Tires and inflation. The great mystery for many motorcoach owners. I found it a bit confusing when we first took delivery of our coach. Our dealer had inflated our front tires to 120 PSI and our rear tires to 95 PSI. The factory had installed a sticker which showed the steer at 120 PSI, the drive at 90 PSI and the tag at 85 PSI. After taking delivery, we took our coach to KAL tire before a major road trip — at that time I did not carry an air compressor — and they inflated all the tires to 120 PSI. I had the coach weighed on a CAT scale and I made a guess that the tires should be 110 PSI on the front and 85 PSI on the rear. When we were at the factory earlier this year, Newmar weighed the corners of the coach and inflated the tires to 110 PSI at the front and 75 PSI to the rear.
What tire pressure should I use? Unfortunately there isn’t a one-size-fits-all answer to that question.
The tire pressure you should use depends on the type of tire and the weight of the coach. There is no substitute for getting the coach weighed, preferably on all corners.
Assuming that you are running a coach with a tag, here is the process that Michelin recommends to obtain axle and wheel position weights. Not necessarily easy to do on a CAT scale. Before we had our corners weighed, we did a rough calculation based on axle weights only.
We’ll use our coach as an example.
We run on Michelin XRV 305/70R 22.5 LRL tires.
Our coach was weighed with 3/4 tank of fuel, 2/3 fresh water, empty grey tank, 1/3 black tank and no passengers.
Our corner weights:
7,290 lbs. left front, 7,115 lbs. right front
8,270 lbs. left drive, 8,645 lbs. right drive (duals)
4,275 lbs. left tag, 4,345 lbs. right tag
Tire pressures need to be the same across an axle which means that we use the heaviest tire weight on an axle to determine the load/inflation for each side of that axle.
Michelin provides the following table for our tires:
A cold tire pressure of 110 PSI will support a weight of 7,300 lbs. on a single front tire. Our heaviest single front is 7,290 lbs. therefore 110 PSI would be the target tire pressure for our coach.
A cold tire pressure of 75 PSI will support a weight of 9,530 lbs. on the one pair of duals on the drive axle. Our heaviest weight is 8,645 lbs. on our drive duals therefore 75 PSI would be the target pressure for our coach.
A cold tire pressure of 75 PSI will support a weight of 5,375 lbs. on the single tag tire. Our heaviest single tag is 4,345 lbs. therefore 75 PSI would be the target pressure for our coach.
How do you get the required tire pressure for your coach?
You need the tire model. You need the load/inflation tables from the tire manufacturer. Michelin provides an online PDF which you can download here.
And you must know the actual weight of your motorcoach. You cannot determine the correct pressure for your tires unless you know the actual weight of the coach. Preferably the corner weights.
The consequence of running tires underinflated or overinflated can be deadly.
We carry a precision tire pressure gauge suitable for our type of tire which means that it is accurate to at least 120 PSI which is our maximum load. We carry a portable air compressor that can inflate tires to at least 120 PSI. We have found through experience that we cannot count on the tire pressure services at fuel stops to provide enough pressure to inflate our tires to 110 PSI.
Before any trip on our coach we check our cold tire pressure and we use an air compressor to inflate the tires as required. We visually inspect our tires for any signs of wear or distress.