In that case: convert to N/S and E/W speeds and average these.

In Excel speak, N/S speed is "=COS(degrees*3.1415926/18

To obtain angle and speed back from the two averages, use

"=SQRT(nsavearge*nsaverage

and

"=180/3.1415926 * ATAN2(nsaverage, ewaverage)"

(I don't have Excel available right now, so please check the angle formula against simple exaple data - maybe you need to swap the parameters)