Lets talk about air.

[Chiropteran]

So my car, a Ford Fiesta, rates the tires for 32PSI, according to the sticker in the door frame. But the actual tires themselves are rated for 51PSI. How dangerous is it to fill them to, for example, 45PSI? Well over the car's recommended PSI, but well within the limits of the tires themselves.

A related question, 3 of my tires seem to need to be refilled constantly. For example, on the way back from a wedding the low tire pressure indicator came on, so I stopped and filled every tire to 35PSI. 3 weeks later and the indicator is on again, and 3 of the tires were down to 20-25PSI, filled them all back up to 32PSI, yet the 4th tire was still at the 35PSI level from when I filled it last month. What causes some tires to lose so much pressure while another tire keeps the exact same pressure? Tiny leaks? Is there any good way to fix this, other than replacing the tires?

 

[JCH13]

So my car, a Ford Fiesta, rates the tires for 32PSI, according to the sticker in the door frame. But the actual tires themselves are rated for 51PSI. How dangerous is it to fill them to, for example, 45PSI? Well over the car's recommended PSI, but well within the limits of the tires themselves.

A related question, 3 of my tires seem to need to be refilled constantly. For example, on the way back from a wedding the low tire pressure indicator came on, so I stopped and filled every tire to 35PSI. 3 weeks later and the indicator is on again, and 3 of the tires were down to 20-25PSI, filled them all back up to 32PSI, yet the 4th tire was still at the 35PSI level from when I filled it last month. What causes some tires to lose so much pressure while another tire keeps the exact same pressure? Tiny leaks? Is there any good way to fix this, other than replacing the tires?

It's not necessarily dangerous, but you may start to wear the center of the tire tread prematurely. You'll suffer a bit in ride comfort too.

It would seem as if three tires never got seated on a clean bead. That time frame is far too fast for a tire (let alone three) to drop 10-15psi. I would have a tire shop reseat/reseal those tires. The dealer you bought it from (I assume) should do it for free I think, it's that bad.

 

[leper84]

First question- not really dangerous but not smart. The tires will be inflated to a point where your car doesn't have the weight to maintain a good contact patch; instead of the entire tread rolling flat the center will balloon out. So you'll have less traction and experience faster, uneven tire wear. But you'll still be well within the pressure and load limits for the actual tires.

As far as rolling resistance, I wish I could find the chart I'm thinking of online. It graphed out fuel economy gains/losses due to tire pressure. Basically while you can experience massive fuel economy gains from returning an under-inflated tire to correct pressure, once you go just a tiny bit over oem recommendation you hit a giant wall of diminishing returns. So while going from 25psi to 35psi would net you (random number) 15% fuel economy improvement, going from 35psi to 45psi might only net you .5%. So any gain you might make in fuel economy would be negatively offset by burning through tires faster.

If you really wanted you can go up to 35-38psi without much drawback, and the car might even feel a bit more nimble, but I would stop there.

As to your second question- if the one tire is maintaining the pressure, you have an accurate gauge and the others are loosing that much air you probably have leaks in the other tires. Most likely you picked up some nails. Take it somewhere you can trust, they can check it out and tell you what you have and if its repairable or not.

 

[SpatiallyAware]

This very topic gives me nightmares............

You should, in general, stick to the pressure on the doorjamb for any passenger car on stock tire size situation.

The tire company makes tires for a lot of vehicles, not just yours. They're rated for up to 51psi because they might be used on a trailer or some other application... It's not the recommended pressure for your vehicle, it's the max pressure for the tire independent of application.

As jch said, that's way too much air to lose. Either bad bead seat or valve stem issue.

 

[waggy]

always inflate to sidewall man! i know what im talking about!

 

[JCH13]

always inflate to sidewall man! i know what im talking about!

In case anyone's meter is broken... :whiste:

 

[Chiropteran]

Last time I filled the tires I bought a portable automatic electric air pump. This was Saturday Oct 13. I'm waiting for the low pressure light to come on again and I'll see just how much pressure was lost in what sort of time frame.

The current tire configuration is from March of this year, I had a non-repairable flat rear tire. I had both rear tires replaced, as I wanted to keep an even tread, and kept the old "good" tire as a spare in case of a similar puncture in the future. The recent low pressure problems started probably 5 or more months after this, does it still indicate they might have seated or sealed them incorrectly?

 

[Zenmervolt]

(1) I agree with everyone that ~10 PSI in 3 weeks is a lot of air to lose. A bad bead seems unlikely given that they all held air fine for several months after the initial mounting but I'd still keep checking. One thing to remember is that tire pressure should be checked "cold," that is, without having driven the car for quite some time. If you filled all the tires to 35 PSI after having driven for several miles, it's possible that much of the "loss" of pressure was just from the tires cooling down again. Still, I'd start checking for leaks around both the bead and the valve stem. Get some soapy water and spread it around both the valve stem and the bead of the tire (where the tire meets the rim of the wheel) and look for bubbles. If you don't find anything, just keep watching the tire pressures and see if the leakdown gets worse.

(2) As for what to actually set the pressure at, there are actually very few hard and fast rules. The big one is never above what the sidewall shows. However, that doesn't capture the full story. If you inflate the tires to the sidewall pressure, you'll greatly change the car's handling and lose a lot of ride comfort. The sidewall pressures are meant strictly as structural limits, not as recommendations for what to use for a given application. I don't recommend inflating the tires to sidewall pressure in your case because there's such a big difference (19 PSI) between the recommended pressure and the sidewall maximum pressure.

That said, you can definitely experiment a bit using the manufacturer's recommendations as a baseline. The manufacturer recommendation is based on a set of design compromises including ride comfort and may not always be best for a particular situation (for example, one of my cars has two separate recommendations depending on whether the car is likely to be fully loaded or only partially loaded). Leper84 gave what I feel is a good recommendation when he said to not try anything much beyond 38 PSI.

ZV

 

[desy]

I usually go door panel and add 3 psi as I HW travel a lot

 

[mmntech]

I usually go door panel and add 3 psi as I HW travel a lot

This. Doesn't hurt to over inflate a bit but always go by the car manufacturers recommendations. I've only ever see tires inflated to 50psi in commercial vans and trucks.

 

[jagec]

I usually go door panel and add 3 psi as I HW travel a lot

That's exactly what I do. Plus, if the weather starts turning cold and I haven't checked the tires lately, I can justify being lazy and putting it off for a few more days.

I've only ever see tires inflated to 50psi in commercial vans and trucks.

Which is part of why driving them feels like being strapped to an office chair and pushed down a cobblestone alley.

 

[nedfunnell]

Could be valve stems. I had a leaking tire hold air like a champ after changing the valve stem.

Edit: I knew it was the valve stem and not the bead because a soapy-water test revealed bubbles forming around the stem. It was a fairly fast leak.