If your car suddenly feels thirstier filling up more often without a change in driving habits it’s worth checking something small but critical: the gap between the electrodes on your spark plugs. An excessive spark plug gap means that space is too wide, making it harder for the spark to jump across consistently. That inconsistency doesn’t just cause rough running it directly reduces fuel efficiency, sometimes by 3–5% or more. You’re not burning fuel as completely, and the engine compensates in ways that waste gas.

What does “excessive spark plug gap” actually mean?

Spark plug gap is the measured distance (in millimeters or thousandths of an inch) between the center electrode and the ground electrode. Every engine has a manufacturer-specified gap usually between 0.028" and 0.060", depending on ignition system design. When that gap is set too wide say, 0.070" on an engine designed for 0.045" the ignition coil must generate significantly higher voltage to fire the plug. If the coil can’t reliably deliver that extra voltage, the spark either misfires or fires weakly. That incomplete combustion leaves unburned fuel in the exhaust, which the oxygen sensor detects and prompts the engine control module to add more fuel in response. The result? Worse fuel economy, even if you don’t notice obvious symptoms like hesitation or stalling.

Why would someone end up with an excessive spark plug gap?

It usually happens during DIY spark plug replacement or cleaning. Using pliers or a gap tool incorrectly can stretch the ground electrode outward. Some people assume “a little wider is better for performance,” but that’s outdated thinking modern coil-on-plug systems are tuned for precise gaps. Others skip checking the gap entirely, trusting the plug is pre-gapped correctly (many aren’t, especially economy brands). And over time, normal wear can widen the gap slightly but it rarely widens enough on its own to cause noticeable fuel economy loss unless the plug is severely aged or damaged.

What symptoms go along with poor fuel economy from excessive gap?

Fuel economy drop is often the first sign but it’s subtle and easy to miss. More noticeable clues include occasional stumbling under light acceleration, a faint “ping” or hesitation when climbing hills, or a rough idle that comes and goes. These happen because some cylinders fire inconsistently. You might also see a check engine light with codes like P0300 (random misfire) or cylinder-specific codes like P0302. If you’ve already ruled out other causes, it’s smart to run a targeted misfire diagnostic instead of replacing parts blindly.

How do you spot an excessive gap before it hurts your MPG?

Visual inspection is quick and reliable. Remove one plug, clean off carbon deposits gently (don’t sandblast or wire-brush aggressively), then measure the gap with a proper wire-type gap gauge not a coin or blade-style tool. If the measured gap is more than 0.005" wider than the spec listed in your owner’s manual or service guide, it’s excessive. You’ll often see signs like a rounded or eroded ground electrode tip, or visible pitting on the center electrode. For step-by-step guidance, see our visual inspection procedure.

Common mistakes people make when adjusting spark plug gap

• Bending the ground electrode sideways instead of straight back toward the center this risks cracking the ceramic insulator or weakening the metal.
• Using needle-nose pliers to force the gap open or closed this deforms the electrode and changes its heat range behavior.
• Assuming all plugs in a set have the same gap even new plugs vary slightly, so check each one individually.
• Setting the gap based on a friend’s car or an old forum post instead of your vehicle’s exact year/make/model/engine code.

What should you do next?

First, confirm your engine’s correct gap using your owner’s manual or a trusted repair database like Mitchell OnDemand or ALLDATA. Then inspect and measure every spark plug even if they’re new. Adjust only with a proper gap tool, applying gentle, even pressure. If you’re experiencing hard starts along with lower MPG, an excessive gap could be contributing; hard starting is another common effect of inconsistent sparking. Replace any plug with cracked porcelain, heavy erosion, or oil fouling no amount of gapping fixes those.

Quick checklist before your next fill-up:

1. Find your engine’s factory-recommended spark plug gap.
2. Pull one plug and measure it with a wire gauge.
3. If it’s more than 0.005" over spec, check the rest and adjust or replace as needed.
4. After reinstallation, reset any long-term fuel trims by disconnecting the battery for 15 minutes (or follow your vehicle’s specific procedure).
5. Monitor fuel economy over three full tanks not just one to confirm improvement.

For reference, the Society of Automotive Engineers (SAE) notes in SAE J549 that spark plug gap tolerances tighter than ±0.002" are required for optimal combustion stability in modern engines underscoring why even small deviations matter.