First, I dont know the proper tire size on a 1980 GMC K25 with 4wd and heavy duty suspension(whatever that was), but what was it? In radial sizing. The previous tires on my truck were narrower and taller, but the tires I put on were free and had a lot of tread, so it was an obvious choice on a truck with no working speedometer. Second, I use 245 75 16 BF goodrich KO2 all terrain TA. Is there a way to know my proper tire pressure that would give full use of tread? My truck weighs about 10,500 in the winter, maybe about 10 or just under 10 in the summer without the plow on. Im assuming. I have no actual idea, just estimating extra steel on top of 8400 on the door. Id just like to know so I can make full use of my tread and not wear tires bad if I were on a road.