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Proper tire pressure for bus/RV?

1964 Champion bus, (motorhome conversion) mileage unknown.

Weight 18800.

Dual tires in the rear. Currently at 80 PSI and they look low. Marked “Michelin D-20 Type X

10.00 - 20X MTC 4371 Regroovable MTC”

Front tires Michelin Radials marked

“HN2j MEAX 423 Radial

10.00 - 20 X 2A”

Please don’t say “Read the manual”. It was bought at auction for basically scrap metal price. The manual is a piece of paper that says “1964. Runs.”

Thanks in advance for any help.



Keep reading the tire side walls, they should also have the max psi listed. keep in mind this is the maximum and you may not need that much.

Do the tires even look new enough to be concerned with ?
Look for numerous surface cracks in the sidewalls as an indicator of aged rubber.

I’m not yet finding that size in a michelin search, they sound old already.

10.00 X 20 are standard semi-truck tires. Look on the sidewall for a Load Range Rating. E, F, G, something like that…80 psi should be pretty close for your new prize…Once you know the load range, you can determine maximum pressure…

Load Range “D”, 8 ply rated, 65 psi max
"“E” 10 ply rated, 80 psi max
"F" 12 ply, 95 psi max

I found the guideline on the Goodyear site a whiles back. There’s truck tire info on the last couple of pages.

Ed B.

Sorry, but in a 10.00R20:

Load Range F max pressure = 90 psi
Load Range G = 105 psi
Load Range H = 120 psi.

Plus, the OP needs to know how old the tires are:

First locate the letters “DOT” on the sidewall of the tire. Nearby will be the DOT code. DOT codes are 10 to 12 digits long. BTW the digits can be numbers or letters.

The first 2 digits are a code for the manufacturing plant.

The next 2 digits are a code for the tire size.

The next 3 or 4 digits are a code for the type of tire.

The last 3 or 4 digits are the date code. The format is week/week/year/year or week/week/year. These are always numbers.

Starting in the year 2000, the date coding used was 4 digits. That means the largest number you should see for the year is 10. Before 1999 the format was 3 digits. 1999 and 2000 are transition years, so you will find both 3 and 4 digits.

A word of caution: If you have a 3 digit date code, it is quite possible that the tires could be from the 1970’s, 1980’s, or 1990’s, and there is no way to be absolutely sure which.

The date code only has to be on one side ? and it is permissible for there to be a partial DOT code, so long as one side has the complete code.

Late addition: I just noticed that the information posted on the front tires has the DOT code - including the date = 423 - which means that tire is from at least 1993! I think these ought to be changed!

First, a big thank you to everyone for your sharing your time and knowledge.

If the tires are 20 years old, but have no or little mileage on them, should they still be replaced? I assume rubber ages, whether it is being used or not.

It looks like the previous owner started to convert the bus to an RV around 1990, then abandoned the project and let it sit for 20 years. (Brand new in the box appliances inside, manufacture date 1989.)

10 years, maybe (some would say certainly), 20 years, most would say certainly replace them.

The load range/psi chart is your best source for pressure. The tires are likely tube type and the rims very likely 3 piace. The brakes are likely hydraulic over vacuum over hydraulic with twin cylinders on the front and possibly on the rear wheels. The emergency brake is likely on a drum at the output shaft on the transmission. I say all this to qualify me is saying you need the advice of an experienced medium duty GM mechanic from the 60s to successfully/safely bring that thing back to streetable condition.

Peace of mind is worth a few hundred bucks(though it may likely be ‘grand’), right? I agree, they should ALL be changed out before you head out on any major road trip.

My advice:

Finish the conversion. The last step should be to drive S-L-O-W-L-Y to a truck tire shop and get the tires replaced.

This assumes you don’t have to drive the bus anywhere to get work done. But if you do, then replacing the tires NOW might be the better way to do this. What you do not want is a tire failure. These tires contain both a large volume of air and high pressure - a very lethal combination. People have been dismembered when dealing with these types of tires.

BTW, an even better idea might be to convert the bus to modern tubeless tire and rim systems. This avoids all the problems with split rings and exploding assemblies that used to plagued these types of tires.

I promise not to say “read the manual.” Instead, I will say “read the placard.” Probably inside one of the compartment doors is a metal placard that has this kind of important information. It also might be on the driver’s door frame. This will tell you the proper air pressure for the tires.

For tires this size, the cost is likely two or three thousand dollars, especially if you need to buy six of them.

While you are working on this baby, you might consider switching the rear dual tires to “super singles.” Super singles are the current “big thing” in trucking. You basically replace a set of dual tires with a single wide tire. You get better traction than you do with dual tires since you get a larger contact area with the road.

I hadn’t thought about the split ring problem. New (used) wheels and new (new) tires would be a great idea in that case.

Again, thank you to everyone for helpful input. Without a doubt, I am going to need a lot of good advice to get her back on the road.

Rod, I’m impressed. You clearly have worked on these before.

The previous owner converted it to air brakes, and they seem to be working well. They charge up to 110 PSI. They have lots of stopping power. And when she’s parked with the engine off, it holds pressure for a long time.

The engine was replaced with a big Cummins V8. A retired trucker told me it sounds great. It does leak oil.

For what it’s worth, I have a '72 Dodge 1-ton water truck I keep in Mexico…It has 20 year old 8-ply Nylon cord tires and I drive it every week on a rough dirt road…With 800 gallons of water on its back…

If the tires on your project vehicle have ever been allowed to become flat and left that way for any length of time, I would indeed replace them. If you intend to do travel at interstate speeds, I would replace them too…You just can’t trust the bond between the rubber and the belts/cords anymore…(30 years)

Sorry, Whitey, but trucks and buses do not have tire placards - plus, I don’t think placards were required until the late 1960’s.

You are way ahead of what I had envisioned in this IWOMH. This might really be a worthwhile project. Air brakes and a diesel make it a whole new ball game. Good luck.

Maybe Champion Bus, Inc can help you determine the correct pressure:

Are they castspoke? If they are split rims switch over to 11r 22.5 for safety

Ah. I stand corrected.