Spark Plug Gap
 

Just wondering how critical gapping new plugs are vs installing straight out of the box in regards to power and mpg's.

Very, you need the correct gap.

Most of the time the plugs that I get already are set to the correct gap (well the specified gap for the truck).

I used to take the word of part clerk and telling me that it is already gapped to the right specs for my vehicle.

One day I checked the gap just to make sure and not all of them have the same gap and I have to adjust it myself.

Everytime I order spark plugs, I always tell them to write the gap specs on the side of the spark plug box and check it for myself.

It can make a world of difference. The gap range is usually about 10% of the gap, and I always set it to the LOW end of the gap, I've found that the engine typically runs better that way. The voltage required to jump a given gap is a linear function, so a 10% increase in gap size requires 10% more voltage to jump the gap. But you should have plenty of voltage to spare if your ignition system is in working order. And, as the plugs wear, that gap opens up (albeit very slowly), so you may as well start on the low end of the range.

Jason