What on-road tyre pressures are right for you?
Words by Mark Allen
When it comes to on-road tyre pressures, many people figure that once they’ve pumped their tyres up, that’s it… set and forget… but nothing could be further than the truth.
It’s foolhardy to not keep an eye on your tyres. Taking a look at them each time you walk past them is a good habit to get into. No, that doesn’t replace using a tyre pressure gauge, but if you know what your tyres should look like when they are in good condition and set correctly, then any significant changes should be easily picked.
That quick look can tell you a lot; are the tyres flat, can you see cracks, a sidewall bulge, tread chipping or a nail sticking out of the rubber. Can you hear air escaping, are they bald, do they look different to the last time you looked at them, are the tyre valve caps in place and are the wheel balance weights still on?
These precautionary checks should be done every single time you walk past your 4X4, especially while on remote outback trips. The sooner you pick up on a problem, the safer you’ll be, plus it’ll be easier and cheaper to repair.
But, what pressure should you set your tyres to start at when you’re driving on the road? When it comes to checking your tyre pressures, you should do them at different times, like first thing before driving, when you’re unloaded, again when you’re loaded, at the beginning of a hot day and at the beginning of a cold day.
Of course, there is no one answer to all situations, nor is there a set of exact answers given the variables between vehicles, load, tyre construction, road type and ambient temperatures. But, there is a good rule of thumb, called the 4psi Rule, taking note that it’s not set in stone and the rule can be twisted or varied depending on the variables. It’s an excellent way to ensure your tyre pressures are set close to correct before driving, regardless of most other variations – remember it’s not set in stone.
To start with, make sure your tyre pressures are set to the manufacturer’s recommended cold inflation settings; that will be a good starting point from which you’ll need to adjust higher or lower.
The 4psi Rule
The 4psi Rule says your tyre pressures should change by approximately 4psi from ‘cold’ pressures to ‘hot’ pressures. Now go for a good long drive for at least half an hour to get your tyres up to a constant temperature, again keeping in mind the tyre temperatures will vary depending on all those differences listed, so keep driving until there are no changes in temperatures. Park and measure the pressures in each tyre – do all four tyres as the weight distribution from front to rear will see different temps and pressures.
If your tyre pressures have increased by 4psi then you’re spot on with your starting tyre pressures and there’s nothing else to do – easy. But if your tyres have increased in pressure by more than 4psi, then your cold start pressures were too low, which has allowed temperatures and therefore pressures in the tyre to build up too much. Likewise, if the pressure increase is less than 4psi, then your cold start pressure was too high.
To make this all even harder, or more confusing, many swap the 4psi Rule to read 6psi Rule, but the same rules apply as above for measuring the difference in pressures from cold to hot.
I’ve been playing with this system for more than six months with my Troopy. With my setup and using 285/75R16 tyres and utilising an ARB TPMS system as part of a long-term trial, I’ve watched my tyre pressures rise on all four corners. For me, the 4psi system seems to work best, and I’ve managed to work out what my tyres should be set at from cold, depending on what load I have in the vehicle at the time, and what they are expected to raise to once up to operating temperature.
Long term, you should also be noting the tread wear on each tyre. Note if wear is even, central or on the outer edges of the tyre. This should give you an indication if you’ve been mostly running your tyre pressures correct or not. Also, take into consideration that your ride quality will vary, for better or worse, depending on tyre pressures and that this general rule of setting tyre pressures is for on-road use only. Hit this link to Unsealed 4X4’s guide to off-road tyre pressures.
Once you’ve hit the rough stuff, running lower tyre pressures aid in grip and comfort, but won’t have temperatures rise as much due to the much lower speeds.
Given your tyres are the first and last contact between you and the road; lack of attention and maintenance will get you into trouble… sooner or later.