What are the specs on your 1989 whitegmc Volvo truck?

How are we supposed to know? Presumably, you have a WIA64, although it could be a different model. Maybe it has a small cam Cummins, or maybe it has a Big Cam, or maybe it has one of a number of Caterpillar diesels which were available that year. Maybe it has an Eaton Fuller Transmission, or maybe it has a Rockwell, or maybe it has the proprietary Volvo 14 speed synchromesh transmission. Maybe it's direct drive with gear ratiOS between 2.64 and 2.93, maybe it has a transmission with a .87 overdrive ration and rear end gear ratiOS between 2.93 and 3.08, maybe it has a transmission with a .74 overdrive ratio and rear end gear ratiOS between 3.25 and 7.something. Your guess is as good as ours. Better, in fact, since we don't even know the application.. it could be a line haul tractor, a day cab tractor, a vocational straight truck.. you can try calling a Volvo dealership. They're typically a PItA to deal with, but they might be able to tell you something. It's also possible to call Volvo directly in Sweden, although I can't tell you how much information they retain on US-made vehicles under the White-GMC badge.