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Old 16th January 2009, 19:03   #16
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Some inputs from carbibles.com

Car Bibles : The Wheel and Tyre Bible Page 2 of 2

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Whats the "correct" tyre pressure?

How long is a piece of string?
Seriously though, you'll be more likely to get a sensible answer to the length of a piece of string than you will to the question of tyres pressures. Lets just say a good starting point is the pressure indicated in the owner's manual, or the sticker inside the driver's side door pillar. I say 'starting point' because on every car I've owned, I've ended up deviating from those figures for one reason or another. On my Subaru Impreza, as outlined above, I got much better gas mileage and no difference in tyre wear by increasing my pressures to 40psi. On my Honda Element, I cured the vague handling and outer-tyre-edge wear by increasing the pressures from the manufacturer-recommended 32/34psi front and rear respectively, to 37psi all round. On my Audi Coupe I cured some squirrelly braking problems by increasing the pressure at the front from 32psi to 36psi. On my really old VW Golf, I cured bad fuel economy and vague steering by increasing the pressures all-round to 33psi.
So what can you, dear reader, learn from my anecdotes? Not much really. It's pub-science. Ask ten Subaru Impreza owners what they run their tyres at and you'll get ten different answers. It depends on how they drive, what size wheels they have, what type of tyres they have, the required comfort vs. handling levels and so on and so forth. That's why I said the sticker in the door pillar is a good starting point. It's really up to you to search the internet and ask around for information specific to your car.
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The Max.pressure-10%theory

Every tyre has a maximum inflation pressure stamped on the side somewhere. This is the maximum pressure the tyre can safely achieve under load. It is not the pressure you should inflate them to.
Having said this, I've given up using the door pillar sticker as my starting point and instead use the max.pressure-10% theory. According to the wags on many internet forums you can get the best performance by inflating them to 10% less than their recommended maximum pressure (the tyres, not the wags - they already haves inflated egos). It's a vague rule of thumb, and given that every car is different in weight and handling, it's a bit of a sledgehammer approach. But from my experience it does seem to provide a better starting point for adjusting tyre pressures. So to go back to my Subaru Impreza example, the maximum pressure on my Yokohama tyres was 44psi. 10% of that is 4.4, so 44-4.4=39.6psi which is about where I ended up. On my Element, the maximum pressure is 40psi so the 10% rule started me out at 36psi. I added one more to see what happened and it got better. Going up to 38psi and it definitely went off the boil, so for my vehicle and my driving style, 37psi on the Element was the sweet spot.

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The other alternative-do not mess up with your tyre pressure at all

So - raising the pressure can extend a tyre's life because there is now less rubber contact with the road, the tyre is stiffer and therefore heats up less so lasts longer and less friction with the road gives greater MPG. Also, less sidewall flex will give a more positive feeling of steering accuracy but it can result in less ultimate grip and sudden unexpected loss of grip at the limit of adhesion. Raising or lowering tyre pressures too much either side of manufacturers recommendations could be at the expense of a less safe, more uncomfortable vehicle. So should we take all vehicle manufacturers recommendations as being absolutely correct? Remember that thousands of hours go into the development and testing of a car. If you've dicked around with your tyre pressures and still don't think it's right, go back to the door pillar sticker and try that again - you could be surprised.
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Old 16th January 2009, 19:08   #17
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Originally Posted by Surprise View Post
One of our members has pointed out in a "Street experiences" thread that higher psi levels as possible reason for the tyre burst on a highway.
On the contrary, experts recommend slightly higher pressure for sustained highway driving at high speeds. The reason is to reduce the chances of the sidewall getting overheated.

Under normal ciscumstances, the pressure build up due to heat and friction won't be more than 2-3 psi for speeds between 100-120.

Also, tubeless tyres don't burst so suddenly as tubed tyres.
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Old 17th January 2009, 10:35   #18
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I highly recommend using your own tyre pressure gauge. Buy a high quality gauge from the after-market / ebay; they are really not that expensive.
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Old 17th January 2009, 11:22   #19
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Originally Posted by GTO View Post
I highly recommend using your own tyre pressure gauge. Buy a high quality gauge from the after-market / ebay; they are really not that expensive.
GTO, how to make sure that the tyre pressure gauge is accurate ? And are there some well known brands ? I bought this foot pump sometime back, and though it works fine, its gauge is totally useless. So, I am thinking of buying a tyre pressure gauge.
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Old 17th January 2009, 12:50   #20
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Originally Posted by lancer_rit View Post
GTO, how to make sure that the tyre pressure gauge is accurate ? And are there some well known brands ? I bought this foot pump sometime back, and though it works fine, its gauge is totally useless. So, I am thinking of buying a tyre pressure gauge.
Moroso is an excellent brand. You will not regret your decision. cheers:

With tire gauges - stay away from the cheap stuff. Especially stuff like "accu-gauge" and all that. If the word "accu" is part of the name, DONT buy it
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Old 17th January 2009, 15:51   #21
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I fill 30psi for all the tires in my swift. Profile being 195/60/R14. But now am thinking that I ll get 32 psi in the front and 30 psi at the back.
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Old 17th January 2009, 19:33   #22
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@GTO - indicative prices please??

