Yes, as you should be. My speedo is off.... Don't know how or why, but alas it is. I have the stock size tires now. As far as I know there are 3 ways to fix it.

First is the speedo itself taken to a specialty shop.

Second is a gearing change down on the engine

Third is to change the tire size.

I would like to go the tire route to fix the problem for multiple reasons.

Tom