I can't speak for your experience... but from a physics (or some kind of math <img src="/forums/images/graemlins/notooth.gif" alt="" /> ) standpoint, height should be the bigger factor. Putting on taller tires has the same effect as putting in shallower differential gears... it lowers the RPMs and, as a result, your vehicle is putting out less power at the same speed. Width, tread type, etc will also have an effect in the form of rolling resistance. I know just going from the stock 31x10.50 to a 32x11.50 came with a noticable decrease in power and braking ability.