If your car’s fuel economy has dropped for no obvious reason like a clogged air filter or low tire pressure it’s worth checking something small but critical: the spark plug gap. An incorrect spark plug gap doesn’t just cause rough idling or misfires. It directly affects how completely fuel burns in the combustion chamber, and that changes how many miles you get per gallon.

What does “incorrect spark plug gapping” actually mean?

Spark plug gapping is the process of adjusting the distance between the center electrode and the ground electrode the tiny gap where the spark jumps to ignite the air-fuel mixture. Every engine has a factory-specified gap (usually between 0.028" and 0.060", depending on make, model, and ignition system). If that gap is too wide or too narrow, the spark either won’t jump reliably or it will be too weak or too short-lived to fully ignite the mixture.

How does an incorrect gap lower fuel economy?

A gap that’s too wide may cause intermittent misfires, especially under load or at higher RPMs. When a cylinder misfires, unburned fuel exits through the exhaust. That’s wasted fuel and measurable MPG loss. A gap that’s too narrow creates a weaker spark with less energy, leading to incomplete combustion. You’ll get more carbon buildup, sluggish throttle response, and lower efficiency over time. Real-world drivers often report drops of 1–3 mpg after installing plugs with the wrong gap, particularly in older distributor-based or coil-on-plug systems.

When do people run into this problem?

Most often during routine spark plug replacement especially if using non-OEM plugs or reusing old ones. Some mechanics skip gapping entirely, assuming “pre-gapped” means “correctly gapped for your engine.” But pre-gapped doesn’t mean “right for your application.” For example, a plug labeled “0.044” might be fine for a 2015 Camry but too wide for a 2007 Civic with a weaker ignition coil. You’ll also see this issue after manually adjusting a gap without verifying it with a proper wire gauge not a coin or screwdriver.

What mistakes make the fuel economy impact worse?

• Using a flat-blade tool to bend the ground electrode instead of a dedicated gap tool this can crack the insulator or weaken the electrode.
• Assuming all plugs in a box have identical gaps even within the same batch, variation up to 0.003" is common.
• Not rechecking the gap after installation, especially if the plug was handled or dropped (a bent electrode changes the gap silently).
• Over-tightening plugs, which can compress the gasket and shift the electrode position slightly.

Can you tell if gapping is wrong just by driving?

Sometimes but not always. Mild over-gapping may only show up as hesitation during hard acceleration or reduced highway MPG, with no check-engine light. Severe under-gapping often feels like sluggish low-end power and increased exhaust odor. If you’ve recently replaced plugs and noticed any of those changes, it’s worth pulling one and measuring the gap with a round-wire feeler gauge. Don’t rely on visual estimates 0.005" makes a real difference in spark energy.

What should you do next?

Grab your vehicle’s service manual or look up the exact gap spec for your year/make/model/engine code not just the plug part number. Use a proper wire gauge (not a blade-type) to measure and adjust. If you’re unsure whether your current plugs are contributing to poor fuel economy, start by checking one plug from each bank or cylinder head. If gaps vary by more than 0.002", that’s a strong sign they need adjustment or replacement. You’ll find more detail about why gaps drift and how ignition system design affects tolerance in our guide on common causes of incorrect gapping. For examples of how over-gapping plays out across different engines, see the vehicle-specific consequences breakdown. And if you’ve widened a gap yourself and now notice hesitation, that’s covered in depth in what happens when you widen the gap by hand.

Quick checklist before installing new spark plugs:

1. Find your engine’s exact gap spec not the plug’s box label.
2. Measure every plug with a round-wire feeler gauge.
3. Adjust only with a proper gapping tool never pliers or a screwdriver.
4. Re-measure after adjustment, and again after gently seating the plug in the head (before final torque).
5. If fuel economy hasn’t improved within two tanks of driving after correction, consider other factors like oxygen sensor health or fuel injector cleanliness.