All measurements in MM, Rim 15" = 381MM

235/70 Original Equipment Tires:
width profile rim   total   %from stock
 235    70    381    710        0
 275    60    381    711        0.1  ie exact

So, retrofitting 275/60s on 15X8 to a car originally equipped with 235/70 is the way to go.

225/70 Original Equipment Tires:
 225    70    381   696         0
 255    60    381   687        -1.3  very close
 275    60    381   711         2.2  a little off

2.2% would mean that your speedometer would read 100 but you'd be doing 102.2.  Whereas -1.3% would mean your speedometer would read 100, but you would be doing 98.7.

In conclusion, if you are putting the 15X8's on a 235/70 equipped car, definitely go with the 275/60's.  If they are going on a 225/70 car, you could go either 255/60 (under) or 275/60 (over).

Editor's Note:  Caution!  From experience, I have found that 275-60's will rub on the front wheel-well molding with SS springs installed and using Corvette style 15X8 wheels.  I don't have any information on the Suburban/Pickup 15X8 wheels.

Chris Thompson