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I have always overinflated my bike tyres by 4 PSI and car tyres by 2 PSI. Less rolling resistance = better mileage but slightly bumpy ride. But then I am a sedate driver/rider. Would not advocate this for spirited ones!

23 psi is too low. You are asking for trouble by running your tyres at this pressure.

Usually a difference of 2-3 psi on either side of the manufacturer recommended pressure would be tolerable. Anything more than that would be trouble, unless the situation demands it. For e.g. in sand you may want to lower the pressure to 20psi to get better grip/traction.

For normal road usage, if the manufacturer has recommended 30 psi, you would be able to manage with pressures between 28 - 32 psi. Considering error of reading with the pressure gauges you may want to pump the pressure up by 1 psi.

What is the right pressure for the tyres (205/55/R16) of the Skoda Superb petrol?

What is correct pressure for the tyres (195/65/R14) Michelin XM1+ of Fiesta 1.4 TDCI ZXI? I used to inflate with nitrogen. But the front right tyre alone getting deflates within a week of time.

A rather strange incident this morning opened my eyes to the dynamics of tyre pressure and tyre heating. I had to pick up my aunt this morning from the airport. BIAL being a 43 km drive from my place, I had thought of getting the tyre pressures checked in the morning before hitting NH7, but then had to ditch the idea as (ok, I confess) I started 10 minutes later than planned. Having inflated the tyres last Saturday to the company recommended 33 psi, I expected the cold tyre pressure to be around 30-31, which I thought would anyway increase to around 33 at NH7 speeds.

Back at home from the airport, I had a break of 15 mins before starting for office to have a quick chit-chat with my aunt and cousin and (ok, I confess again! lol:) to allow me to sink my teeth into some goodies brought in from our hometown. Now, I mostly fill up air at a Shell outlet close to my home where the newly installed digital air pressure gauge seems to be (well, a subjective thing anyway) quite accurate. I drive in by the air gauge, and to my dismay, the attendant, instead of pumping in air into the tyres, started deflating them. Surprise! Surprise!! The same digital gauge which had shown 33 psi last Saturday, had read an incredible 36 psi for the front tyres! :eek: I dismissed the reading, attributing it to a gauge error, and to be on the safer side I asked the attendant to fill in 34 psi instead of 33 anyway.

Back home in the evening, I thought of digging up some long forgotten high school physics to find a plausible reasoning for the apparent anomaly. What came next was indeed a big eye-opener. Now, those of you who still remember the gas laws (or Charle’s Law if you so prefer) would reason that assuming the volume of the air inside the tyre remaining constant (a convenient and safe assumption for all practical purposes), we have

P1/T1 = P2/T2

where T1 and T2 are in Kelvin (degree C + 273).

Or, 31/293 = 36/T2

where I assume the cold tyre pressure this morning to be 2 psi below the last filled 33, and cold tyre temperature of 20 degrees C.

(Not using metric units for the pressure does not matter anyway as we are working ratios and the imperial and metric units for pressure are proportionate). A quick calculation in Excel to solve for T2 yields

T2 = 340.26 K, i.e., 67.26 degrees C.

I am not sure if 67 degrees for a hot tyre is realistic, that too after cooling for 15 mins, but my wheels sure feel quite hot even after brief highway runs in summer, so probably they indeed run that hot.

Now coming to the main point – some people, including a well-known tyre manufacturer’s website point out that you should over-inflate your tyres by 5 psi before highway runs. So, taking P1 as 38 (33+5) and T1 as 293 (in the cold, remember?), and assuming that the tyres will again heat up to at least 67 degrees (thus T2 = 340), P2 ends up at a whopping 44.13 psi for the stock JK Tornados on my VXi !! Now can someone assure me that this is not unsafe? Or are we compromising our safety by blindly following some ‘advice’ without reasoning them out? What do you think guys? BTW, please spare me the brickbats for the longish post and if at all I sounded like your high school physics teacher for a brief while!

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I have to ask what may be considered a "dumb question" (feel free to quote this on the "Quirky comments from non-petrolheads" thread :uncontrol ):

My car's manual has two categories of recommended tyre pressure: Laden and Unladen. Can anyone explain what it means?

- Does it refer to laden/unladen when filling, or laden/unladen when running?

- If I am going to drive alone, that can hardly be considered "laden" - so should I go for the unladen pressure?

- If I am going to have only one passenger in the front seat and no luggage, is that laden or unladen?

My Ikon's recommended tyre pressure for the rear is 41 psi (full load), which is quite close to the "44 psi max pressure" marked on the tyre itself ! Maybe the Ford guys feel that the pressures do not rise by more than 3 psi due to heating up on the highway runs. I personally have not been exceeding 38 psi with full load, it's been fine with the tyres so far.

Laden/unladen refers to the number of passengers/luggage loaded in your car. If you are travelling alone and always light, fill the pressure stated for unladen and if you travel with people and heavy luggage fill the pressure under laden. :)

After going through the entire thread, it was recommended that the tyre pressure should be checked when the tyre is cold. How much driving in the morning do you people consider cold? I have to drive around 2 kms for the nearest petrol pump to check tyre pressure.

Thanks in advance.

From the Bridgestone India website: "Consider laden if load exceeds 300 Kgs. else consider unladen".

The document attached below has been taken from the same website, and lists (so-called) recommended tyre pressures for various vehicles sold in India.

As long as you aren't doing any high speed driving, I think the tyre condition still qualifies as cold after just a couple of kms.

Cheers,
Vikram

I have this same dilemma. So I put in +2 psi all around . Then use my hand held gauge to correct the pressure the next morning before moving out.

Does tire pressure (higher or lower) have anything to do with causing tire sidewall bulges? or is it something else.

I put 32 psi for front and 30 for rear for my petrol Indica. I always try to fill the air in the morning and that also from the nearby petrol station. Recommended tyre pressure in the manual is 30(front) and 28(rear).

For my alto it is 32 psi for all.

Tyre pressure recommended by the manufacturer is often the cold tyre temperature. As mentioned above, it is recommended to fill a little more and correct it the next day early morning using a tyre pressure guage/TPMS.

Some claim that the tyres run cooler when filled with Nitrogen. It is completely false as I've seen with my TPMS (can read temperature and pressure accurate to 2 decimal places while running) that Nitrogen filled tyres warm up the same way as Normal Air filled tyres and they also exhibit pressure variations with temperature.