Stupid question, but I'm about to blow a chunk of money on silicone couplers and I'm looking for the answer. My IC's input/output sizes are 2.5" if measured by the OD, and 2.25" if measured by ID... Annoying. Seems to make more sense if measured by OD but I'm just making sure.

Anyways, help is appreciated.

 

Stick a coupler on your throttle body. If it goes on, it's 2.5". If it doesn't, it's 2.25.

Most ICs are 2.5" in and out.

 

You sure on that ID? Typically, they use 16 gauge on the input/output. 16 gauge is .065", so that's about 1/8" difference between the OD and ID.

In the end, all tubing is measured by OD. Pipe is measured by ID(kind of). If your tubing measures at 2.5" OD, then you should buy a 2.5" coupler.