You already have a taller top gear than I have on any of my road bikes. If you can spin 100 rpm in that gear and still want more, by all means, get yourself a 53 ring. But I'm thinking you need to work on the motor and the spinning mechanics to get the most out of the gearing you have. Another factor may be wind resistance. It is pretty hard to push a bike to 30 mph while sitting upright. Drop bars would help with that.