I guess I really can't put a percentage on the dfference. But I can tell you is that with the 86_88 in high moisture corn(30-35 percent) with a six row head very rarely could we get over 3.5-3.7 MPH, but with the 99 5+ MPH is no problem. Beans we went form a 20ft head running from 3.5-4.0 MPH to a 30ft going 5+ MPH. Now where we live(central WI.) corn runs form 145 to 180 bushels per acre, beans 20 to 50.(we have seen 200 bushel corn and 65 bushel beans, but not very often) I guess what I am trying to say is that with the 86_88 capacity and horsepower where the limiting factors, now they are nonfactors Field condidtions and keeping empty trucks and wagons is the problem now. After having a 99 I couldn't image going back anything smaller.