Overinflating duals on my RV.

[pete c]

I have a '90 Toyota Odyssey MH.

The recommended tire is the 185R14D. The D signifies that it has a higher load rating. I also believe it means it is 8 ply. I currently have 195/75/14s on the rear. I believe they have a C rating, meaning they are "light truck" tires.

These tires are in quite good condition. Little tread wear, no cracking from age. I will replace them with the proper D rated tire eventually, but my inner cheap SOB just won't let me throw away tires that look to have a decent amount of life remaining.

From reading on Toyota MH web sites, I have found that the biggest problem with the tire I have on there now is that if you allow the pressure to drop much at all, the duals will start rubbing together. This generates heat and eventually tire failure. In addition to monitoring pressure religiously, I was thinking about bumping them up, maybe 10 lbs. Will this cause any trouble? It should help with the potential rubbing issue. I am more concerned about possible blowouts. I am not so worried about wear, as the limiting factor on RV tires tends to be age ,anyway. They simply aren't driven enough.

Another thought was the pressure monitoring valve stem caps. Are they any good? Are they expensive? Seems like they would be good insurance.

[Soichiro]

I use the pressure monitoring valve stem caps, and they work well and are only about $10 for a set of four. The limiting factor is that I've only seen them available in 32 and 36 PSI. So if your recommended PSI on your trailer is above 36, they won't help you so much. They certainly do simplify checking your tire pressure, though, so they would be good for this purpose.

[ecomodded]

I bet the load range D tires are 80 psi max or more?
I believe over inflating your current tires will aid in the load caring abilities of them and provide more stability for their sidewalls, ie: will not get squashed down so much.
Of course they will roll easier too

[CapriRacer]

There are a number of issues - but first we need you to do some homework.

Your vehicle ought to have a placard that lists the original tire size and the proper pressure for that size. This is important because of this:

A 185R14 Load Range D has a load carrying capacity of 1874# at 65 psi.

An LT195/75R14 Load Range D has a load carrying capacity of 1710# at 65 psi - 160# less.

If you are using the tire at the same pressure, you are putting more stress on the tire and it is more likely to fail.

In some respects this answers your question, you need to use that +10 psi to get the load carrying capacity.

- BUT -

Is your tire a Load Range C? Then any increase in pressure over 50 psi doesn't increase the load carrying capacity.

HOWEVER, when I looked up LT195/75R14's I only found Load Range D's (except for the Standard Load Passenger car tires - and using those would be very dangerous as they do not have the load carrying capacity needed). So it's really important that we establish what you have. It will be written on the sidewall.

You will be looking for either a "Load Range" or "Standard Load". We need to know which you have.

We also need to know what the placard says for original tire size and inflation pressure.

[pete c]

A little more info. The tires are cooper trendsetter SEs. P195/75R/14. Load rating is 1400 lbs. Max pressure 35 PSI.

The camper weighs right around 6000 lbs. Assuming 70% of that weight is on the rears, each tire has a little over 1000 lbs on it, so I should be OK.

I think I will try running them at 40 PSI and get a set of the 36 PSI valve stem pressure monitors.

[Saskwatchian]

The P in P195/75R/14 stands for Passenger car. Your tires have lighter construction and less protection than proper truck tires. I wouldn't use them but you might be OK if you have good roads.

Truck tires meant to be run as duals usually have two max load ratings. One for single and one for dual use, which is generally about 10% lower. Since car tires aren't designed to be used as duals I would go even farther and drop your safe range from 1400 to 1200lbs each.

You are still within the safety margin if you really do have 70% of the weight on the rear but I would seriously consider driving to a highway scale that weighs individual axles to find out what the axle loads really are.

I would also consider buying some spacers for the duals if you plan on keeping the tires. A 1cm spacer will give you the clearance needed between the tires but keep in mind the outer edge of the outer rear tire will be about 15mm further out than stock.

Even with the spacers I would be concerned with the tires rubbing on turns etc with their light casings and low pressure. I have never been a fan of running pressures higher then the rating and feel it makes you more susceptible to flats.


So in general I wouldn't do it but if you are consider spacers and keep a close eye on your tire pressures.

[CapriRacer]

Pete,

Please tell us what the vehicle tire placard says for inflation pressure and original tire size. This is important to determine what the vehicle manufacturer intended for load carrying capacity.

Also, you should be aware that passenger car tires (like yours) have to be derated 10% when they are used in a light truck application (like yours are). So you need to proceed carefully. It would be really easy to overload the tires.

[pete c]

I will see if I can find such a placard. I do know for a fact that the recommended tire is the 185R14D. That comes from many different knowledgable toyota motorhome owners. The RV companies bought these as cab and chassises from toyota, so they could list something different as they could have ended up as box trucks or whatever.

I will see if I can find a highway scale to get a better idea of what the actual weight is.

The spacers is an interesting idea, but, rather than spend the money there, I think I would just put it into proper tires. Also, with the tires at normal pressure, I measure a little over one inch clearance between them. So, clearnce shouldn't be an issue assuming I stay on top of proper inflation pressure. And if I can get a set of the monitors for 10 bucks, that would be money well spent.

[ecomodded]

I keep my tires at 50 lbs (rated at 42)
Under inflating tires has been causing the SUV roll overs.
Over inflating tires has been causing better gas mileage.
1 inch space between tires sounds awfully close, my 1 ton dodge dually had 4 inches space,that routinely got rocks stuck between the two i'll add.