-1990s). Modern fuel-injected engines adjust fuel to actual airflow — a dirty filter reduces performance but barely affects fuel economy. (See our dedicated air filter article for full details.)

Myth 5: Overinflating Tires Saves Fuel

Slightly — but at the cost of traction, handling, and uneven tire wear. Correct pressure as specified by the manufacturer is always optimal.

Myths 6–10

• Tailgating semis safely saves significant fuel (you'd need to be dangerously close)
• Manual transmissions are always more efficient than modern automatics (false for CVTs and modern 10-speeds)
• Turning A/C on highway uses more fuel than windows open (roughly equal)
• Synthetic oil gives dramatic MPG gains (1–3% is realistic, not 10%)
• Premium gas gives better MPG in any vehicle (only in vehicles designed for it)