Garmin Forerunner 305 (Cadence)???

Does anyone use the Cadence accessory for the Forerunner 305? I have been using one but since I started using it, it does not give me accurate distance. For example routes that I use to run are longer than on the bike using the 305. 10 mile route running shows up as 9.78 cycling. A big difference. My bike computer shows a distance similar to running with the 305.<br><br>
Does anyone else have this problem? <img alt="confused.gif" src="http://files.kickrunners.com/smilies/confused.gif"><br><br>
I know you can manually put in your tire size. I let the Garmin compute it automatically. Does anyone know where I can get the number for the tire size 700x23?<br><br>
Thanks for the help.<br>
David