A couple of trips to the bathroom.

Let's see...for an average 150 lbs cyclist, the difference in mass is about 2%. That my friend, is noise in the real world...unless you're doing a time trial.

Want more proof? Buy a cheap $5 speedometer. Put a 5 lbs bag of rice or bean in your backpack and go out for a ride. Come back and let us know if you lost more than 1/2 mph.

http://www.dealextreme.com/details.dx/sku.24075