Max on those tires is about 42psi, I would start about 35 and just watch how they wear.

 

You should have a pressure reference on the side of the tire, normal pressure would be at 32psi

 

The pressure rating on the side of the tire has very little to do with the pressure you run, it's ONLY the maximum pressure the tire is built for. Actual pressure depends on the weight of the car.

 

Radial tires have roughly twice the heat dissipation as bias-ply tires. Heat buildup is what can lead a tire to fail. Bias ply and radial tires build up pressure at highway speeds so that 24 PSI in a bias ply might build up to 30 or more while driving and can't shed the heat very well. If you run 24 in a radial it's never going to build to a proper highway pressure and running too low increases heat which can result in a blow out from heat. For example, my F150 specs on door jamb is 41 PSI. It will build to around 45-46 PSI on the highway. Tires stay warm enough to keep cornering traction. Cold tires don't corner well.

 

running too low increases heat which can result in a blow out from heat.

This is 95% of our trailer tire failures at work. Run so low on air with a heavy load, you can see the lines on the tires from where the bead of the rim wore it through.

One new mechanic insisted the pressures on these tires should be lower than whats on the tire for heat. I do get that, but i told him, "let me know when you find a tire around here that failed due to over-inflation." Havent seen one yet.

I wouldnt be afraid of running a tire rated at 42 lbs at 30 or so on our cars, but there is a little trade of for whatever better ride you may get with mpg's and tire wear, which, i mean, depending on how much you drive, might be fairly insignificant.

 

Take it for a ride on a warm sunny day when you stop immediately exit the vehicle & check tire temp @ both edges & center using an infrared pyrometer. Colder center = add air if + / - a few * = good to go. If one edge is hotter / colder than the other you have an alignment issue.
Or:
Use the *** hayseed method, you need to be on a paved driveway or road this method don't work on dirt, make a wide chalk line across the tire thread, drive the car a bit then stop & look @ the chalk line, worn on the edges = add air, worn in the middle = let some air out.

 

I was asking this same question last month and got a range of answers and none of them gave me a warm and fuzzy feeling. The best info I found was on the internet from other Nitto tire owners. 35 PSI is right where you want to be. At one time I was running identical sizes to what you are running now and 35 PSI was correct then also.

 

Refer to my post about a month ago. I was looking for a softer ride which entailed a discussion about the correct tire pressure. All my life, I was putting in 35lbs in all my cars. I put on Bilstein shocks to get a smoother ride but to no avail. A tire expert who has owned a tire shop for 40 years told me I wasted my money. All tires should be pressured at 30-32lb no matter what the size of the tire or weight of the car. He put that much in dump trucks. He says that will also allow for a softer ride. I did that and now am experiencing a softer ride. I have huge Mickey Thompsons 28 x 12 x 18's on the rear of my 57 and I used to go with 35/37lbs but now I put in 32lbs. Hope this helps you.

 

A tire expert who has owned a tire shop for 40 years told me I wasted my money. All tires should be pressured at 30-32lb no matter what the size of the tire or weight of the car. He put that much in dump trucks.

That has to be the dumbest tire expert in the history of tires.

 

I think 30lbs is not overinflated. 30-32lbs would be just right for all of the reasons you state.

 

Not to offend anyone, but in my mind when you go to 17-18 inch wheels and tires, (and especially 18-20 inch), you're usually using a tire that, by design has a stiffer sidewall. Which means the ride is going to be stiffer, (by design). I know that what your using or installing for suspension components plays a big part in your ride also. However, my goal is to have a car that handles, corners and is responsive as good as I can get it. I've been using 17-18 inch tires and wheels for over 20 years now. I guess I thought a stiffer ride was part of the deal and I accepted that. But not to the point that a stiff ride wears me out every time I drive my Chevy. And as part of that, my thought is to inflate tires enough to get the best wear and mileage. If a softer ride is my goal, I'd go back to 15 inch wheels and tires with more sidewall to cushion the road bumps. Just saying....

 

It's all about the required volume of air in the tire to safely support the weight. Just look up information for the load index number on the tire listed after the size on the sidewall, and also in fine print on the tire what pressure it takes to support a given weight. Most GM cars have always used the appropriate load index size on almost all of their cars and required 30 psi. The bigger the air chamber the less pressure required. Low profile ties usually have less volume. Load index is what is all about. I have been installing tires for 20 years.
My 275/60-15 tires have a load index of 107 rated at 2149 pounds per tire at 44 psi. I have a 3000-pound car that I run them at 28psi which is more than enough.

 

I run 37 psi in my Merc and Lindsay’s Prius. Bertha‘s and Maude’s tires are at 35 psi.

 

Thanks for everyone's responses. I've enjoyed reading them and have learned quite a few new things.
Shadetree55 explained to me why bias tires are so underinflated. More rolling resistance=more heat=higher psi buildup.
I think I'll lower my tires a bit to around 28-29 psi like 55 Rescue Dog recommends and see if I can tell any difference.
(I will add some psi back if I know I will be traveling at highway speeds for long distances.)

I've got to admit I've never paid any attention to the number after the tire size.
I just Googled 'tire load index calculator' and it looks like there are a lot of them out there.
I need to educate myself more on the load index of tires and play around with some of those online calculators.

I checked my tire pressures earlier and I'm averaging about 33.5 psi, but my car currently has no weight on the tires.
My rear tires have a load index of 110V and the fronts have a load index of 97H.

 

You need to check with the manufacture. The max air pressure is what is shown on the sidewall for the max load that is also listed on the sidewall. When I worked in the tire industry years ago, I had the air inflation scale for each manufacture and tire we sold. You may want to find a dealer for the brand tire you are running. They should be able to get that info for you.

 

I spent a little bit of time today reading some Google links and playing with a couple of calculators.
With the load index of my tires, I think will keep driving it like this for a while, and then I will try and run 27psi rears & 30psi fronts and see how I like it.

 

Since my tires and wheels are basically C4 Corvette sizes I run them at 35 psi which is what they should be on our 90 Vette. My 55 and our Corvette are within 200 lbs. of each other as well so a very similar load also. I have had this set of tires on my 55 for 4 or 5 years now with no unusual wear patterns. You should be fine with pressures in the 30 to 35 psi range.