I have 205/40/17 and it says my speedo should be off .1% but its def off by more....unless those things coppers set up on the side of the road to say "Your speed is" are off cause this summer I did like "40" by one according to my speedo, but that thing said 37 Oh and my car only had 35k or so on it at the time, so I dont think the speedo would be off yet