Are you gonna do gears or any taller tires ever? If you do, your speedo will be off again. In my case, I am not done yet so I haven't messed with anything yet, but i did do the math so I know how fast i am going. What I did was figure the circumference of the stock tire, then of my tall tire, then divide the smaller into the bigger and it will give you a percent. Your new tires take that much more to make one revolution, so in theory your speedo would be that far off.
Man.....this is confusing as hell. All I know, in my case my 33's are 12% bigger than the stockers (235/75/15), so when my speedo is reading 100 mph I am going 112 mph and so on. I checked it and it is right and helps my figure my MPG easier.
Or, someone could tell you something useful to help you! If it didn't take me so long to type this I would just delete the whole thing...LOL
Good luck!!