If you have calibrated your speedometer for your actual tire size, then figure it's "accurate" and the other guy is off. Otherwise, go to mapmyride, put in the ride there, and see what it says.
I've been known to ride to the ride or ride around the parking lot while waiting, and either could have run the other guy's milage up just a tad.
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"be careful this rando stuff is addictive and dan's the 'pusher'."