I did a bunch of research and calculations a few years ago, using the most accurate comparison I could find -- the revolutions-per-mile data from tire company web sites. You'll find that tire sizes are indeed nominal, and the actual revolutions per mile vary depending on manufacturer and tire model, even for the "same" size. So the following is approximate.
Going from 215/75R15 to 30x9.50R15 is a change of approximately 6%.
Going from 215/75R15 to 31x10.50R15 is a change of approximately 10%