Wheel Offset
 

Good day all I’m looking at a set of rims from OE wheels there 18”. 10.5 rear and 9.5 front the offset the recommend for a 1996 c4 is 56 rear and 54 front is this correct?
https://cimg5.ibsrv.net/gimg/www.cor...0d573c877.jpeg

I would go with 50mm on the rear since the 17 x 11 GS had 50mm. 56mm may work but you would have to fit wheel/tire assembly to be sure.

To end the suspense, the direct answer to your question is that both of the offsets being recommended will be fine on your car.

The 18x10.5 et56 will fit. It gives you 7.95" of backspacing, and it will fit just fine on a C4, front or rear. You might prefer the look with a bit less offset or a 1/4" spacer to set it out just a bit further, but either way will fit well. For reference, the 11" et50 fitment that Kevova mentions gives backspacing of 7.96". It also fits fine on the front or rear. It would add the entire extra 1/2" of width to the outside edge.

The factory offset for 9.5" wheels was 56mm. You have plenty of space to play with different offsets at that width. 54mm is close to the stock offset, and it's probably the most offset you'll ever want at that width. But you could try less offset or up to a 1/2" spacer if you wanted to play around to find the best look.

Conversation regarding spacers etc ain't actually relevant for the OP. I believe the OP mentions 10.5 @ 56 and the 9.5 @ 54. Neither 'if factual' are issues for the OP and the OP has a newer than '88 C4!

***11 @ 50 and 10.5 @ 56 would be identical I believe for back-spacing and so long as the OP has no issues with inner clearance outer rim edge is very close to maybe original.

It's relevant if those are the only offsets he can get in the wheels he wants to buy! But yes, as you and I both posted, the offsets mentioned will work just fine. I note that I got them backwards in my response, so I will make the appropriate edits. Regarding inner clearance, that's determined by backspacing and the tire's section width. On a 88-96 C4, ~7.96" of backspacing with a 315 tire is about the max one can go without having issues with inner clearance. The sizes and offsets mentioned by the OP should be fine.

To the OP, I forgot to mention this, but have you considered running the 10.5 et56 both front and rear? That would fit well up front, too, and gives you the option of running a 285/35 or 295/35 tire at all four corners.