Tire Size/Tire Pressure Question

[EastTnFarmboy]

The sticker inside the door of my 2000 Dodge 3500 Diesel 4x4 Dually provides recommended tire pressures for 235/85/16 size tires. The tires that had just been put on it when I bought it are 215/85/16.

I am going to be towing my tractor this week on a 20' + 5' gooseneck so I want to get the pressures right.

I understand that you are supposed to add 10 pounds to the recommended pressure if you will be traveling at highway speeds for extended periods. They also provided a chart inside the owners manual that provides pressure recommendations under various conditions. However, all of them are based on 235/85/16.

The sidewalls of the truck tires say max of 65psi. The booklet suggests that you can fill to 10psi more than the max printed on the tire if you will have a load at highway speed for extended periods. I guess max doesn't mean max?

Based on the suggested psi for 235/85/16, the fronts would need to be at 75 and the rears at 60 under load at highway speed.

How do I adjust for the 215/85/16?

(The trailer tires say max 80 and that is where they are.)

Thanks!

 

[Egon]

If the load rating for the tires are the same just use the chart you have. It should work out. On my truck I run at maximum tire pressure of 80 psi.

 

[workinallthetime]

just a foot note, make sure you check and adjust your tires when they are cold. pressure goes up when you drive, and dont go above the max pressure !!! the sticker on your door is what ford, dodge, gm thinks will give you the best ride under normal conditions. i raise my back pressure 5-10 lbs when towing and lower them a bit when we get snow

 

[Mahana79]

I've got 238/85r16's on my duramax dually but it came factory with 215/85r16and i run the rears at 80psi all the time, mainly due to the fact that it is a pain to adjust pressures when needed. i keep the fronts at 75psi. however mine are Load Range E 10 ply, the max for mine are 80 psi, if you say yours state a max of 65psi that sounds more like a Load Range D 8 ply and i think is a rather light tire for a Dually

 

[Dmace]

I only use what it says on the truck door for the stock tires as that is what they are for. Even if the new tires are the same size but different brand, I go by the tire markings. In the summer I keep it about 3-6 psi below the max and in the winter about 7-10 psi below the max.

I wouldn't run it at the max 80psi if I were doing a lot of highway driving, especially if it were 50 miles or more at once non-stop. You check the tire pressure when cold and after a good 20 miles or so at 55mph or more, those tires are going to heat up and the tire pressure will increase. I think it is 1 psi for every 10 degrees the tire temperature goes up.

This means when you stop from a long highway drive, those rears could be at 85-90 psi plus. This could become dangerous but most likely it will not.

Here is a good article from Tire Rack about tire pressure fluctuations.

 

[WTA]

Always make sure the tires you have are the correct load rating and maximum pressure rating as suggested by your truck manufacturer, usually it is on the door sticker.

That sticker was developed with a lot of testing. It is the best for fuel economy, handling, tread wear, driveability and many other reasons. It doesn't just ride the best there. I've never heard or seen anywhere that it is ok to go 10 psi more than the door sticker. Regardless never go more than the sidewall on the tire says.
I worked at firestone back durring that big recall. The tires were failing because a certain truck manufacturer had a snafu on their recommended pressures. 26 psi instead of the 32 to 35 they should have been at. The tires weren't all that great either but combine that with too low a pressure and you get way too much heat on the highway and something had to give.

I got some new oversized mud tires on my f150 4x4 a long time ago and this was before I got into doing alignments and learned the truth about tire pressures. The bone head that put them on set the pressures at 50 psi like it said on the side wall but the door sticker called for 35 front and 41 rear. That truck drove like a tank and would barely stay on the road with those pressures. Every time I found a rut it would follow it almost uncontrollably. I let the pressures down to the recommended level and it drove perfect again.

With one exception that I can remember in recent history the vehicle manufacturers always get it right on the door sticker and you really shouldn't deviate from that.

If you have a different sized tire than it calls for than you have other problems to worry about. Computer calibrations, speedometer, ABS system and all knds of things could not be working right or inoperable now. Especially the ABS. I'd really recommend having the correct size tires on your truck especially if it is a newer one with computer controls. On many of them, GM and Ford I know, the dealer can recalibrate your computer for the tires you have as long as they are an optional size for it. They can also tell you the correct pressures for them. Check with the manufacturer dealer, not some used car lot.

 

[jayhaitch]

Check out my post titled "Picking tire pressure help" in this Trailers/Transportation section.

My Ford Expedition came with P series tires rated for 35 max. I replaced them with same sized load range E LT series tires, rated at 80 psi max. I took my empty Expedition to a weigh station, got the GVW and each axle GAW. I now was able to calculate the load on each tire based on what I'm hauling, towing, etc.

What is the GVW of your dually? Probably quite a bit! What is the hitch weight of your trailer? What is the max load range of your tires at 65 psi? I'd take it from there.

I've decided on 50 psi for my Expedition. It rides well, no abnormal wear after a few thousand miles. I suspect you'll be running at 65 psi based on your load, but "do the math" as they say.

Also, to clarify the extra 10 psi if sustained running above 75 mph: 10 psi above what you've set, NEVER above the max sidewall pressure cold. No need to worry about what the tire does when hot. The tire manufacturers have taken that into account.

 

[KICK]

you ever try this trick to see how much a tire contacts the road

find a nice empty parking lot and have a piece of tire crayon.

make a stripe across the tread of each tire and then drive a few feet and look at each tire and see how much of that tire crayon stripe is still across the tread. adjust up and down as necessary.