The average distance between and Mars is the average Sun-Mars distance 1.523679 AU = 2.279391 E+08 km, nearly.

The average Earth-Sun distance AU = 149597871 km. The average Mars-Sun distance is 1.523679 AU = 2.279391 E+08 km. The difference is nearly the least Earth_Mars distance, when both are on the same side of Sun and are nearly aligned (conjunction) When they are on the opposite sides and are nearly aligned (opposition), the distance is the sum. This is nearly the greatest. So, the average in-between distance is the mean of the sum and difference = Sun-Mars distance.. .