The OEM GM 6 lug rally wheels were available in 15x7 and 15x8.
The 15x7's were used on vans and base model trucks, 15x8's on Suburbans, Blazers and 4x4 trucks.
The width is measured with the tire off and is where the tire bead locks, not the outside total width of the wheel itself. That may be what is throwing you off
If yours are wider they could be an aftermarket wheel, or they could be wheels that have been modified.
As for figuring what tire will work on your truck... bolt the wheels on and do some measuring. Measure thrice, buy once.
