Changing Load Range tires on your truck

General Discussion
1

Came across this on another forum. According to this, when I replaced my P265/70-17 (placard) tires load range B or C, not sure which with LT265/70-17, I should have increased my tire pressure from 35psi (placard) to around 50 psi or higher just to have the same load carrying capacity.

I have used google to try to find anything else that supports this or provides a chart as to what pressure I should be using now. BTW, I typically run about 37 and I haven’t had any issues with the tires, they are about half worn now.

2

I am going to Bat Signal the real tire expert that posts here . @CapriRacer Here is a question for you.

Frankly I don’t know why you did not just put the same type tires on and how often do you load this thing down with real heavy loads ?

3

The P metric tires I see in that size are all 115 load rated or 2679 lbs.

The LT metric tires I see in that size are 121 / 118 rated. The 121 is as a single, the 118 as duals. So as a single it is rated at 3167 lbs or 20% higher.

I’d say you could safely run your LT tires at 37 psi in unloaded condition but you’d have the ability to up the pressure and safely carry more load…or have a bigger safety margin…if needed.

Maybe the tire company’s respond to that 2012 warning was to bump up the LT load ratings so they’d be able to match a common P-metric load rating at lower pressures.

@CapriRacer would likely know the back-story on this if there was one.

4

First, the rule of thumb is that if you replace P type tires with LT type tires of the same dimensions, you need to add 15 psi to get the same load carrying capacity.

If you want to verify that, you need to find the tire load tables for each size. Since this is a Toyo bulletin, here are their load charts: Toyo Load and Inflation Tables

The key to this is that P type tires mounted on a light truck, SUV, or van have to have their load rating reduced by a factor of 1.1. So this becomes a math problem and the shortcut is to use the 15 psi rule of thumb!

And BTW, the load tables actually come from tire standardizing organizations. The one in the US is The Tire and Rim Association. They publish a book every year - about an inch thick - with all different kinds of tires (farm, mining, passenger car, trucks, etc.) including load tables and sizing standards. These load tables do not change from year to year because once published, they are used by tire manufacturers, vehicle manufacturers, and government regulatory agencies. So they have to be right the first time they publish them.

Also, P type tires come in SL (Standard Load) and XL (Extra Load). LT’s comes in Load Ranges (C, D, E, etc.)

And lastly - Why the difference in pressure? LT tires are made from a stiffer rubber than P type tires. That stiffer rubber tends to crack under more deflection, but is needed to prevent the rubber from flowing under the higher unit loading in LT tires.

Ya’ see, rubber is a viscoelastic material and does not behave like we normally think solids behave. Rubber will permanently deform under high shear stress, So rubber is tailored for its use and in some cases, made stiffer so cold flow is not an issue in the application. That is what goes on in the difference between P type tires and LT type tires.

6 Likes
5

Thank you CapriRacer, I too had considered LT tires. Now I will stay with the P tires.

6

First, thanks to @CapriRacer for the information.

I bought the truck used and it had a mixed bag of tires on it, two were C rated and 2 were E rated. One tire was brand new, E rated so I bought three more matching tires. Never liked them. Fortunately they didn’t last very long, you’d think that tires with almost zero traction would last forever, but not these.

When I replaced them, I got Cooper AT3 tires. I like them a lot. They were available in P and LT versions, I chose the LT version because even though the Silverado is an LT, it is still a big heavy truck. The tire people practically defaulted to the LT version and I did not object.

But they only put 35 psi in the tires, going by the placard on the B pillar. You would think that every tire shop in the country should be aware of this but I didn’t even see anything on Tire rack, only that E rated tires could be inflated to 80 psi if you wanted. Nothing about having to inflate them above the placard if the OEM were P instead of LT.

Edit: it appears I should run 50 psi. I only do local driving with this truck.

7

I will only buy P’s from here on out. I really don’t tow or haul anything other than bikes, aluminum boats, and lightweight stuff. The ride is so much better. I think most half ton trucks come with P metrics. Or maybe they just used to…no idea what they put on those barges nowadays.

8

I received a set of load range E tires and installed them on my Ram 1500 8 years ago, inflated to 35 PSI when carrying ATVs and towing a camper. Load range D and E tires have a harsh ride quality but are very puncture resistant compared to passenger tires on gravel roads.

265/70R17 has a load limit of 1890 lbs, that is 3780 lbs for the rear axle, about 500 lbs more than the weight of the rear axle of my truck.

At that pressure the load limit is 9880 lbs, greater than the GVR of a 2500 truck. Have you ever loaded the truck this much? I have carried 2000 lbs of gravel 5 miles, too lazy to increase the tire pressure.

If the load was 200 lbs over the rating I really don’t care. I frequently see 1/2 trucks loaded with top soil or gravel loaded well above the vehicle rating equipped with passenger tires. There regulars here that have claimed to have carried more than 2000 lbs in compact pick up trucks that came with P195/75R14 tires.