Can someone educate me about the dangers of the extra 5 PSI?
The tire pressures on the vehicle placard are based on a set of assumptions plus the handling of the car. If the assumption don't match your conditions, then adjusting the pressure is correct:
The first two assumptions are always used:
Ambient temperature 65F (18 C). This is for the high of the day. If the temperature gets hotter than that you have to increase pressure. If the temperature is much lower, then inflation is typically done indoors with warmer air and you also have to adjust the pressure upwards. There are tables for this, but if the placard pressure is 35 psi and the high of the day is 100 F (35 C) then a 4 psi increase is in order.
A pressure check each driving day before driving (very slowly) for more than one mile (1.6 km). If you don't check the pressure daily and don't have your TPMS set to alert when the pressure goes down over one psi (by default TMPS are set for 25% less than the placard pressure) then you need to add a few psi to combat laziness. The idea is that you never ever want the pressure to go below the vehicle placard pressure for normal on-road driving (Note that track, rally, drag, and other sports use and/or driving on gravel roads are another matter entirely).
Speed: This varies by car. A typical assumption for family sedans is a maximum sustained speed of 65 mph. However, sports cars anticipate higher speeds and the placard pressure already takes care of this.
Load: Typically the loading is based on the way the engineers think the vehicle will normally be driven. This may not be the GVW (gross vehicle weight).
In addition the handling of the vehicle, particularly in an emergency situation, is taken into account.
A few rules:
1. Never exceed the maximum psi stated on the sidewall of the tire. Note that this is when the tire is cold. If it goes over while driving, that is fine.
2. After driving for an hour or two on the highway the tires should have reached thermal equilibrium. The pressure shouldn't be more than 8% to 10% higher than cold inflation pressure. The exception is if there is a large temperature change during the day, in which case 15% is okay. If it goes higher than that, then there is a problem that needs to be looked into.
3. Heat, not pressure, kills tires. Regardless of the pressure set cold, the tires will flex and inflate to about the same psi after a couple of hours of driving. The difference will be that the tire with low pressure will be far hotter and will wear more quickly. In the worse case it will affect the compounds used in the tire and they may de-bond.
4. Always use a good pressure gauge to check the tires. Don't rely on the tire store or filling station gauge. Get a good pressure guage such as
50406 or model
50404. Note that cheap digital gauges are no more accurate than cheap analogue (dial) gauges. A good digital gauge is about twice the price of a good analogue gauge.
Note that there is a lot of garbage about tire pressures out there and myths abound, some have been carried forward for over 50 years--a bit of common sense is in order.
