Exactly. It's the wheel size and sidewall height that change. If the tire diameter was less, you would have all sorts of issues with the stability control nannies and your speedo would read significantly lower than your actual speed.The tires should have very nearly the same diameters as you go from 18" to 19" to 20". 18" get 45 section tires, 19"s get 40, 20"s get 35. Torque will be the same - nothing lost or gained.