The i.d. of the two is very close. the o.d. of the 2.5 is, well 2.5. The wall thickness of my 2.5 is 1/8 so that is why it is wider overall. It is not an exact fit but a little weld-on 16 would make it just right. Your choice, I have a 6 ft peice lying around that I don't forsee ever needing all of. I can cut it to whatever length you guys want and ship it USPS Priority for $5.00. I measure 3 and 5/8" for the riser.