Hey Bill, yep it is about 10". I did the same as you are planning and had Stockton Wheel put the artillery wheel on new centers. You do need to get the offsets correct though for 255s. I had that size before these wheels and they were a bit too close to the bed. Occassionally the opposite tire would rub the bed when the other tire went over something like a steep curb or such. Didn't hurt anything since it was flat and going slow. When I did these wheels, I moved them out. They are close to the rim now. No rubbing anywhere so I'm OK. No way I could put anything like cable chains for snow though.
Dennis
