[Dalaigh]

Picked up this straight and solid, but needs TLC C3500 today.
Plan is to build a clean work truck. It's a 98 w/ 454/4l80e.


I don't have a jack that will lift the back to figure the gear ratio out, but does anyone know how to find (guestimate) gear ratio off of speed, RPM and tire size?
With the factory 225/75r16 at 90km/h the truck is running down the road at near 4000RPM. This seems high to me, and it's going into OD.