What are you doing for a spare tire? I have heard it is not good for the rear axle to use a smaller, like the OEM, tire. On my 3500, there is no way a 37 tire would fit under the bed, so I would just have to keep it in the bed. If I kept it at home and needed it, there is no way my wife could pick it up and put it in her vehicle. Or, just take a chance and not worry about having a flat. Over the last 20 years or more, I have never had a flat on any of my trucks that have had the E rated tires/10 ply.