Tyre pressure in psi

masterboy123

My car got 225/50 R17 tires and car recommendation says 250 kpa (36 psi; machine at my petrol pump don't support kpa), see attached pic. But when i did fill up all tires with 36 psi, car instrument showing only 240 kpa.

I am wondering if 37 or 38 psi should be the right pressure? Just 2 persons in car and no luggage.
Car model : C class w205

http://imgur.com/a/4mdiU

Henry Ford III

36psi is equal to 248kpa.

CiniO

Manometer on petrol stations are often not very precise even when new, and after all abuse they get over the months or years, they might be way out...

I wouldn't trust them - better buy your own small reliable manometer to verify air pressure.

masterboy123

Ok
Is it safe to put 38 psi in all tires?
In my previous hatchback car it used to be only 30 psi in all 4 tires.

P.S. I can't seem to find thread "Post Your Car Picture" . Is it gone?

H3llR4iser

Different car, but I have the same size tires (225/50 R17 94W) and keep the fronts at 39psi and rears at 36psi (2.7 and 2.5 bars, respectively) as the manual recommends (2.5-2.7 for the fronts, 2.5 for the rears). So I wouldn't see any issue running yours at 38, considering the type of car, performance and mass are similar, and the fact mine is FWD and your Mercedes is RWD.

I would also second the "get your own manometer" hint; If you can find a petrol station that has the digital, automated inflation stations, they tend to be relatively more reliable than the standard "can't even align the needle in this light" hand-operated ones

Also, first time I see tire pressures recommended in kPa by a manufacturer

masterboy123

This is very helpful information.
Ok will hit the pump station and get them filled up with 38 psi. In my car manufacturers manual it says all tires to be at the same pressure if only 2 people in the car.
This car is from Germany, perhaps they use KPa there?
Thanks

H3llR4iser wrote: »

Different car, but I have the same size tires (225/50 R17 94W) and keep the fronts at 39psi and rears at 36psi (2.7 and 2.5 bars, respectively) as the manual recommends (2.5-2.7 for the fronts, 2.5 for the rears). So I wouldn't see any issue running yours at 38, considering the type of car, performance and mass are similar, and the fact mine is FWD and your Mercedes is RWD.

I would also second the "get your own manometer" hint; If you can find a petrol station that has the digital, automated inflation stations, they tend to be relatively more reliable than the standard "can't even align the needle in this light" hand-operated ones

Also, first time I see tire pressures recommended in kPa by a manufacturer

H3llR4iser

It definitely has to do with the SI used instead of the Imperial one, as people tend to stick to here (Ireland finished its metric conversion in 2005, yet many people haven't got the memo - including lazy car-manufacturer who recycle UK manuals!). What I mean is that usually European cars seem to express the pressures in Bar, or at least the ones I owned did.

By the way, 100 kPa are exactly 1 Bar - I'm telling you because many manometers sold here have the dual measure unit; So if you set it to Bar, 2.5 are exactly the 250 kPa you are looking for, provided the measurement is correct; I believe you're looking to see "250" on your dashboard, which I would hope was more accurate than the pumps

mikeecho

H3llR4iser wrote: »

It definitely has to do with the SI used instead of the Imperial one, as people tend to stick to here (Ireland finished its metric conversion in 2005, yet many people haven't got the memo - including lazy car-manufacturer who recycle UK manuals!). What I mean is that usually European cars seem to express the pressures in Bar, or at least the ones I owned did.

By the way, 100 kPa are exactly 1 Bar - I'm telling you because many manometers sold here have the dual measure unit; So if you set it to Bar, 2.5 are exactly the 250 kPa you are looking for, provided the measurement is correct; I believe you're looking to see "250" on your dashboard, which I would hope was more accurate than the pumps

I grew up in an ireland, where road signs were in miles, later changed to km, but speed limits still in mpg, speedo in mph.
Gallons, feet, yards, psi, pints, lbs, oz, Litres, ml, km, Kg, g, etc etc.

I understand both, and for different circumstances, different measurements in different circumstances make more sense to me.

Tyre pressure = psi
House size = square foot
Weights = Kg (i get stone and lb, but not oz)
Volume= L or Gal (never understood fluid oz)

its just the way my mind works.


How much is a gallon of petroleum spirit in old money :pac:

Henry Ford III

Manometer? That's an old fashioned yoke with a U shape made of glass and liquid that gets moved by air pressure.

A digital tyre pressure gauge is a much better bet. You can buy one in Halfords.

mikeecho

Henry Ford III wrote: »

Manometer? That's an old fashioned yoke with a U shape made of glass and liquid that gets moved by air pressure.

A digital tyre pressure gauge is a much better bet. You can buy one in Halfords.

you're right..
http://www.halfords.ie/webapp/wcs/stores/servlet/SearchCmd?srch=tyre+pressure&action=search&storeId=11101&catalogId=15551&langId=-1


how much were you willing to bet?

CiniO

Henry Ford III wrote: »

Manometer? That's an old fashioned yoke with a U shape made of glass and liquid that gets moved by air pressure.

A digital tyre pressure gauge is a much better bet. You can buy one in Halfords.

I actually meant something like that:
http://www.halfords.ie/workshop-tools/garage-equipment/tyre-inflators-pressure-gauges/halfords-essentials-dial-pressure-gauge

Digital one is probably good as well, but can't think of any reason why digital would be more precise.

masterboy123

Thanks everyone.

I filled tires with 39 psi as tires were relatively warm.
I will get the digital tyre pressure guage soon