Doing this can affect braking distances and handling ability, not to mention tyre wear, wet performance and the fact that if you corner hard on a hot day, your tyre could blow out. Very dangerous, i wouldn't do it.
I have ran most of my vehicles over the years anywhere from 5-10 psi over the "label" pressures. Keep in mind, those labels are set for a certain vehicle weight as well. If you are going to be taking a trip, and your car is packed full of people and stuff, you will WANT more pressure in the tires to compensate. Dangerous? Yeah, if you go way over the sidewall rating pressure. Those are the maximum recommended cold pressures for your tires, not the recommended pressures for the vehicles front/rear gross weight.
Perfect example, my current caravan is I believe 34 PSI according to the label. I run them at 38. The sidewall states 44 for a maximum. I often have the van packed with 3 kids and the other half, not to mention the occasional roof rack bag even full along with the rest of the interior. A bit more helps for the trips, and I find it helps my mileage. It will give you a bit harder ride when the vehicle is carrying less stuff and sure it may sacrifice a bit of traction, but most of us aren't driving our cars on Nascar tracks either. I have never noticed any traction problems in most weather. Winter is probably the one that might be affected most. But you should be running snow tires anyway! lol
(by the way, tip for bad weather traction, lower your pressures a few PSI (say 5-15) if you are having trouble with being stuck and need to get going, but be sure to correct the pressure asap later. It will give you more traction though)
One thing to be sure to do, is check your tire wear when running higher pressures, to be sure you are not wearing the center of your tread. If you are (and I have not seen anyone with that issue in a while, and I certainly never have) you are running way too much pressure. Same goes for too low, you wear out the outer sides of your tires leaving a lot of tread in the middle. And if you have uneven wear, (insides bald, while outsides are good) get it ALIGNED and the wheels BALANCED along with new tires, don't just throw new tires on and say done..
Having your car properly aligned will ALSO help your mileage.
4 PSI more is perfectly okay. A few weeks ago, I had a "Low Tire" warning on the dashboard when the temperatures dropped about 50 degrees. I went to the gas station to put some air in but I didn't have a Tire Pressure Gauge. This is my first car so I didn't know how much air would it take to add the additional 6 PSI to get rid of the warning. So I just filled each tire holding the thing in for 15 seconds.
When I went to the dealer to check out why my car was only getting 14 MPG when it's suppose to get 21 city/28 highway. The guy checked the tire pressure and it was 56-65 PSI on the tires. It's suppose to be 35 front and 38 rear. I actually drove two weeks with it like that and had no problems. But the service technician said an extra 2-3 PSI would be better. Now, I fill the front to 37 and the back to 40.
Always keep in mind the heat factor. Long drives in the summer on a hot road at high speeds will increase tire pressure by several pounds. If you are over inflated that pressure will build up and you could have a dangerous situation, which can cause a blow out. Too low air pressure in the colder months will give your car more grip but at the expense of over wear on your tires. Stick with what the manufacture says, as that compensates summer driving.
I'm not an expert, but I disagree with a few things here:
@Draigflag "very dangerous." <- Tire manufacturers specify their Max PSI with buffer safeguard to protect themselves from lawsuits. This about how often non-car-guys fill their tires and don't pay attention. Happens all the time, and 4 PSI over is easy to obtain. Tire manufacturers build this in to their Max PSI ratings. The tire will be fine...all day long at 4PSI over.
@DTMAce "(by the way, tip for bad weather traction, lower your pressures a few PSI (say 5-15) if you are having trouble with being stuck and need to get going, but be sure to correct the pressure asap later. It will give you more traction though)" <- Depends on the type of bad weather. For snow/ice, u actually want higher tire pressures so the tire is more rigid and does now flatten out as much. This way you have more lbs/sq. in on the tire, and you cut through the snow better and have more weight on a single point trying to grip the ice.
@PayHoe "Tyre manufacturers want pressires high as it improves handling and tyre longevity. car manufacturers want lower pressures as it imcreases comfort." <- Not necessarily. ~Correct~ tire pressure improves handling (I assume you are talking about normal conditions, not Snow/ice). Too high pressure and you don't get enough heat in the tires, thus they won't grip as well as usual. A ton of too much pressure and you won't have the full contact patch of the tire in contact with the group at all times either. Too low pressure can overheat the tires to where they get greasy and lose traction. Too little traction will make the car feel sloppy in transitions and contribute to body roll as well. To low pressure also can heat up one side of the tire too much as compared to the rest of the tire. You want uniform temps across if you can manage it.
Bster13 <- currently hypermiling while running 50PSI in 44PSI max tires just fine.
Personally i try and keep mine at the recommended pressures. Imagine you caused an accident, completely your fault, the Insurer sent an assessor out and he found that your tyres were "over/under inflated"
Now they have and excuse not to pay out, as you are responsible for the maintenance of the car. Just a thought, as some insures will use any excuse not to pay out.
@Draigflag. To each his own. I'd argue that the majority of us over-inflating our tires are hypermilers doing the speed limit or less, thus much less likely to cause or be involved in an accident. I'd say speeding is a bigger cause of at-fault accidents.