I received an email from a lurker who asked about converting from an 8R19.5 to a 225/70R19.5. Rather than answer this question again and again, I'm posting this as a resource. Would the moderator(s) please make this a sticky or whatever?
Here's the scoop:
1) If you have a 5.25" width rim, you can NOT make this exchange. The width ought to be stamped on the wheel somewhere and usually it is on the back side and written this way: 19.5 X 6.00RW (with the letters indicating the type of rim flange).
2) If the space between the rear tires (the duals!) is currently less than 1.7", you can NOT make this exchange. Please note that this is the result of adjusting for the width of the rims. My guess is that this is a deal breaker pretty much all the time. The fix would be to replace the wheels with something with a bigger offset.
3) You will need 0.3" more clearance. The problem here is that tires flex and suspension move. The worst condition is when the suspension is completely collapsed and the steering wheel is fully turned - and even then you have to account for tire distortion. I can't tell you how much as each situation is different - but if I had to guess, 0.3" isn't very much, so I think this is not much of an issue. But this ought to become part of your inspection procedure.
4) The diameter difference, will mean the vehicle will indicate a faster speed than it will be traveling. An indicated 60 mph is really 56 mph.
5) Choose the same Load Range and use 15 psi less.
Here's the backup info for the above:
An 8R19.5 has a diameter of 33.8", a width of 8.0" in a 6" rim, with an allowable rim rim of 5.25" to 6.75", needs a dual spacing of 9.1", and has a load carrying capacity of 3525# at 110 psi (Load Range F single).
A 225/70R19.5 has a diameter of 31.9", a width of 8.9" on a 6.75" rim, with an allowable rim width of 6" to 6.75", needs a dual spacing of at least 10.0" and has a load carrying capacity of 3640# at 95 psi (Load Range F single) and 3970# at 110 psi (load Range G single).
So there you have it.