Tires: tire size, rim width, carrying capacity

Question
I have 8r19.5 kills power band.eng runs grate.need shorter tire to get on power band .

Answer
Al,

You have to be concerned about load carrying capacity as well as diameter.

An 8R19.5 has an overall diameter of 33.8" and a width of 8.6" on a 6.00" rim with an allowable rim width range of 5.25" to 6.75".  The minimum dualled spacing is 9.1".  Load carrying capacities are (single) 2680# at 80 psi (LR D), 3000# at 95 psi (LR E) and 3305# at 110 psi (LR F)

A 225/70R19.5 has an overall diameter of 31.9" (6% reduction) and a width of 9.3" on a 6.75" rim with an allowable rim width of 6.00" to 6.75".  Load carrying capacities are (single) 2755# at 65 psi (LR D), 3195 # at 80 psi (LR E), and 3640 # at 95 psi (LR F).

So if you have a 5.25" rim, there is nothing that will fit.

But if you have a 6.00" or a 6.75" rim, then you can use a 225/70R19.5 provided you have at least a 9.4" dual spacing for a 6.00" rim and 10.2" if you have 6.75" rims (if you have dual's).  If so, then you can also use 15 psi less and one Load Range lower than you currently have.