My Garmin shows I am going about 2 MPH slower.  I have also noticed this on my boat that is 2 to 3 MPH different and the Garmin in my car is 1 MPH different.   Am I the only one and which one to you go by?   I guess in the long run it doesn't make that much difference I am going to get there anyway.

My Garmin(s) and bike computer are fairly close. some days the computer is ahead of the Garmin and some days behind but never more than 1%
 
Speedometers in cars are usually not very accurate. I have no idea what kind of speedometer is in a boat.
 
sometimes I ride with two Garmins and one bike computer and they never agree on speed but they agree on distance.
 
A little story: 15 years ago I ended up with three bike computers. (From my son who just joined the marines)I spent the winter calibrating all three to agree, 0% in 10 miles.   Come summer and on any given day two would agree and one would be way off.  But it was never the same combination.

It is the Garmin 705.  I didn't set up the Cateye so I am not sure how it was set.  I might try the Garmin on the V3 or Stratus to see how it works.   I was talking about the average speed but the distance is the same.  It also has the same time.  I guess it would be just as easy to do the math.

Average speed is also affected by what happens when you stop. You can set the Garmin to go into pause mode below a certain speed. Computers always seem to take longer to go into pause mode when you stop. That will make the average speed different between the two. If the distance is showing the same, then the Cateye is set well, and if you do a ride where you don't stop at all, the average speed should be very close between the two.