7 laps on 60' lines is 1/2 mile. so the math is fairly simple

(7 * 2* pi *60 = 2639 feet, 1 mile = 5280' so 1/2 mile = 2640', close enough

)

A bit more math and you get that speed in mph = 1/2 * 3600/ time for 7 laps = 1800 / time for 7 laps.

Notable numbers for combat models:

24 sec = 75 mph

22.5 sec = 80 mph

20 sec = 90 mph

18 sec = 100 mph

15 sec = 120 mph

(Same numbers work for 8 laps on 52.25' lines)