Irish_Alley,
His rear tire size is wider and taller than the front. The first three digits is the width of the tire in mm. So the front tires are 285mm and the rears are 315mm. The second set of numbers depicts the sidewall height, so the front tires have a sidewall height that is 40% of 285mm, and the rear sidewall height is 35% of 315mm. Since the wheel diameters are the same front and rear, the rear tires will actually be taller than the front.
REVINKEVIN,
Unfortunately I cant say if those sizes will fit with a lowered truck. A lot of that will depend on the offsets that you choose.
Jake