Motoring Discussion > Tyre pressures - Wheel Size x 2 2 method Miscellaneous
Thread Author: VxFan Replies: 23
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 Thu 11 Sep 14 10:36 
 Tyre pressures - Wheel Size x 2 + 2 method - VxFan
It's been mentioned several times on another forum I read that if you use "Wheel Size x 2 + 2" then that'll give you an approximate tyre pressure that's close enough.

I personally think it's noncense and to go by either what your handbook says or many of the tyre pressure charts out there that are available online and at most garages.

If I were to use the above method for my 17" wheels then that would make my tyre pressure 36 psi, but the handbook says 32 psi.

If I applied it to my motorbike, then the rear tyre would be 38 psi (correct pressure is 28 psi) and the front 36 psi (correct being 24 psi).

Anyone else heard this method before? No tyre fitter I've ever mentioned it to has.

For starters, the pressures of the front tyres will vary depending on whether you've got a diesel or petrol engine because of the weight difference.
Last edited by: VxFan on Thu 11 Sep 14 at 10:37
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 Thu 11 Sep 14 10:41 
 Tyre pressures - Wheel Size x 2 + 2 method - Manatee
I've never heard it.

Maybe it's one of those "get you home" things, but I can't see when it would be helpful - more likely that you have the pressure recommendations but no gauge, than the other way round.
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 Thu 11 Sep 14 10:45 
 Tyre pressures - Wheel Size x 2 + 2 method - Zero
I look at the pressure chart stuck to the drivers door jamb.,....
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 Thu 11 Sep 14 11:04 
 Tyre pressures - Wheel Size x 2 + 2 method - ....
Never heard of it but it gives me the Eco setting stated by the manufacturer for my car 38psi.

It works for the front wheel of my bike 36psi but under inflates the rear 36psi vs 42psi.
Last edited by: gmac on Thu 11 Sep 14 at 11:05
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 Thu 11 Sep 14 11:10 
 Tyre pressures - Wheel Size x 2 + 2 method - rtj70
Never heard of this either. My car knows what pressures should be in for full load or part load and monitors it for me.

If I use this method my 18" wheels would be 38psi. They are meant to be more like 36psi.
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 Thu 11 Sep 14 11:26 
 Tyre pressures - Wheel Size x 2 + 2 method - Fullchat
I heard this one from one of our vehicle inspection guys. It is designed as more of an approximation rather than an exact calculation.
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 Thu 11 Sep 14 11:33 
 Tyre pressures - Wheel Size x 2 + 2 method - Crankcase
Sounds like that "double it and add thirty" thing you use for rough temperature conversion.
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 Thu 11 Sep 14 11:35 
 Tyre pressures - Wheel Size x 2 + 2 method - ToMoCo
Recommended for the 15" wheels on my pug is 31 PSI. I always think this feels / looks a little soft.

I generally run at 34 PSI.
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 Thu 11 Sep 14 13:17 
 Tyre pressures - Wheel Size x 2 + 2 method - TheManWithNoName
I always use superior quality air and run mine at 2.3 bar. I don't want none of this low quality psi rubbish.

;-)
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 Thu 11 Sep 14 13:55 
 Tyre pressures - Wheel Size x 2 + 2 method - Slidingpillar
Rubbish frankly. The vintage car has 19" wheels and weighs less than 8 cwt. Tyres pressures are 19 front, and 25 rear. This method says 40 psi.

Now I know that's being silly, but the Landrover I used to have was on 16" wheels and was 28 always on the front and 40 on the rear, with 48 if fully laden or towing. That gives 36 psi. And underinflated rears can be felt.

Most tyre shops have the inflation data for most cars.
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 Thu 11 Sep 14 14:34 
 Tyre pressures - Wheel Size x 2 + 2 method - WillDeBeest
Most cars have their own inflation data inscribed on them somewhere; fuel flap on mine, doorframe on Z's. There's simply no need for a rule of thumb here, let alone one that's plainly wrong in so many cases.
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 Thu 11 Sep 14 15:21 
 Tyre pressures - Wheel Size x 2 + 2 method - rtj70
My car had it's first MOT a few weeks ago. When I got it back all 4 tyres were under inflated. They weren't before the MOT. After re-inflating them back to 2.5 bar they have not lost pressure - pressure obviously changes with temperature so they change a bit when driven but no punctures. And it was all 4 that were down to the same low figure.
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 Thu 11 Sep 14 15:41 
 Tyre pressures - Wheel Size x 2 + 2 method - Armel Coussine
>> My car had it's first MOT a few weeks ago. When I got it back all 4 tyres were under inflated. They weren't before the MOT.

Garages and mechanics are very odd about tyre pressures. They know I suppose that most drivers mimse most of the time, and it's certainly true that soft tyres make for a smoother ride (although they wear out faster, get overheated and lose grip sometimes to a dangerous extent). The fact that a reckless insouciant driver might cause an accident is of no concern to the average dumb grease monkey, because no one can prove that they let air out of the tyres unless there's a witness who has it in for them.

