How do I determine the new tire pressure,
when I switch from 255/70R/16 to 265/75R/6?

Thanks

 

The PO did the same tire switch to my truck. I run 40 psi. TP is always stated on the side of the tire. You will have to look carefully. You can always ask the tech who does the installation about the proper tire pressure for your particular tire.

What tires are you going to buy?

 

265/75r/16

 

Run what it says on the sticker on the door jam of the truck. Usually 35F/35R. The pressure on the tire is just the maximun allowed by the tire maker. The truck will drive the best on what the door sticker says and the tires will wear the best this way.

 

I have a huge problem with running what the door sticker says when you switch tire sizes. The sticker on the door was made for the best ride on the stock tires, nothing more. When you go to larger tires you will most likely need to put more air in them or they will be severely underinflated. This will cause the tire to wear on the outside edges and could result in the tire rupturing. In my 31's (265/70/17's) I run 35-40psi, when I ran 33's I ran closer to 45psi and the tires wore perfectly. I even put 60psi in the 33's for a while but the ride was just too stiff.

 

Uh...that is NOT true.

How do we measure presure? Pounds per square inch! PSI has NO RELATION to the volume of air in the tire. If a bigger tire has the same PSI as a smaller tire, it has still has much more air in it.

If you have a larger tire, you have more surface area touching the ground. Thus, more surface area x more PSI would equal a harsher ride, and the tire being overinflated.

Ever wonder why a conventional car tire carries ~32 PSI, while a SMALLER temp spare carries ~50? What about small trailer tires? I have a trailer with a 2k lb axle, and both of those tires are supposed to be at 95 PSI. The smaller the tire, the more pressure it requires to hold a given weight

 

PSI is relative/proportional to tire size/volume. Look at bicycle tires, some need 100psi and I now large earth moving equipment need very little psi, but much volume.
I still think that you have to look at the tire and not the sticker because different companies require different psi, of course. The original poster is going up one size and the TP maybe different from factory settings.
I just us the max setting stated on the tire, just so I am not under inflated.
I do prefer a stiffer tire, since I usually carry a load.

 

Good point.

BUT...being overinflated is almost as bad as being underinflated. I read a great post a few months ago in the '04-'07 forum about calculating air pressure based on maximum load rating of the tire in question.

Say the max load is 1500 lbs per tire, and the max PSI is 40 PSI. That means it's rated at 1500 lbs at 40 PSI...but if you are in a lighter car that puts, say, 1200 lbs on each tire, you need less pressure. The person who mentioned this suggested using a proportion equation.

For example: 1500 PSI 1200 PSI
40 PSI = x

Cross multiply and solve for x, and you should have a good PSI.

This is especially true for those who use heavier, LT rated tires...many of these are designed for heavy duty pickups(i.e. F250, F350, F450) and if filled to max pressure, would be WAAAY overinflated.

 

so - does that mean the 1st train is going 50 mph and the 2nd @ 65?

drj

 

I am not saying over inflate them, anything under the max pressure on the side of the tire is not over inflating. Under inflating is a HUGE mistake, look at all the idiots who didn't bother to check their tires on their Explorers and then wanted to sue Ford when the underinflated tire blew up and rolled their vehicle. Trailer tires are not comparable since a trailer holds much more weight on its tires than most vehicle tires. Anyway my main point is the door sticker has nothing to do with your new different sized tires and underinflating them is the most dangerous thing you can do.

As for F250+ trucks not having the tires filled to the max psi on the tire I think that is incredibly stupid, although a lot of people don't use those trucks as they are intended, for hauling and towing. So if you are going to use it like a car you could get away with that, once you start towing and hauling you damn well better have those tires at the max inflation or they are going to blow sooner or later.

 

You're right, underinflating is much worse than overinflating...by the way, the problem with the Explorers was that Ford recommended 25 PSI back then to make the center of gravity lower....huge and very dangerous mistake!

BUT.....

There's a difference between overinflating to tire specs, and overinflating for the weight on the tire.

Too much pressure for too little weight will cause the center of the tire to wear unevenly, and the sides will get almost no wear, as the center section of the tire will bow out, and put too much pressure on the center.

Nothing whatsoever to do with the max pressure the tire can handle, but it can cause unstable handling. Consider, your truck would be running on ~1/2 the tread width it was designed to run on, as the pressure would push the sides of the tread away from the pavement. This would narrow the track width of the truck and very well could cause it to roll when a lower pressure would cause it to simply skid.

I agree it's much better to overinflate a bit than underinflate, but running at max PSI for the tire, without regard to the vehicle application is not the answer.

 

the 1st train was doing 50 MPH, but only because it left the station in the rain, the 2nd train 65 MPH due to the larger head light.......

 

hehehe...thanks guys!

 

not to mention that the max tp on the tire is stated as max cold, and tires heat up and the pressure inside increases when the tire gets hot there by putting the TP over the maximum limit of the vehicle. The stiker pressure still applies to the aftermarket tires because the vehicles sprung weight is what is resting on the tires, this weight hasnt changed so the recomended tire pressure doesnt need to change, tire pressure has to do with the weight of the vehicle more than anything else, smoothness of ride and wear and gas mileage are less of a factor than the weight of the vehicle, putting larger tires on simply means that more air is required to get up to pressure as someone above stated, you can but larger tires on and run the exact same tire pressure and youll be fine, you wont have horrible tire wear or anything

 

Exactly. This is why it says MAX pressure, not recommended pressure!!