Campbell,
I'm not sure you understand the naming conventions with tires; not all 70 profile tires are the same height. The first number is section width, in millimeters (the part of the tire most typically in the middle between the rim and the road, where the tire bulges, measured across the tire) The second number is the profile (the distance from road to rim); this is a percentage of the first number. So in the case of 255/70/16s, the profile is 70% of 255 mm. To calculate the size, I suggest you figure them out on your own, as many of the online calculators drop significant digits, and don't accurately represent the sizes IMO. Simple enough procedure:
Convert 255 mm to inches: Multiply by 0.0393701 --> 10.039 inches, this is your section width.
Next, mutliply this number by the profile, in this case, 70%. --> 7.0273 inches, this is the profile.
The overall height of the tire is calculated by adding the profile to the rim size, twice (since the tire goes all the way around) This tells you your final tire size:
255/70/16 is closest to 30x10x16
Not sure if this is what you wanted, you may already know this; but I wanted to point out that a 235/70/16 tire is very different from a 325/70/16 ...
-Alan
