That formula will work just as well for your dually, if you have a tach and know the other stuff. If I remember right, a 10:00-20 is about 41" tall and makes around 498 revolutions per mile some thing like a 235/70-15 will get up toward 700. That explains why a big truck with 4.11 gears will run as fast as a car. The small block should turn 4,000 rpms. If it has a 36" tall tire, it would go 60, that might still be exciting on the way home in a barn truck.
My 65 C-80 with the 409, 10:00-20's and 7.17 gears will turn 4,000 rpms before the governor shuts it down. At that point, it's running around 68.
