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Tire Pressure Compensation


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Old 08-29-2011, 12:32 AM   #1
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Condition: Setting tire pressures, particularly for vehicles being operated in cold temperatures.

Repair Procedure: It is important to have tire inflation set to the proper pressure for best fuel economy, handling characteristics, and tire longevity. The advent of Tire Pressure Monitoring Systems (TPMS) adds to this importance as an underinflated tire will illuminate the vehicle's warning indicator, and some vehicles need the pressure at the proper value when the TPMS system is initialized.

In many parts of the country, winter brings temperatures substantially lower than summer, or the repair shop's ambient temperature. The technician must be aware of this and compensate the tire pressure for the environment in which the vehicle will be operated.

BMW offers a reminder that for every 10F drop in ambient temperature, tire pressure can decrease by 1 psi. If tires are inflated in a heated shop, the pressure can drop by as much as 3 psi when cooled to the outside temperature.

Lexus offers a chart to assist the technician in evaluating the correct compensated pressure value (Figure 1).

On this chart, Plot 1 (solid line) is used for tires that are "cold": the vehicle has been parked outside and not driven for 4 hours.

Plot 2 (dashed line) is used for "warm" tires: the vehicle has been driven on surface streets for about 30 minutes prior to inspection.

Plot 3 (dotted line) is for "hot" tires: the vehicle has been highway driven for at least 60 minutes prior to inspection.

(see attachment)

The vertical axis (4) is the adjustment pressure in pressure in psi.

The horizontal axis (5) is the DIFFERENCE in temperature.

For example: the shop ambient temperature is 68F, and the outside temperature is 14F. The difference is 54, the temperature value to use on the chart.

Using this example, if the vehicle was parked in the shop overnight, or tires were just mounted ("cold" tires), the pressure would be increased about 4.9 psi over the cold specification on the placard on the vehicle (the intersection of 54 and the Plot 1 line).

If the vehicle had "warm" tires, the pressure would be increased about 6.7 psi above the cold specification on the placard (the intersection of 54 and the Plot 2 line).

A vehicle with "hot" tires would be increased about 9.0 psi above the cold specification on the placard (the intersection of 54 and the Plot 3 line).

When the tires cool to the outside ambient temperature, they should be at the correct inflation pressure.
Attached Images
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