What exactly is average speed? Can you provide an example?
Is the formula r = D/t?
That formula can be used in this sense: If you traveled $D$ miles in time $t$ hours, you could say your average speed for the trip was $r = D/t$. That measure doesn't take into account any time taken for any stops or changes in speed along the way, but it does tell you that if you had traveled at exactly that average speed for the whole trip, you would get there in the same $t$ hours. If you have varying velocity it becomes a calculus problem.