thanks for that.
Did a search, seems to have answered my question. Attesa will tolerate up to 3% diameter difference.
Based on this caculator http://www.1010tires.com/TireSizeCalculator.asp and factoring in the wear on the fronts i think i might be able to get away with fitting the 255 40 17.
If not i have a 4d controller, so ill turn if off all together