QuestionMy motorhome has 225/70/19.5 tires on it now. Think about 8r tires. I know there a little taller and not as wide. Will this hurt the motorhome and will these tires be safe.
AnswerBill,
A 225/70R19.5 has overall diameter of 31.93", has an allowable rim width range of 6" to 6 3/4", with a minimum dual spacing of 10". It has a load carrying capacity of (I'm only going to quote singles for simplicity) 2755# at 65 psi for a Load Range D, 3195# at 80 psi for a Load Range E, 3640# at 95 psi for a Load Range F, and 3970# at 110 psi for a Load Range G.
An 8R19.5 has an overall diameter of 33.82" (That's 6% larger!), an allowable rim width range of 6 1/4" to 6 3/4" (OK, there!) with a minimum dual spacing of 9.1" (OK, there, too!) It has a load carrying capacity of 2835# at 80 psi for a Load Range D, 3195# at 95 psi for a Load Range E and 3525# at 110 psi for a Load Range F.
This means that you have to run 15 psi more pressure to get the same load carrying capacity (which means a less comfortable ride), there might be some clearance issues (The worst case is fully turned with the suspension fully compressed.), your speedometer will read 6% low (you'll be going 64 when your speedo says 60 mph) and there is going to be less torque to climb hills, but you'll get better fuel economy when it is flat.
So the next step is to find the vehicle placard. It is usually located on a doorpost or in the glovebox. It will tell you the original tire size and the proper pressure for that size. Pay particular attention to the Load Range - you have to run the same Load Range (and 15 psi more).
OK, those are the facts, but there are some other things to consider.
Metric sizing is cutting edge technology and a lot of effort is going into making those work. Inch sizing is very old technology and not much effort is going into those (except as spillover from the work on metric tires.) I would tend to think this gives the edge to metric sizing.