What exactly is "cheap", and what is not?
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Old 18th January 2009, 19:16   #23
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I am at the reccomended 32 on my 235 & 265 19' on the mondeo. Still have to be really careful because the tires are not at all suited to our roads.
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Old 19th January 2009, 13:33   #24
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Quote:
Originally Posted by BaCkSeAtDrIVeR View Post
@GTO - indicative prices please??

What exactly is "cheap", and what is not?
I bought mine on ebay USA. Seen several in the range of $10 - $40. The high quality examples last a long time, my '03 is doing a great job till date.
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Old 19th January 2009, 13:48   #25
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Quote:
Originally Posted by GTO View Post
I bought mine on ebay USA. Seen several in the range of $10 - $40. The high quality examples last a long time, my '03 is doing a great job till date.
I bought one for about USD 10/- in 1983. Still works with accuracy!
PCL, Tyre Pressure Gauge, Made in England by Pneumatic Components Limited, Sheffield.
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Old 19th January 2009, 13:54   #26
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I dont know if I am correct or not, but I think if constant high speed driving in is going to be done,, then the tyre pressure must be kept lower.

Air expands on heating as we all know. Now as we drive at high speed for long distance, the air inside tyre is going to heat up and expand. So if we fill in more pressure, then there are chances of tyre burst.

If I am wrong, please update me.
We keep 28 psi/33psi ( front/rear ) in M800 and 30 all-round in Baleno.
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Old 19th January 2009, 14:06   #27
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Incorrect.

Underinflated tires OVERHEAT like crazy because they are constantly flexing due to lack of air-pressure. My cousin came over to my place one day and I noticed that his tires were SMOKING. He had only 5psi in them and underinflation was the cause of all the heat and damage.

Driving agressively heats up tires far faster than simple high-speed driving.
The recommended tire pressure is always COLD. Dont worry about how driving will affect your pressure. Besides, the pressure wont go up more than 4-5psi tops. Just stay within +10 psi/-4psi of the recommended pressure for your car.

Overinflated tires have low rolling resistance and you risk reducing your car's performance overall and uneven tire wear. This is relatively rare - but you will see some people inflating their tires to the brink of destruction in the name of hypermiling

I hope that helps.

Quote:
Originally Posted by aaggoswami View Post
I dont know if I am correct or not, but I think if constant high speed driving in is going to be done,, then the tyre pressure must be kept lower.

Air expands on heating as we all know. Now as we drive at high speed for long distance, the air inside tyre is going to heat up and expand. So if we fill in more pressure, then there are chances of tyre burst.

If I am wrong, please update me.
We keep 28 psi/33psi ( front/rear ) in M800 and 30 all-round in Baleno.
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Old 19th January 2009, 14:08   #28
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Quote:
Originally Posted by aaggoswami View Post
I dont know if I am correct or not, but I think if constant high speed driving in is going to be done,, then the tyre pressure must be kept lower.
Not correct, Aggo!
Normal heat produced in a tyre does cause the air to expand, but only up to a point where the heat build up and the heat loss reach an equilibrium.
Under-inflation makes the sidewalls flex more than normal and the heat produced in this case is far greater. This leads to rapid pressure build up and a burst.
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Old 19th January 2009, 14:35   #29
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If its stock tyres size, we can go with the manufacturer's recommendation as the reference point and decide either to over-inflate or under-inflate.

But if we upgrade the tyres, there is no concrete data where we can zero in on any particular psi levels. Hence, Its more important to find out whether the car tyres were under-inflated or over-inflated.
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Old 19th January 2009, 14:50   #30
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Quote:
Originally Posted by Surprise View Post
But if we upgrade the tyres, there is no concrete data where we can zero in on any particular psi levels.
You can always find the correct pressure for your car by looking at tyre tread wear. If there is more wear near the center then the tyres are over-inflated. If there is more wear near the outer edges then the tyres are under-inflated. It is always better to err on the side of over-inflation though.

You can make your own marks on the tyre and observe the tyre wear over a few weeks.
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