gear ratio change from larger tires? that's a new one. not sure what that's all about. the gear ratios within the tranny as well as between between the tranny and motor as well as tranny and differential will be the same. no matter how big the tire size difference is. that the only drivetrain mod you have?
with a larger wheel/tire diameter, there is more circumferance per revolution, thus the higher speed per motor RPM.
how to get it fixed? i have no idea. try calling a Nissan dealer/repair shop.