Tires: tires, 8-19.5 Tire Substitution

Question
i have a motor home with 8r-19.5 goodyear tires what else can i put on the motorhome? or do i have to use the same tire? they are hard to come buy or very high priced.   thanks

Answer
Al,

These types of tires come in Load Ranges (LR) - and along with the tire size, the load range determines the maximum load carrying capacity of the tire and the corresponding pressure this occurs at.  I'm going to assume it is a LR F.  

An 8-19.5 LR F has a load carrying capacity of 3525# at 110 psi, but when dualled up the load carrying capacity is reduced to 3305# (also at 110 psi).  It has a width of 8" measured on a 6" rim with an allowable rim width range of 5.25" to 6.75", and an overall diameter new of 33.8" (highway tread), and a dualled spacing of 9.1"

I have responded to several folks about making a change and I have suggested that an alternative would be a 225/70R19.5 LR F.  That size has a load carrying capacity of 3640# at 95 psi (dualled: 3415# @95), a width of 8.9" on a 6.75" rim with an allowable rim widtnh range of 6.00" to 6.75", , an overall diameter of 31.9" (highway tread), and a dualled spacing of 10.0"

What does this mean:

1)  If you have a 5.25" or a 6.00" rim, a 225/70R19.5 will not fit.

2)  A 255/70R19.5 is going to make the vehicle sit about 1" lower.

3)  A 225/70R19.5 needs about 1/2" more clearance on each side.  (Keep in mind the worst case for clearance is fully compressed AND fully turned.)

4)  The dual spacing (the distance from the center of one of rear tires on a corner to the center of the other) needs to be 10" minimum, and 1/2" wider than for an 8-19.5.

5)  BUT if all the above works, you can use a lower inflation pressure.

I am going to guess this is not going to work.  In particular you'll find #1 and #4 will prevent you from making the switch.