IRC ratings are produced as a Time on Time (ToT) number, but you can easily convert an IRC rating (TCC) to Time on Distance (ToD) if preferred:

ToD = 600 / TCC

examples:
TCC = 1.100.  ToD = 600 / 1.1 = 545.5 seconds per mile.
TCC = 0.900.  ToD = 600 / 0.900 = 666.7 seconds per mile.