85 90? sounds slight bit high. 85 sounds about right for a 440 Jag. I don't knoe it is a fancooled 440. So you don't have a speedo(I am guessing that is what you call a tach, car experience helped that huh?) and are just guessing the speed? How high in the clutch is the belt riding? if it doesn't ride up in the clutch far that will affect your performance. Shift the outer sheave on the secondary clutch and alittle and push in. slide the belt outwards first and try and see how the belt is riding up. If it is low on the sheave(not near the top) it will lose plenty of top end. Maybe you have the right size belt just not the right width? I don't know check it out.