1950 Track was 3.5 miles long
1952 Track was 5.2 miles long
1966 Warehouse Turn re-routed
1983 Backstraight (16-17) re-routed, 4.86 miles long
1987 Turn 1 revised and 2-3-4 loop around Flagpole added, 4.11 miles long
1991 Current Turn 2-3-4-5-6 added, turn 10-11-12-13 revised to current configuration, 3.7 miles long
1997 Turn 8-9 revised to "Fangio Chicaine" in order to provide more of a margin of safety to the regular road traffic adjacent to the track on driver's left. Turn 15-16 revised to current configuration which is mostly ignored as drivers tend to go wide at turn 15.
1998 Turn 7 revised from Hairpin to Safetypin per the request of Mario Andretti and the CART (now OWRS) organization. They only run at the track for purposes of `spring training' or early testing.
see
http://www.marshalspost.com/circuit/80.html for an illustration that has relavance starting with 1991.