My car says to inflate tires to 30 psi. Fine and good, but I just got 4 new tires a few weeks ago and when I checked the pressure (using one of those cheap, $1 pencil tire pressure gauges) I noticed my tires are all in the 35-40 psi range. The car had been sitting without being driven for several hours when I tested it, but it is warm outside.
Is there a reason they’d overinflate them? Is an extra 5-10 psi something that can cause issues? Should I let some air out to get them closer to 30 psi, or is an extra 5-10 psi not going to have any real issues for traction or tire lifespan?
There is a balancing act between ride and rolling resistance.
As my tyre dealer explained, the extra 5psi/35kpa gives you a fraction harder ride and a fraction better fuel economy. When I put the care into the dealer for service they set the pressures to makers recommendation. When I periodically check tyre pressure I run them at the higher mark.
They have to over inflate to force a seal on the wheel, but they should have then bleed down to the recommended pressure. I’m not sure I’d trust a cheap gage though. I’d go back to the tire place myself.
There are tradeoffs, of course. While an extra 5 psi will reduce the rolling friction (and thus give you better fuel economy), the ride is harder (as you mentioned), the treadwear becomes uneven, and the braking performance may be adversely affected.
I don’t have experience with changing car tires, but I’ve changed dozens of motorcycle tires and found that the tire beads snap into their final position on the rim without a whole lot of pressure - maybe 20 psi.
As for why a tire shop might deliberately overinflate, I can think of a couple of reasons:
It saves time. The mechanic doesn’t have to go look up the correct pressure for your particular car, either in a book or in your car’s door jamb.
Underinflation is dangerous, overinflation much less so. To cover for any potential inaccuracy in their tire gauge, generous overinflation helps them avoid liability for catastrophic tire failure due to underinflation.
There’s also a potential accidental reason. If they’re using a tank of compressed air at room temperature, the air expands and cools as it fills your tire to 30 PSI. Then it warms up to room temperature, increasing the pressure. Using this adiabatic process calculator, I came up with the following:
State 1: Supply tank, 200 psi and 70 F:
State 2: Tire filled to 30 psi, but not yet warmed up: temperature of air inside tire is -152F
State 3: Tire and air inside have warmed to 70F, pressure is now 51 psi
This assumes the tire is inflated instantaneously, leaving no time for the air to warm up during the fill process. A real fill process takes time, allowing the air that’s already in the tire to warm up a bit and limiting the total air mass that can be delivered before the mechanic sees 30 psi on his gauge. And maybe they don’t use a 200 psi tank, just a 150 psi tank, so the initial adiabatic temperature drop isn’t so large. And maybe the tank is well above 70F (because the shop uses a lot of air and the compressor is running a lot), so that when that air gets into your tire, its temperature isn’t as low as -152F. So there are a number of real-world factors that could explain why you see 35-40 psi (and not 51 psi), after your mechanic has filled them to 30 psi.
Ideally, tires should be inflated to the pressure indicated on the tag on the front door frame of the car and not what is shown on the sidewall of the tire. I agree that you should take it back to the shop and have them check it with their gauge.
Driving for any significant distance heats the tires and warms the air within, increasing pressure. Unless the OP is prepared to sit at the dealer for an hour or so to let the tires cool down to something near ambient temp, any measurement the shop makes will be inaccurate (relative to the “cold” measurement one is supposed to use).
OP seems able to use a tire gauge and let air out of their tires; the most expedient approach is to take care of this at home when the tires are cold. If there’s concern about the accuracy of the cheap $1 pencil-type gauge, I’d suggest spending just a little more money for a bourdon-tube gauge or a digital gauge, either of which should provide decent accuracy for less than $20. Here’s a good candidate:
Note that this gauge has a release valve on the side. This holds the pressure so you can read it after you remove it from the valve stem, but it also lets you conveniently bleed tire pressure down while you hold it on the valve stem.
I find the two tire shops and the oil change place I go to all seem to inflate to around 37 psi, no matter what the sidewall says. My wife’s car has recommended 34. My car is 36. My old car was, I believe 33 in the front and 32 in the back? Or was it the other way around? But whenever I’d get my vehicle or my wife’s vehicle back, it’d be 37-38 psi (as measured by a boudron gauge (Those pencil gauges are absolutely useless to me) or the car’s internal tire pressure sensors.) I’ve never much worried about it – it’ll give you a slightly harder ride, and at some extreme – I believe – it can cause a little less grip on the road, but at that range I’ve not noticed a thing.
If the car is parked outside, the tires in the sun can show a significantly higher pressure than the tires in the shade.
Also, as mentioned, tire gauges are very inaccurate. If you own one tire gauge then you (think) you know your tire pressure. If you own two or more tire gauges, then you can never be sure of their pressure.
I notice ~1 psi difference for every 10F change in temperature and about 1-ish more psi from driving around. This is based on the live readout on my wife’s car. Her car was filled to 37 or 38 psi just two weeks ago when it was around mid 70-ish after we changed out a tire, and the pressure range has been 35/36 psi (55-60F days) - 39/40 psi (nearly 90F days and highway driving.) The recommended tire pressure as per the sticker on the door frame is 36 psi cold, and the max pressure according to the tire sidewall is 49 psi.
I had one shop UNDERinflate mine. Like down to 5-10 psi. When I went back & confronted the manager, he just gave weak excuses about his employees’ incompetence or malfeasance.
My experience with this is that independent shops can be hit or miss about tire inflation, but dealerships are the worst. Every time I’ve taken a car to a dealer for anything maintenance related, they would deflate the tires to 5 under recommended psi. Every time I’d call them out on it and they would claim it was for “better handling and safety”. yeah sure buddy whatever…
I agree with this. But for completeness sake it should be mentioned that accidently putting way too much pressure into a tire (e.g. 2X) is dangerous.
I read this story a number of years ago, and it’s so horrific that I’ve never forgotten it. I just could not imagine a tire exploding on my lap. Since reading it, I have taken care to not overinflate when adding air to my tires.
The link to the story seems to be not functional in some way, for me anyway, the address bar shows the headline for it, but there isn’t any story. Anyway, here is a different link to the story on another site that I found.
Shouldn’t be quite that much, but in that ballpark. Ambient temperatures will be around 520-530 F above absolute zero, so a change of 10 F is a little less than 2%. A 2% increase from a base pressure of 35 PSI would be 0.7 PSI.
yeah sure buddy whatever…