2.5"

I am sure.....I measured them.

 Calvert Racing says 2 3/4", so I'm still wondering.
 
 SKeown

2.5 on the 20 tubes as was said.
the spring/shock plate holes are 3" center to center.

remember you need a little bigger U bolt to be able to slip over the 2.5" tube I think that is why Calvert says 2 3/4 Course I have only used the 3" tube 9" rear so I really can't be sure.
 

 I told Calvert Racing that the OEM shock mounting plates had 3" C to C mounting holes, they said that 2 1/2" U bolts would be needed in that case. I will know in a few days, for sure.
 
 SKeown

The U bolts on my car are not bent square like a u bolt from a hardware store, they may be 2.5 at the loop but flare to around 2.75 at the bottom . maybe a little more.
I had to pinch them together a little to get them in the shock plates.
 
2.5 or 2.75 it is not going to make much difference. I think ether one will work.
But what do I know.