Ok, the 215 is milimeters. The 75 is aspect ratio for the sidewall. To convert 215/75r15 into a XX x XX x 15, just convert it from milimeters into inches.
First number first, the diameter.
multiply the width, 215, by the aspect ratio, .75(because it's a percent), then by the number of sidewalls 2(top and bottom of the wheel), then divide that number by 25.4 (because there are 25.4 milimeters in an inch). You should end up with
(215*.75*2)/25.4 = 12.7"
then add the wheel diameter
12.7+15 = 27.7" or 28" diameter.
Now the width:
215/25.4 = 8.5"
Alternatively, find the width first, then multiply by aspect ratio then by 2 to find the diameter.
So your tire in a floatation size is 28x8.5x15.
To take a different tire, say a 33x12.5x15 and convert it into metric, do the opposite.
Let's find the width first.
12.5 * 25.4 = 317.5 which would probably round down to a 315. Now we need to find the sidewall height.
Total height minus wheel height divided by two gives you the height of the sidewall.
9" of sidewall.
Aspect ratio is the percentage where sidewall is divided by tread width. So 9/12.5=72. We know there is no such thing as a 315/72r15, so we'll have to round again. 315/70r15 turns into a 32.4" and 315/75r15=33.6" diameter.
Your guess is as good as mine, but I'd bet on the 315/70r15.