OK, here's the scoop:
An 8R19.5 is 8.0" wide on a 6" rim (with an allowable rim width range of 5.25 to 6.75", a diameter of 33.8" and a minimum dual spacing of 9.1", and a load carrying capacity of 3525# at 110 psi (single, Load Range F)
A 225/70R19.5 is 8.9" wide on a 6.75" rim (8.6" on a 6" rim), with an allowable rim width range of 6" to 6.75" and a diameter of 31.9", a minimum dual spacing of 10.0" ( 9.7" for a 6" rim) and a load carrying capacity of 3640# at 95 psi (single, Load Range F)
A 245/70R19.5 is 9.8" wide on a 7.50" rim with an allowable rim width range of 6.75" to 7.50", and a diameter of 33.3", and a minimum dual spacing of 11" with a load carrying capacity of 3860# at 95 psi (single, Load Range F)
I think the 225/70R19.5 is a much better choice. It is much more likely to fit.
- BUT -
- If the rim is 5.25", then the 225/70R19.5 is a No Go.
- The 225/70R19.5 needs 0.3" more clearance between the frame and tire. This can be difficult to check as the worst condition is when the steering wheel is fully turned AND the suspension is fully collapsed. I'd advise getting someone who is knowledgeable about truck suspensions to look at this.
- And the minimum dual spacing could be a problem and needs to be checked.