TYRE PRESSURE AND ALTITUDE

I recently travelled by car to Pretoria from Cape Town. I checked the cold tyre pressure at the coast and it was reading 2,1 bar. After reaching Pretoria and allowing the tyres to cool, I checked the pressure again and this time the reading was 2,3 bar.

I am aware that barometric pressure changes with altitude and have the following question: what pressure should I adjust the tyres to in Pretoria to ensure that it will again be 2,1 bar when back at the coast? It sounds logical to then ensure that the tyres are 2,3 bar again before I embark on my homeward journey. RENÉ SWANEPOEL Bellville An interesting question that gave us pause for thought. Your last statement rings true if the atmospheric conditions are similar to the day you drove to Pretoria. For some background, a tyre gauge measures gauge pressure, which is the pressure above the atmospheric pressure where the measurement takes place. • The atmospheric pressure at sea level at standard atmospheric conditions is 101,25 kpa, or close to 1 bar (100 kpa). • Pretoria is at an altitude of 1 330 metres with a standard atmospheric pressure of 87 kpa, which is 13 kpa (or 0,13 bar) lower. Therefore, the new reading should have been 2,1 + 0,13 = 2,23 bar in Pretoria. • Johannesburg is at an altitude of 1 700 metres, with a standard atmospheric pressure of 83 kpa, which is 17 kpa (or 0,17 bar) lower. The new reading in Johannesburg would be 2,1 + 0,17 = 2,27 bar. If you inflate your tyres to 2,23 bar in Pretoria (or 2,27 bar in Johannesburg), they should again read 2,1 bar back in Cape Town. In our opinion, rather start on the correct pressure and, once you reach your destination and allow the tyres to cool down, readjust them.