I am trying to get an average from following sql statement but it returns wrong number:

v_mileage = 50
min(a_startmile) = 30
v_mileage-(min(a_startmile) is equal to 20

(vehicle.v_mileage-(MIN(a_startmile))/6.5) as average

gives me

45.3846

which is wrong.

Is there a trick to get an average in an sql statement?