I believe (but I am not reading this anywhere) that the original tires for the 84 were indeed 255 50 VR 16. The ZR did not come out until the 88 model with the 17" option.
Do you have any pictures of your rims? That would really help explain what is going on.
The 84-87 had the saucer saw blade looking things. The 84 had an unusual offset pattern where the front rims were like 8" and the rear were like 8.5 or 9" wide. In 85 they started moving towards the same size all the way around with specific left and right rims (to keep the saw blades all going in the same direction).
The 84 and 85 had black painted centers. The 86 got a silver natural center, the 87 got a grey painted center. This is about the only way you can tell the difference between these year models at a glance.
The Z51 optioned cars got the wider rims and you can see the wider difference by looking at the center hub. As soon as you have seen two side by side, you can judge from a distance which one you are looking at.
The Gator back was the most revolutionary tire in decades to hit the market. Up until 84 it was only a rare option on the European exotics. Hitting the road in 84 on the Vette, then the IROC Z in 85/trans am, the world was amazed at the overall look, feel, and performance. With the introduction of the ZR rated tire, pretty much all of that was standard equipment.
Alright, I have had too much coffee this morning, and am writing a thesis now on tires. I will get off my soap box now.
Chris
p.s., Look at the specifications tab that Rob has built in here for full details on the options available on your car.
Not very expensive, and are quite grippy. Funny thing is that I just traded a Maxima w/ Kumho 712s in for my Vette last Monday. These tires are more popular than I had thought. 
