Rodsswag, disregarding speedo accuracy for a moment, keep in mind that taller tyres compromise gearing and off-road performance. This will be especially notceable at 'step-off' in tight or hilly conditions (especially when towing) when you need every ounce of torque to move the mass. In particular, clutches really suffer.
Much better IMO to stick with the vehicle's largest available option tyre in a quality off-road, LT construction of your choice/need