Personally I keep tyres on cars I drive at the high speed/heavy load pressures. Safety and speed come before comfort every time. I don't expect garages to mess with them, but usually check the pressures when they've been in the garage, just in case.
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 Fri 12 Sep 14 16:51 
 Tyre pressures - Wheel Size x 2 + 2 method - Dave_
>> Most tyre shops have a chart with the pressures for most cars

But they never use them. I don't think I've ever had tyres fitted and set to the correct pressures, from budget partworns from a back-street outfit to Mercedes-specific tyres at the main dealer. I always check the pressures at the first opportunity, the two fronts that were recently fitted to the Merc were 40% over-inflated.
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 Fri 12 Sep 14 20:25 
 Tyre pressures - Wheel Size x 2 + 2 method - Slidingpillar
From memory, I've usually been asked what pressures - and being the sort of person I am, I can trot the numbers straight out. But most tyre shops have the chart on the wall anyway.

Oddly, the only place that has got the pressures really wrong was a dealer! The Defender 90 at delivery had the back tyres at 110 pressures - pumped up far too hard, and 110 had 90 pressures - far too soft. The overpressure was not that noticeable, but the underpresssure caused serious handling problems.
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 Fri 12 Sep 14 20:57 
 Tyre pressures - Wheel Size x 2 + 2 method - Armel Coussine
What's the matter with people?

It's a simple fact that garages and dealers leave tyre inflating to moronic monkeys who don't know scheisse and don't give a damn. Whether or not you know the moment you drive the thing - I usually do - it's commonsense to check and adjust pressures whenever someone else has been messing with the motor. You shouldn't have to of course, but good mechanics are rarer than hens' teeth.
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 Fri 12 Sep 14 21:10 
 Tyre pressures - Wheel Size x 2 + 2 method - carmalade
Can the manufacturers make tyre pressures any more confusing?First we had psi,then we had to deal with Bar,now I see they are using Kpa. Complete chaos?
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 Fri 12 Sep 14 22:00 
 Tyre pressures - Wheel Size x 2 + 2 method - WillDeBeest
Some people must be easily confused. You don't work for NASA, do you?
};---)
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 Fri 12 Sep 14 22:21 
 Tyre pressures - Wheel Size x 2 + 2 method - Slidingpillar
psi makes a lot of sense for tyres. The vintage car is 1.3 bar at the front, and 1.7 bar at the back. Doesn't sound that different, but 19 psi and 25 psi makes it clearer.

131 and 172 in kpa - kpa sounds like bar times 100... Hmm.
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 Fri 12 Sep 14 22:30 
 Tyre pressures - Wheel Size x 2 + 2 method - Slidingpillar

It's a simple fact that garages and dealers leave tyre inflating to moronic monkeys who don't know scheisse and don't give a damn. Whether or not you know the moment you drive the thing - I usually do - it's commonsense to check and adjust pressures whenever someone else has been messing with

Agreed, but both my errors were on delivery of a new car. The fronts were acceptable pressures, maybe not perfect but nothing to get excited over. I'd be surprised if the error came from the factory, although it might.

I did once get a selection of gauges checked for me by trading standards as they have a rather super duper one as a reference.

With a background of motor sport, I'm quite used to playing around with pressures, up for circuits and hillclimbs and as low as will stay on the rims or regulation pressure for trials. Bet Bathtub Tom knows what I mean.
Last edited by: Slidingpillar on Fri 12 Sep 14 at 22:31
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 Fri 12 Sep 14 22:53 
 Tyre pressures - Wheel Size x 2 + 2 method - bathtub tom
>>as low as will stay on the rims or regulation pressure for trials. Bet Bathtub Tom knows what I mean.

12 PSI!
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 Sat 13 Sep 14 10:06 
 Tyre pressures - Wheel Size x 2 + 2 method - martint123
On the bike the manufacturer states pressures 29/29 psi
Current tyre maker says 36/42 psi for the same bike.

Bit of a difference!
Last edited by: martint123 on Sat 13 Sep 14 at 10:06
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 Sun 14 Sep 14 12:31 
 Tyre pressures - Wheel Size x 2 + 2 method - Armel Coussine
Anyone interested in motor sport since the fifties knows what you mean Sp. A lot of successful trials cars were specials, and very weird and wonderful (not to say horrible) some of them looked. Was there an off-the-peg one other than the Dellow? I always thought Dellows were pretty little cars.

I often wondered whether the energetically bouncing passenger trials cars carried was a real help in regaining or improving traction.

What a good unit that sidevalve Ford 10 engine was.
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 Sun 14 Sep 14 13:11 
 Tyre pressures - Wheel Size x 2 + 2 method - Slidingpillar
My technique I've called making hay while the sun shines basically treating speed as a commodity, going fast on the grippy bits, and shedding speed on the slippery bits. Sort of basic if you think about it, but you would be surprised how few folk do it - even regular competitors. Trying to maintain speed in slippery bits is often the cause of failure so you start the section with speed to spare and thus shedding a bit is no problem.

I learnt a lot in one of my jobs and could usually be relied upon to get a totally unsuitable car to the tops of hills although the chap who washed the cars was a bit perturbed to find copious mud on the roof.

As for makes of cars, I always thought the Dellow was an only child, but the later Liège apparently is good but there are rather few of them.
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