1min 20s is 80 seconds. 1mi in 80 seconds. 1mi in 55 seconds is 80/55=16/11=1.4545 as fast.
1mi in 80 seconds is 1mi in 80/3600 of an hour, so mph=3600/80=45mph.
1mi in 55 seconds is 16/11 * 45=720/11=65.4545 mph.
Or, 55 seconds is 55/3600 of an hour, so mph=3600/55=720/11=65.4545 mph.
The difference is 20.45 mph approx.