Get a cyclocomputer that allows you to independently reset individual data fields. I do this every year when I go to Tucson AZ to climb Mt. Lemmon: It's a 20 mile climb, round-trip from where we stay is usually 60 miles. After we get to the top of Mt Lemmon I reset the Average Speed field but don't reset the Trip Distance field. Then I descend down a 20 mile mountain, and sprint the piss out of the last 10 miles home. Net result: My computer says I did a 60 mile ride with a 38 mph average speed. I win!