I'm certain you meant to say E 245 75 17 in your last line.
255 65 17 = 94.408 inch circumference (stock)
245 75 17 = 98.863 inch circumference (new)
265 70 16 = 96.152 inch circumference (spare)
Ideally, you would replace the spare as well with a E 245 75 17 more so for the E rating.
That being said it is closer to the circumference of the new tires than the stock tires.
It is not a good idea to run a mismatched spare for a long period of time. If you have to, it is best to use it on a non drive axle (i.e. the front end, 4X4 not engaged).
If you have extra money and plan to do a lot of offload, replace the spare if not, you should be fine, just don't run it for any length of time.
explanation: (cut and paste from Popular Mechanics)
because the spare is smaller than the opposing wheel on the same axle, it must turn faster to keep up with the speed of the car, making the differential work to account for the variation. It's as if the car is constantly in a turn. Leave the spare on long enough and the grease lubricating the differential will begin to break down, accelerating wear between the gears and the clutch plates if it's a limited-slip differential. For all these reasons, manufacturers suggest keeping speeds below 50 mph and using the spare tire only for limited distances if possible.