I would add a caveat to Tom and Ray’s advice however. I inflate my spare tire to a higher pressure than the mounted tires, mainly because I anticipate it might lose pressure between the time I fill it up and the time I use it. I inflate my tires to Honda’s recommendation of 29 PSI, but I inflate my spare tire to 35 PSI, because it is easier to let some air out of a tire than put air in, so if I get a flat, and the spare is at 32 PSI, all I have to do is let a little air out. It’s better than inflating the spare tire to 29 PSI and finding it only has 25 PSI when I need to use it.
NO..Pressure will stay EXACTLY the same. What will change is the amount of tire contacting the road. Tire pressure doesn't change with weight applied. Think about it. When you buy new tires...do they only fill the tire (while it's off the vehicle) to a real low number then when they put the tire on the car..the psi is now correct? NO - PSI doesn't change.
Just being picky, but the pressure does change. When you put the weight of the car on the tires, the tires do compress. As you just said, the amount of tire contacting the road changes. The rubber itself is being compressed, but is also now pushing in on the inside of the tires. As a result, the volume is slightly decreased, increasing the pressure.
PV=nRT. Since n, R, and T don’t change in steady state conditions, a decrease in V leads to an increase in P.
@starman1, if there is a pressure difference, it’s not a big enough difference to see on a non-scientific tire gauge. I’ve tested this theory myself with a digital tire gauge and there was no measurable difference between the pressure in the jacked-up tire and the pressure in the tire with the weight of the car on it. My digital tire gauge measures to the nearest tenth of a PSI, and I got the exact same readings in both scenarios, just like Tom and Ray did.
@Whitey
Yes, this is a case where the difference in pressure is negligible - the word you gave criticism to when the OP used it earlier in this thread.
I didn’t intend to criticize the word, only its misuse. In this case, I agree the difference in tire pressure is negligible. In my opinion, however, uneven tire tread wear that can be seen with the naked eye is not negligible.
At the end of a tire’s life, I would say a barely noticeable less center tread depth compared to outside treadepth is not significant. (Now sorry that Discount Tire did not measure the difference. I did point it out to them.)
That would be SO minor an amount it probably couldn't be measured without a very sensitive tire gauge (which I doubt you could even buy).
I did say I was just being picky on the wording. 40.00 psi is not “EXACTLY” 40.01 psi
You can calculate the weight of a vehicle by it's footprint. It's directly proportional.
And if you run your tires at 50% ABOVE what is recommended…that means the tires will have 50% LESS contact with the road.
Be careful here with your use of applying this proportion relationship. That seems to suggest if someone ran their tires at 100% above what is recommended, they would have 100% less contact with the road. Which would mean their car would be floating on air.
if someone ran their tires at 100% above what is recommended, they would have 100% less contact with the road. Which would mean their car would be floating on air.
Now that would be nice. Think of the fuel savings!
Well, if Honda recommends I inflate my tires to 29 PSI, and I inflate them to 58 PSI, they’re going to be bouncing all over the place when I drive, so at some point, they could actually all be off the ground for an instant. If you took a picture at that instant, it might look like the car is floating on air.
