Is this truck driver under pressure?

Dear Ask a Mechanic

I recently inherited an ’88 Chevy K1500 1/2 ton truck. The tires on it are supposed to be LT245/75R16, but when I got it, I discovered it has P245/75R16 tires on it. My question is: The sticker on the door says the truck says the tires should be at 45 psi. Should I keep these at 45 or should I go down to 30, as they are passenger car tires?

We sell a good number of tires at my workplace, and a switch from a “P” (or passenger tire) to an “LT” (light truck) tire is pretty common, especially as several of the more popular off-road tire models, such as BF Goodrich’s classic All-Terrain T/A KO2, only come in LT sizes.

Deliberately switching the other way is far less common — it is potentially dangerous — although I wish I could say that I haven’t seen vehicles where this had been done elsewhere, either unwittingly, or in an attempt to save money.

As you allude to, a weird quirk of LT tires is that they actually have less load capacity than a P tire at the same air pressure.

The higher permissible pressure is what generally gives LT tires a greater maximum load capacity. For that reason, you can run a lower pressure in the P tires. (The inverse is true when switching the other way; LT tires must be set to a higher pressure to maintain the vehicle manufacturer’s intended load capacity.)

Like you, I’m going to talk in “psi,” as that’s what most of us are familiar with in tire pressures.

I remembered some of those trucks as having P tires, but it turns out that only the 2WD ones did; all 1988 GM half-ton 4x4s used LTs.

I have no idea why. Those factory LTs were load range “C,” maximum 50 psi, an oddball which is difficult to find now. Load range “E,” which allows up to 80 psi and has a far higher capacity, are all that seem to be readily available in that size.

This turns out to work in your favour, as a P tire’s capacity can actually exceed the C-rated light truck equivalent size.

For example, a Michelin Defender LTX in that P size has a maximum capacity of 1090 kg @ 44 psi. Cooper’s Discoverer A/T3 LT LRC (when it was still available) was rated 1000 kg @ 50; a 90 kg advantage for the P tire.

As you’re probably not using that classic truck to haul bricks, I’d recommend running the P tires at 35.

If you’re planning a Home Depot run for drywall sheets, or towing a big trailer, crank them to the sidewall max. If it’s 44 psi, like the Michelin example, you’ll be safely above the capacity the original LRC LT’s were rated for anyway.

Brian Early is a Red Seal-certified automotive technician. Send your questions to wheels@thestar.ca. These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle.