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-   -   Filling the right 'Pound per square inch (PSI)' (https://www.team-bhp.com/forum/technical-stuff/52058-filling-right-pound-per-square-inch-psi-7.html)

My general rule of thumb for hatchbacks and sedans is to keep the pressure at 32 psi in city when speeds are below 80 km/h and for intercity commute aka highway travelling, I keep the pressure at 35 psi in all 4 tyres.

If you are talking about Maruti Swift (both brothers), then its 28 as per stock tyres and label on driver side. I made this mistake of 32 and 35 (going by bunk attendant advice) and had narrow misses while braking. It though gives a good boost to mileage (got 19 in city) with my sedate driving habit.

Stick to correct pressure (cold) to avoid unnecessary drama at high speed. Bad experience for me.

As a maintenance engineer, I vary with this view and would like to brief this in a simple way. Hope it helps:)

Proper air pressure in a tyre helps to distribute the weight of a vehicle evenly across the tyre's tread pattern, so the vehicle is at it's most stable.

Under-inflated tyres tend to show wear on the outside edges of the tread, while over-inflated tyres show wear, down the middle of the tread.

Under-inflated tyre will flex more in cornering and stopping. It becomes less responsive and some of the performance and safety benefits built into the tire will be lost. Simply, it increases the area of contact with road. Often, reduction of air pressure is used momentarily while 'off roading' in ghats to avoid 'rolling'. I have seen some VIPs prefer to keep lower pressure to have a smooth ride, albeit at the cost of tyres ,mileage and suspension.

On the other hand, over-inflated tyres lose traction as shape of the tyre gets deformed by excessive air pressure, decreasing the tyre’s footprint on the road. Simply, it reduces the area of contact with road surface. It becomes stiffer and may not react as expected to road hazards like potholes. Over-inflation can even give a harsh, noisy ride.

Often, it is believed that with increase of load, tyre pressure has to be increased above the norms prescribed by the manufacturer. Actually its not so since air volume requirement doesn't increase in exact proportion with load, above the norms. Hence, standard ratings given by the manufacturer takes care of it, by itself.

So, maintaining the correct air pressure recommended for that tyre for that vehicle is the proven and safe bet. I had the Swift Vxi first generation and 33 psi was the recommended pressure for all the 4 wheels. I changed them after 5 years and 40K km, as a precaution but still tyres were decent enough.

Finally, air pressure be checked , atleast once in a month, if not regularly, while the tyres are cold, say, first thing in the morning. Heat generated by driving, an increase in air temperature as the day goes on, and even the heat of the sun shining on the tyres can temporarily increase tyre pressure, so most accurate reading is got, when tyres are cold. And, finally the well calibrated gauge is all the most important one.

My dad purchased a used grand i10 recently. The stock the size is 165/65 R14. The previous owner had changed to 195/50 R15. What is the recommended tyre pressure? I know that increasing width will need lesser pressure. I know that decreasing profile will also need lower tyre pressure. Increasing radius, I presume will need more pressure. If the recommended tyre pressure is 33psi, what should the new tyre pressure be?

When in doubt, use the 4psi rule.

This is how it works for on road:

Fill in your pressure to whatever you think is right - based on perfect contact patch with group for example in the morning on a "cold Tyre" - very important the tyre to be cold.

Then drive for 35kms at a constant speed averaging around 40kmph. This will warm up your tyres and get them to "ambient" temperature.

Now get out and measure the PSI of your tyres.

If you have an increase of more than 4psi that means your tyres are getting too hot and you have started with under inflated tyres.

If your psi rise is less than 4psi that means you overinflated to start with.

Restart again on cold tyres and adjust pressures accordingly.

EDIT: you may take a week perhaps to get it right. Again, calculations will change if your car loads change.
Also ambient temperature should be the same when measuring cold temp and after the 35km drive (approx 1 hour you need to drive)

I always refer to Tyre Pressure Sticker placed on underside of B- Pillar, Driver Door or Fuel Cap. For exact location of Tyre Pressure Sticker refer to owners manual. It is not advisable to change Tyre Pressure specified by Vehicle Dynamics Engineer. If a car is being overloaded change Tyre Pressure accordingly otherwise under normal or full load specified by Car Manufacturer it is normal to use Recommended Tyre Pressure.