When measuring the spark plug gap, a __________ should be used.

When measuring the spark plug gap, a wire or round feeler gauge should be used because it provides a precise and consistent measurement of the gap width. These gauges are specifically designed to fit into the gap, allowing you to determine if the gap is within the manufacturer's specified range. Using a feeler gauge ensures accuracy, which is crucial for the proper ignition of the fuel-air mixture in the engine, leading to optimal performance.

It's important to note that the other tools mentioned may not provide the required precision needed for this specific task. A tape measure is typically used for measuring larger distances rather than small gaps. A wrench is intended for turning nuts and bolts but does not offer an appropriate way to measure a gap. Lastly, while a caliper could potentially measure the gap, it is not the standard tool for this purpose and may not be as effective in quickly checking the gap size as the feeler gauge.