Simple offset question
 

The more negative the offset the more poke right? So in essence it gives the illusion of a wider wheel. If this is the case, would a 9.5 -15mm be equivalent to a 10.5 +10? Or an 11+23m

More negative moves the outside edge of the wheel further away from the hub, yes.

And the outer edge of those three wheels will end up in about the same place. (With my quick guestimation math)

*edit* waaaayyy out. Are you going for the super Mario Cart look, our are these just hypotheticals?

Ya me and yoshi are a team. Jokes aside, pandem rocketbunny.

Yes, more negative more poke.
No, divide that in half because half of the wheel is on the inside and half is on the outside.
Offset is from the center line.

9.5 -15 is roughly equal(on the outside) to a 10.5 -3(or -2)
9x +0 = 10x +12
9.5 +34 = 10 +40
so 12mm(1/2 inch) for 1 inch width change
and 6mm (1/4 inch) for 1/2 inch width change