speed limits
Us 131 south from Cadillac is true interstate-70 mph until you get to the north side of Grand Rapids. US 127 is mostly interstate, except for about 20 miles from north of St Johns to Ithaca, which was recently changed to 65 mph (from 55), it is still 4-lane with some crossroads. If it matters both roads are in very good condition and 131 is a little more scenic. As in the farms are hillier.
Richard