new tires? Size? 2001 expedition ford

my original tires are 255 70R16 can i go to 265 70R17 on just the back 2 wheels or do i need to repace all 4 wheels at the same time? How much larger of a tire can i use? i just need good tires for mostly road use and occasional towing.

in other words do i have to stay with 16 inch tires?

You can change the size as long as all 4 are the same size. Then you will need to have the PCM calibratred to have the tire new tire size match everything else so the speedometer, odometer and even the ABS to work properly. So weigh the cost and time factor involved versus staying with 16 inch tires including cost of 17 inch wheels.

ok , i have a set of 26570R16 tires available, those should not require any recalibration. is that correct? can i just use those 2 on the back and 2 25570r16 on the front without problems?

The tires need to be the same size. I went from a 225 to a 235 and my speedo was off 7mph at 60 mph. So, replace them all with the same size. If the size is different from original, the PCM needs recalibrated or you need to know the difference in speed. If the front and rear are different sizes, the ABS will also go crazy.

I guess in order to answer your question well I would ask first "why?" do you want to go to 17s.
The other post is absolutely correct in saying that all wheels should be the same as they may cause handling problems not to mention the cost of recalibrating the PCM.
I would stick to the manufacture's wheel size on the sticker inside the driver door jam