Automobile Repair-Tire pressure and Tire pressure gauge

car tire

Tire pressure

Proper tire inflation is very important. All tires should be inflated to the pressure recommended in the vehicles owner manual. Cars sold recently that meet U.S standards will have a notice, often on the driver’s door frame but sometimes elsewhere, like in the glove compartment, showing what the front and rear pressures should be, and they should be listed in the owner’s manual. This should be between the minimum and maximum pressure ratings listed on the sidewall of the tire. Improper inflation may result in poor/unsafe handling, blowouts, poor gas mileage or excessive tire wear. Contrary to some expectations, blowouts are much more frequently a result of underinflation than overinflation; as tire pressure lowers, the sidewalls of the tire flex more during rotation causing more heat, which builds up and damages the tire. In contrast, tires are manufactured with a substantial margin of safety for overinflation.

The manufacturer’s suggestions are a good place to start, if it is intended to adjust pressures optimally. Historically American cars would usually have the specified tire pressure somewhat low, for a more “cushiony” ride, but this practice is reduced of late. If suggested pressures are not available, or if the vehicle is modified, Usenet quotes an algorithm from Oscar Pereda, an engineer for BFGoodrich, as a good starting point for pressure in pounds per square inch:

(Vehicle Weight in lb/100) psi + 2 psi at heavier end + 2 psi all around if suspension and alignment are stock.

If equipment is available to measure the temperature of the tire tread, they should be inflated so as to achieve even temperature distribution across the treads of all four tires; higher temperature in the center of the tread compared to the edges indicates overinflation, while the opposite indicates underinflation; higher temperatures at one end of the car similarly indicates that that end is overinflated with respect to the other end.

Tire pressures can then be altered to tune handling. If, after the pressure is set up, the car seems to lack adhesion at one end, then air can be added to the tires on that end (one or two pounds at a time) until balance seems to be achieved to suit the driver. As an example, in the rear-engined Chevrolet Corvair, the normal oversteering tendency of a rear-engined vehicle was combated by keeping front tire pressures down to twelve psi, which was safe because of the low weight the front tires carried on that car; if the front tire pressure was raised to what would be normal for a front-engined vehicle, the car would become dangerously unstable.

In the case of tires with very thick tread, such as aggressive mud or snow tires, the tendency described above to build up heat and deteriorate is that much greater, and therefore they should be inflated closer to the maximum pressure given on the sidewall, altered as necessary for safe handling, as described above.

The almost universal adoption of radial tires has made adhesion on wet pavement less of a problem; however, where bias-ply tires are still used, the speed at which “hydroplaning” occurs, i.e. the tire loses contact with the pavement and begins to slide on the film of water, can be easily calculated as the square root of the tire pressure, times ten psi. If high speed driving on wet pavement with bias-ply tires is contemplated, this is something that must be taken into account.
You will need a tire gauge. You can get a cheap pen style gauge for between $1 and $5 (USD) at a gas station or automotive shop.

To check the tire pressure, locate the valve stem, take the cap off it, and firmly push the gauge onto it, and then remove the gauge and check the reading. The gauge should only hiss briefly while pushing it onto the valve stem. If it hisses more, then you aren’t pushing on it hard enough, and are not getting an accurate reading.

Spare tire

Proper inflation of the spare tire is also very important. All tires will lose some pressure over time, but since the spare tire is usually hidden from view, many people overlook it. If you don’t check it regularly, your spare tire could go flat, and cause further problems when you have a tire failure.

The spare tire is often different than the regular tires. If the vehicle has a full-size spare, it’s usually the same as the tires that were originally installed on the vehicle, which may differ from the tires currently installed. If it’s a mini spare, the difference is obvious. So, make sure to check the pressure ratings on the sidewall of the tire to find the proper tire pressure.

If you don’t already know how to access your spare tire, you should check your car’s manual. There are too many different configurations to explain it in this book.


Tire-pressure gauge

A tire-pressure gauge is a pressure gauge used to measure the pressure of tires on a vehicle.

Since tires are rated for specific loads at certain pressure, it is important to keep the pressure of the tire at the optimal amount. Tires are rated for their optimal pressure when cold, meaning before the tire has been driven on for the day and allowed to heat up, which ultimately changes the internal pressure of the tire due to the expansion of gases. The precision of a typical mechanical gauge as shown is ±3 psi (21 kPa). Higher precision gauges with ±1 psi (6.9 kPa) uncertainty can also be obtained.

Built-in tire pressure sensors

Many modern cars now come with built-in tire pressure sensors that allow all four tire pressures to be read simultaneously from inside the car. In 2005, most on-board Tire Pressure Monitoring Systems (TPMS) used indirect pressure monitoring. The anti-lock brake sensors detect one tire rotating faster than the rest and indicate a low tire pressure to the driver. The problem with this method was that if tires all lost the same pressure then none would show up against the others to indicate a problem.

Regulations on tire pressure

Since September 2007 all new automobiles below 10,000 lb (4,500 kg) in weight sold in the United States are required to incorporate a Tire Pressure Monitoring System, which is capable of monitoring all four tires and simultaneously reporting under-inflation of 25 percent of cold placard pressures in any combination of all four tires. TPMS known as Direct TPMS are capable of TREAD Act legislation requiring simultaneous pressure measurement for each tire pressure.

Early TPMS sensors required batteries but the latest TPMS technology eliminates all sensor batteries.


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