what is my gear ratio

I would argue it doesn't matter what gear set you have or don't have. You are right that you need a larger gear to slow the speedo down. I have used a rule of thumb of 3 mph per gear tooth for decades with good results. Since your trying to correct a 6 mph error, I'd remove what you have (the tooth count will be clearly marked on it), add two to it and buy that. (have a 30 tooth, buy a 32) This is not an exact science but a simple rule of thumb to get close(r).