As other have said, just let the receiver calculate it. Your eTrex can display that value if you select it as one of your data fields. Also, it would be interesting to see if Strava will upload an eTrex file. If so, then Strava will give you that data point also. Good unit you have there. Haven't heard of anybody using an eTrex as a bicycle computer, but it sure has the capabilities.

- - -

p.s. On one of the display screens of my GPS receiver, I have "average moving speed" displayed. It has been a motivational tool for me. When about 2/3rds into a planned ride, I might check that value and if I see it just below, or right at a benchmark value, then I'll put in a little more effort to get and stay above that. For example, near the end of a ride, but with enough miles left to make a difference, if I see my average moving speed at, let's say, 14.8 mph ... then I'll stop slacking and try to get it at or above 15.0 mph, then try to keep it there till the end of the ride. Sometimes it works ... sometimes not. Depends on how much I'm hurting, how many miles are left, and which way the wind is blowing.