If you are really afraid of the dogs maybe mace would make you feel better. Thing is most mace is like 6', or so, max effective distance, and that is not really considering riding a bicycle at speed and so forth. They have those huge bear mace can foggers, but don't know how you would strap that one on comfortably. One of the riot control mace grenades would probably do the trick though. Seriously, I carry mace sometimes, but it is meant for unruly people in certain areas.
Personally I've had tons of dogs give chase and, so far, there has never been one that can match a decent little sprint, though they can gain some ground while you are getting things wound up (Though they do claim that Greyhounds, whippets & some Afghan hounds can get up to between 38-45 mph. Mmmm. Now, that would be getting it on).
If they come in from the front, just slow down enough to make them think you're going to face off with them, then blow on through to one side and pour it on.
Closest one ever came was one jamming a paw into the rear spokes during a good sneaky side attack (didn't see it coming in time) & it wasn't walking to well after that, but didn't stop to check on it, as it was one of the more vicious ones.
It would be bad to have one make you down though, while a few more were on the way. Guess you would have to force feed them some bicycle bits and pieces.
Actually, there are some roads where I know where the dogs are and look forward to a little race with them (as do they).
This person put some serious thought into it -->
www.kenkifer.com/bikepages/traffic/dogs.htm