The Right Tire Pressure
WARNING: Using the wrong tire pressure in your vehicle may result in a high speed blowout where you could kill yourself, your passengers, other motorists as well as innocent bystanders.
I'm not an expert on tires or auto safety so I can't guarantee what I tell you here is 100% right. I do a lot of research though and hope that what I have found out can be of help to others.
"I bought a vehicle that does not have the standard sized wheels fitted. I do not know what pressure to inflate the tyres to?"
All standard automobiles I've ever seen has a recommended tire pressure of around 2 bar, that's 30psi and can be a good starting point. But increase that to 2.3 bar or 34psi as the vehicle manufacturers recommendation is usually at the low end.
Another starting point is to read the maximum allowed tire pressure from the sidewall of the tire, it could be 50psi, but check you own tires as too high pressure can result in a blowout. Then use this value minus 25%, with a maximum pressure of 50 psi this would give us 50 x 0.75 which is 37.5 psi.
When optimizing your car for better fuel economy you can use the maximum tire pressure minus 10% as your starting point. This will save gas but give you a harder ride.
When you have filled you tires according to one of the above starting points you have to drive some miles, see if you like the feel of it and change the pressure if it doesn't feel right. Lower pressure for comfort, higher pressure for performance driving or fuel economy.
General Advice on Tire Pressures
As far as I know using a tire pressure somewhere between the vehicle manufacturers recommendation (which is often in the low end) and the maximum pressure printed on the sidewall of the tire should be safe as long as you are not pushing it with performance, racing style driving.
As I mentioned above some vehicle manufacturers recommended tire pressure is not optimal (it's usually low) and depending on your driving habits you may want to change it to improve handling and fuel economy.
The starting point when trying out pressures should always be something safe, like the manufacturers recomendation, or according to Christopher J Longhurst of the wheel and tyre bible another good starting point for performance driving is the tires maximum inflation pressure minus 10%.
The maximum inflation pressure can be found at the side of the tire. If it says 40psi, use 40 - 4 = 36psi as your starting point.
Sgt. Dave Storton, Director of the San Jose Police Academy recommends that for best fuel economy and handling you should always use the maximum allowed tire pressure printed on the sidewalls of the tire.
He also says that the San Jose police department uses a pressure of 50 psi in the training vehicles even though the maximum sidewall pressure is 44psi. This improves the fuel economy even more than just inflating to the maximum rating.
Still I don't recommend anyone to use a pressure above the tire maximum as it could put you into unvanted liabilites or give you a leak or blowout if the tire is old.
This actually happened to me, when I had my old winter tires balanced at a tire shop. They inflated them way more than normal, and this must have been to much for them as I woke up the next day to four almost flat tires. There was nothing to do but to replace them with new ones.
Use a reliable pressure gauge
One thing you have to remember when adjusting air pressure is to make sure your air gauge is reliable.
First of all the gauges built into the pressurised air equipment at the gas station is often way off and even though some of them may be calibrated there is no way for you to know which one.
Secondly the pen type analog air gauges are next to worthless. They may be quite right when new, but dirt and oxidation will make them unreliable within half a year. Don't use these!
You local gas station or repair shop might have a real reliable air gauge that you can use (not the one used at the air self service), try asking them.
If they didn't have one or it's too inconvenient to use it you should buy a quality air gauge like the Accutire ABS Coated Air Gauge so you can check your air pressure every month.
Also try the Gas Mileage Calculator
The Tire Pressure Checklist
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