It equates to about a 4/6 drop. We currently have a 255/40R18 on the front and a 275/40R18 on the rear of the blue truck. They fit great with no rubbing and do not require a custom offset. I have also seen another 72 C10 with 20in wheels and they seemed to fit great as well but I (personally) prefer the 18 as it allows us to run a slightly taller tire which also improves ride quality. I will try and attach a picture of some trucks for you to see.
Both of the trucks in the pictures have 18in wheels and have the StreetGrip system.
