It's not easy to outrun a big dog.
They have a huge chase instinct, so I take away the "chase" factor. I immediately stop and get OFF my bike and swing the bike around to place it between me and the dog(s). Then I yell "NO!!! GO HOME!!!! GO HOME!!!!" in a very deep commanding voice and point away and try to look imposing. I get my pepper spray in my hand too. (i keep it bungeed to my handlebars)
So far this has worked like a charm every time for me. They stop and look ashamed and then trot away embarrassed. They wanted an exciting anonymous chase, ....they weren't figuring on dealing with a big human machine combo standing there yelling at them.
If a big dog is vicious and determined to bite you, it will whether you are stopped or riding fast. Pepper spray is a very good idea, but you need to keep it where you can grab it quickly. And don't leave it on your bike if you go into a store, etc.