Again, a 2 psi difference will have a negative effect. * Tire wear and life is negatively impacted with improperly inflated tires. Excessive wear on the outside and inside edges is a sign of an under inflated tire or an alignment issue.
You may even experience sharper cornering and increased fuel economy. So, when filling your tires, the recommended tire pressure is the best compromise between handling, comfort, fuel economy and safety. But it's certainly fine to go over the recommended inflation by a psi or two.
I think most passenger cars can easily tolerate a difference in tire pressures of 2 psi above and below recommended. At 4 psi below recommended, fuel mileage can suffer. Underinflated tires tend to wear out the edges of the tire tread as the tire tends to cup.
How much does temperature affect tire pressure? A good estimate is for every 10° fluctuation in air temperature, vehicle tire pressure will adjust by about 1 psi. So if outside air temperature decreases 30° from your last tire pressure adjustment, expect tire pressure to drop about 3 psi.
2 psi under probably won't hurt a thing and it might give a little more traction in ice and snow conditions. Tire pressures can vary quite a bit at different ambient temperatures. Also running a tire at too low a pressure will overheat the tire and quickly ruin it.
If you have to fill your tires when the tires are hot, inflate them 3~4 psi above the recommended psi, and check again with your gauge when they are cold. It's ok to overinflate a bit when filling the tires, as you can let the air out with the gauge.
Slightly overinflated tires (3 to 5 psi over placard) is sometimes a good thing. It promotes tire wear, fuel economy, steering responsiveness, hydroplaning resistance, snow traction, but the ride quality suffers. But don't do more than 3 to 5 psi (assuming the tire size is as listed.)
The 4psi rules states that “if your tyres are inflated correctly then the tyre pressure measured when the tyres are hot (after travel) will be 4psi greater than the pressure measured when they were cold (before travel)”.
Tire pressure between 32 to 35 PSI is most suitable for most vehicles. However, one can go as low as 20 PSI (although that is not recommended). Anything below 20 PSI is considered a flat tire. PSI stands for Pounds per Square Inch.
As a rule of thumb, a tire will inflate by one pound per square inch every second and a half. Some vehicles require all four tires inflated to the same psi.
Recommended tire pressure
Based on normal tire sizes for each segment, small-sized car are usually 30 psi, medium-sized cars are 36 psi, and large cars are 42 psi. Keep in mind that warm tires normally exceed recommended cold tire pressures by 4-6 psi.
If your tires aren't inflated to the proper tire pressure, they are saggy and soft. The softer your tires are, the more rolling resistance there is between the road and the rubber, and the harder your engine will have to work to move your car.
Your car has a recommended tire pressure that will give the best gas mileage, handling and tire life for that car — and it's written right on the vehicle's door. That's the one you should follow when filling them up with air to the recommended pressure, measured in pounds per square inch, or psi.
Low Tire Pressure Will Cost You
Because low tire pressure distorts the tire's footprint—where your car's rubber contacts the road—tire wear can increase dramatically. This often shows up in unusually heavy wear on both a tire's inner and outer shoulders.
Overinflating your tires can make them more vulnerable to damage. Like an overfilled balloon, tires filled to their maximum capacity are more stiff and inflexible, which makes them more susceptible to damage from potholes, curbs, or debris.
Most passenger cars' psi requirement will be between 30 to 35 psi, but several vehicles fall outside of that range and every vehicle will have specific requirements. Good tire air pressure ensures that tires wear evenly, provide a smooth ride, and increase fuel efficiency.
The recommended psi for 4WD's on bitumen is around 30-35 psi. It is also good practice to read tyre pressure when the car has been at a stand still for around 3 hours, as when tyres get heat in them they tend to expand, and therefore alter the tyre pressure reading.
In contrast, in Australia and China the units “kilo-Pascal (kPa)” and “mega-Pascal (MPa)” have become established. In North America, the preferred pressure unit is “psi (pounds per square inch)”.
If your car is underinflated, then it is going to feel sluggish. Underinflation will also lower the fuel economy of your car and shorten the lifespan of a tire. Overinflating a tire will also shorten its lifespan. The car will also handle more poorly because less of the tire is in contact with the road.
Tires are overinflated during the shipping process, so as to help prevent the flat spot from forming in the tire as it sits for days on in during the shipping process. It is supposed to be part of the delivery check process that the service department deflates the tires to the proper pressure.
In most of these cases, 40 psi should be more than enough, and since most modern tire designs will allow up to 44 psi (in some cases more) this is not a problem. If you have a heavily loaded vehicle, up to 44 or even higher is ok if the tire allows it, but this is a little high for most vehicles.
Answer: 2.5 bar corresponds to 2.5 times 14.5, so 36.25 PSI.
For most passenger cars, the ideal tyre pressure will be somewhere around 2.2 bar (or 35 psi) when the tyres are cold.