It is sort of a moot point to convert to actual miles covered. Maintenance based on miles traved is not an exact science. It is a general rule of thumb that the drivetrain has rotated a certain amount and the fluids should now be changed. Obviously the rotations can vary greatly between vehicle brand/model/physical configuration. With the larger tires, your mechanicals are turning less, so don't stress that you actual miles traveled may be over the magic number for fluid change.
With that said, it is very easy to change your speedometer gear
2003 TJ, 33" BFG AT, 2" BL, 2" BDS spring lift, Truck Lite headlights, Homemade sunrider top.