From what I know,(which isnt much,)I have the 89,my Dad has the 91. I like them both,but I feel the 89 is the much better all arouund convertor. It is looser than the 91,by a little. Off the line,the 89 is quicker,,and smokes a little less than the 91%. It also doesnt jump into gear quite as hard. If you want to come out of the hole quick with the 91,you have to power brake,for 5 seconds to build up boost,then they will rocket out of the hole. Once moving I cant feel any difference between the 89,and 91 at all,they both seem to transfer about the same amount of power,if there is a difference it isnt measurable by the seat of pants ,it would take a dragstrip or dyno to tell the difference. I think the 89 will run cooler in stop and go than the 91,the 91 really strains the engine while going slow,which heats it,and the transmission fluid a lot. IMO the 91,and 93 are for racing only,95% of the trucks should get the 89 IMO. If i had it to do over my dad's would have an 89% in it,without a doubt