So, you upgraded to bigger tires and now your speedometer is no longer accurate. You should probably fix it, but you just spent your life savings on tires and there's nothing left for the speedo gear and you don't have a GPS. Now what? Easy. Just plug your new tire size, your original tire size and any speedometer reading, hit the calculate button and viola, this calculator shows your corrected speed.

Formula : (New Tire Size / Old Tire Size) * Speedo MPH = Corrected MPH