Spark plug gap is mainly determined by the power of the ignition and the mixture. A good starting point on late model vehicles is .045 and should be set with a plug gap gauge. Turbo or supercharged engines should use a smaller gap--.035 to start with.
All vehicles should have a placard under the hood which lists the timing, idle rpm setting and spark plug gap. Ignitions will vary from one car to another as to the amount of available power from the coil. The larger the spark plug gap, the more power necessary to jump the gap.
The larger the gap, the more power due to a hotter ignition of the fuel mixture resulting in more complete combustion. One method used by racing enthusiasts is to increase the spark plug gap by .005 and drive the vehicle. Accelerate and listen for an ignition miss. Continue to increase the gap until you experience a miss, and at that point reduce the gap by .003. This interpolation would give the maximum power and fuel consumption.
Another point on spark plug gap is that the gap increases with mileage as the plug wears. It varies somewhat with the variation of engines, but this is why the gap on all plugs should be checked every 30,000 miles.
Spark plugs are a window into the condition of the engine and the mixture. They have been used from the model T to present day by mechanics to determine how complete the combustion is and the degree of timing. By inspecting the tip of the spark plug, the mixture, condition of rings and valve guides, and the degree of timing can be seen. Looking closely at the ground strap or electrode for color and condition is a major factor and the same with the porcelain center electrode.
A ground electrode that looks melted or rounded indicates too lean a mixture and possible detonation. A light gray color at the 90-degree bend in the ground electrode is ideal. The same color in the middle of the electrode means the mixture is slightly lean with would reduce power output. If the color is concentrated on the tip of the ground electrode, the mixture is too rich.
The center porcelain is also a big indicator. The tip closest to the ground electrode indicates the idle mixture. White is too lean, black is too rich and blistered off white is detonation. Optimum is light gray. The center of the porcelain represents the midrange or part throttle like cruising mixture and the deepest part of the plug indicates the mixture at full throttle.