245-75-17 = 30.47" overall diameter
255-70-16 = 30.06" overall diameter
Difference of .41" overall diameter
From what I have read in the past, for every 2/10's of an inch in diameter, you change the speedometer accuracy ~1%. So, according to the measurements here, there is a change of ~2%
Your speedometer now may read 2% faster at speeds. Which will now show ~1 mph faster at 50 mph(51 mph), ~1.5 mph faster at 75 mph(76.5 mph) and ~2 mph faster at 100 mph(102 mph). All insignificant! At least you'll be showing a faster speed reading for the purpose of speeding tickets.
Your odometer may also gain more miles thus, throwing off your MPG should you keep track of MPG. But, I doubt it as I don't see any differences between my:
Stock Steelies used for winter only, 205-65-16 = 26.49...Although my Pirelli P4 A/S in this size spec shorter @ 26.3 according to Pirelli/TireRack.
Summer only tires, 225-55-17 = 26.74...A difference of ~ .44"
Actually showing my speed to be slower than I am actually going so, I need to be watchfull.