I got a Garmin Forerunner 305 with cadence/speed sensor for christmas. I used it outdoors once and it sync'ed perfectly. And the first 3-4 times I used it, the speed was comparable with my Blackburn Delphi 4.0 with wireless speed sensor (+/- 0.2 mph), but from the 5th time using it, the speed of the Forerunner has been ~2.0mph slower.

I've been using it indoors mostly, so GPS has been turned off. Bike circumference was auto-calibrated at 2096mm (comparable to 2090mm for 700x23 tires). There are no speed dropouts seen on the garmin training center software. GPS has been fully charged before each use.

I initially questioned the blackburn computer, but the speeds matched for the first few uses, then something seems to have changed?

I'm kinda at a lost of what else to try. Any suggestions?

The only suggestion I could offer would be to reset your 305 and see if the issue is resolved. Lp

Take the bike outside for a quick spin with the cadence/speed sensor turned on & engaged and go for a quick spin of 1/4 mile or so. That should re-synchronize your speed sensor.
Good luck and let us know how it works.

OR if you live someplace with ice on the roads:
>>Menu >>Settings >>Bike Profile >> and set your wheel circumference to 2096 (if you have a 700x23c tire (2105 if you have a 700x25c).

Thanks for all the help everyone, I changed the battery and re-sync'ed the forerunner and sensor. Now, the speed on the forerunner is consistently 0.5mph higher than the blackurn cyclocomputer. I think that's a small enough difference that I can live with.