The physics
Average speed is total distance ÷ total time. The odometer and the clock give it to you. It is not the average of the speeds.
vavg = total distance / total time
Two equal distances at v₁ and v₂ give 2v₁v₂ / (v₁ + v₂). Two equal times give (v₁ + v₂) / 2.
Average velocity is displacement ÷ time. It follows the straight arrow from start to finish. On a curve it is smaller than the average speed. On a round trip it is zero.
|vavg| = displacement / time chord = 2R sin(θ/2) arc = R θ