This is important because the tachometer signal comes from the alternator on all 6.5L diesel trucks. If the pulley size is wrong, then the tachometer reads wrong.

Most aftermarket or reman alternators come with a more common pulley that was used on GM gas engines and is the wrong size for a 6.5L application. It doesn't really hurt except for if you want your tachometer to read correctly.

If your tach shows close to 1000 RPM at hot idle, it’s safe to assume you have the WRONG pulley since the computer should be commanding 600-700 RPM

You can also measure across the face of the pulley and reference the next section of this article.