Since most Japanese products (like exhausts for instance) use mm instead of inches, here's a link to a conversion chart that will help you better understand what you're looking at: http://www.reticles.com/conversion.htm

A 2.5-inch exhaust is roughly anywhere from 63 to 65 mm, while a 3-inch exhaust is around 76 to 78 mm.

If you're a turbo guy, go with no less than a 3-inch exhaust.

 

Pretty much the easiest way to do it by te following formula:

convert MM-CM. 10mm=1cm
So the best way to figure out something it to figure out the CM then divide that by 2.4 which is a rough est. of one inch., i.e.

95mm---9.5cm/2.4=3.96in

 

Makes some sense right?

 

The conversion is 25.4 mm to 1 inch....
therefore a 3" exhaust system will tanslate to 3x25.4=76.2mm

 

"Excuse me HKS, Yes, I'm an American and I think I need a 76.2mm exhaust for my Evo...." huhu

 

Being educated using the Metric system, I just cannot think normal using inch/feet/pound and all the other measurements...

 

This website's great: http://www.onlineconversion.com/

 

here's another.

http://www.flash.net/~lorint/lorin/convert.htm

 

Simple...1 inch = 25.4 mm...exactly.