The exhaust pipe diameter on a 1997 Ford Ranger with a 3.0L V6 isn't a single, universally consistent size. It varies along the length of the exhaust system. The diameter at the engine's manifold will be smaller than the diameter at the tailpipe. You'll also find variations depending on whether the truck had any aftermarket exhaust modifications.
To find the exact diameter(s) you'll need to measure it yourself at the specific point along the exhaust you're interested in. There's no readily available single definitive answer.
