Originally Posted by Suspenderguy
Hello all. I'm looking to replace 20 plus yr. old Goodyear unisteels on a 79. They are the 8R 19.5. I have a quote for Hercules 14 ply Stronguard Regional All Purpose tires in a 225/70R19.5. In the forums here I get the impression the size is ok but the speedo might be a little off. My question is has anyone used this particular tire and what is your opinion of them . Thanks for any info.
To clarify it's a 28 ft "79" AS motorhome.
Here's the math:
If you start with 8R19.5's, they are supposed to be 8.0" wide on a 6.00 rim, with an overall diameter of 33.8", a minimum dual spacing of 9.1", and a load carrying capacity of 3525# at 110 psi for a Load Range F.
A 225/70R19.5 is supposed to be 8.9" wide on a 6.75" rim, with an overall diameter of 31.9", a minimum dual spacing of 10.0", and a load carrying capacity of 3640# at 95 psi for a Load Range F.
Assuming for the moment that you have 6.00 wide rims to start with, the 225/70R19.5 will be about 8.5" wide on that 6.00" rim. Since it is common for chassis manufacturers to specify minimum dual spacing rims, the net effect is that there will be a loss of about 1/2" space between the duals. I urge caution in this area.
Also, the diameter change results in 6% change - which means the speedometer will read 60 mph when it is going 57 mph.
And lastly the load. In order to carry the same load the 8R19.5 does at 110 psi, the 225/70R19.5 only needs 92 psi.
So the real danger areas are the rim width (minimum 6.00" for a 225/70R19.5) and the dual spacing (9.6" for the 225/70R19.5 on a 6.00Ē rim)