* Do not fit an odd-size tire on only one or only three wheels, except in case of a get-you-home emergency, because the handling and braking could be weird and dangerous.
* If your truck is a 4x4 or all-wheel-drive, ALL four wheels MUST be exactly the same size (width and diameter), else the transmission will wind-up internally for rapid wear or breakage.
* If your truck is 2WD, you could put a pair of 255s on the front axle and a pair of 265s on the rear axle, to preserve load-carrying, handling and braking.
You have two potential negative issues with going to a 255 width tire: a smaller contact patch and the possibility of not having the necessary load carrying capability for your vehicle. You might be able to safely live with the lesser contact patch but if the load index rating of the 255/70R17 tire is less than the original equipment tire it should not be used.
its the same size diameter ..just a little bit skinnier
No one but you will ever notice, but you do need to do both sides on a axle.
