Two steps to think about
first convert feet into seconds
then convert number of seconds into a time as understood by Excel

Your conversion factor of 1.66666... would be better thought of as 5/3
so feet*3/5 = seconds

next the time bit. If you format a cell as date or time, Excel treats 1as
a day which is 24 hours or 1440 min (24*60) or 86400 sec (24*60*60)
so seconds/24/60/60 formatted as time will display correctly
however because you want mm:ss custom format as [mm]:ss
the [ ] allow the mm to go above 59 without showing hours

If you do want hours [hh]:mm:ss

In full if feet value is in A1
formula is =A1*3/5/24/60/60
Custom formated [mm]:ss

hth RES