There's no such thing as a 78-profile tire, is it? It's either 215/70-15 or 215/75-15. I'm guessing 75, so here's my ratio: 1,1192628751335894399352993847665
You don't need all those decimals, of course...
However, few 31" tires are truly 31", they usually cheat you about 1/2 inch of rubber. The tire is also sueezed flat when the car is weighing on it, and different rim widths and air pressure can cause the tire to grow or shrink in diameter.
The best (and really only correct way) to measure a wheel's rolling lengt, is to do this:
Put your car on flat ground. With a chalk or something, draw a line from the tire and down on the ground, leaving a line on the ground as well.
Drive your car until the mark on the tire hits the ground again, after one revolution. Draw another line down to the ground from this line. Measure the distance between the lines. Now you have a quite accurate measure of how far your tire's actually rolling with current load and air pressure. To get it all correct you should do this with your 215's on as well!
I totally understand if it's too much hassle...