07 F-150 tire size ?

[Drfloppy]

Hello guys, I have a 07 F-150 that has stock 235/75-17 tires on it. They aren't the best in the snow, and I was thinking about changing to a better tire. I found some tires that I think are a good deal, but I don't know what the difference in size will do. Thats where you smart guys come in. The new tires are 255/75-17.
I understand they will be a little taller, but how much will it screw up my speedometer? And in which way ? What other neg effects will these tires have on my truck. Thank you very much for your help.
Dale