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Thread: Oops... Beginner's Mistake...

  1. #1
    Regular Member parapaul's Avatar
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    Red face Oops... Beginner's Mistake...

    Think I've discovered tonight why my MPG has been so much lower than it should be

    Quiet night at work, which is fairly unusual, so thought I'd give the car a quick once over. I'm happy to hold my hands up and say I place too much faith in the car's warnings, and don't usually bother checking oil, water etc. So I've checked all the fluids and thought I'd have a look at the tyres.

    When I had these alloys fitted, they inflated them with Nitrous Oxide. This is becoming increasingly popular because the tyre rubber is less porous to Nitrous Oxide than to Oxygen, so in theory tyre pressure should stay constant for longer. It's been a few months now and to be fair, this is the first time I've checked the pressures myself!

    Anyway - trawled through the owner's handbook to find the charts for tyre pressures. It says: Z19DTH (check) estate (check) 235/35/29 tyres (check) should be 39psi (front) 35psi (rear).

    Now I know the rule of thumb for tyre pressures is twice the rim size, and they were fitted at Demon Tweeks, a large and experienced company, so I was expecting all 4 tyres to be 38ish... Imagine my suprise when I found all 4 tyres at 28psi!!! That's not only wrong, it's dangerous!

    So, having now inflated all tyres to the right pressure, I'm hoping my MPG should increase noticeably from the 'almost 40' I've been getting recently.

    Lesson learnt - check tyre pressures more often!

  2. #2
    Forum Moderator FIL 4822's Avatar
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    [quote=parapaul;563328]
    235/35/29 tyres (check) should be 39psi (front) 35psi (rear).
    quote]



    The tyre must cost a fookin furtune at that size !!!


    Have consideration for the deaf, if you're gunna fart, make it smell !


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    Vectra-C Obsessed Poster Drinky's Avatar
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    Smacked wrists all round me thinks Paul .................Oh and i think you mean NITROGEN not nitrous !!!!!!!

    Back in a V6 !!!!!!!!!!!! And lovin it !!!

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    Regular Member parapaul's Avatar
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    Oh, gosh darn it being so late at night..... Or is that early in the morning?

    Yes, I meant Nitrogen... Nitrous is the stuff we give ladies in labour and wussy footballers with twisted ankles

    And what's wrong with my imaginary 29" tyres, Fil? I did, of course, mean 235/35/19...


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    Regular Member shan's Avatar
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    Good job you never filled the 19" tyre's to 29" pressure rating's.
    If not a big BANG you might have floated away

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    IMO you won't notice a huge difference in MPG if the tyres were filled to 28PSI.
    At 28 they're not flat enough to make it dangerous either.

    Also, IMO filling with nitrogen isn't going to make a huge difference either. The normal air at the compressor is 78% BV nitrogen anyway, and Nitrogen is a smaller molecule than Oxygen, so I can't see how tyres are going to be more porous to O2 as opposed to N2 anyway.

    Just applying logic to the puzzle, and to me it doesn't add up. N2 would be slightly lighter than O2 though, and at 2 bar pressure the gas will be only slightly lighter. Maybe the dry N2 is the key, as opposed to 'wet' air from the compressor. - 'Wet' air as it will contain water vapour.


    Density of N2 at 0C, 1 atm and dry is 1.251 g/L
    Density of air at 0C, 1 atm and dry is 1.275 g/L

    So DRY gases would not make a huge difference at all.

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    Default tyre pressures

    in my handbook, for the 235/35/19 on a 1.9dth, it lists the tyre pressures at 44 front 39 rear

  10. #10
    Regular Member Maverick's Avatar
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    I don't know the physics, but I know Nitrogen is better than compressed air in tyres.

    I did the same with my Astra, on 18" wheels, when the tyres were swapped over, 28psi was put in, should be 39psi fromt and 36psi back.

    Once at the correct pressures, mpg was for sure better, so Parapaul you should see an improvement

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