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Thread: MISSING MPG after fitting 18" alloys.

  1. #1
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    Question MISSING MPG after fitting 18" alloys.

    Hi,
    I am new to this site but i can see you all have a lot of technical knowledge so has anyone any advice on this issue.
    I have vectra c 2.0dti that had 16's on when i got it.
    I use the car for long motorway journeys and on many occassions i filled the tank to overflowing, zeroed the mileometer and drove on cruise at a steady 70mph. Without exception the fuel light would come on after 600miles.
    two weeks ago i fitted 18" bk alloys with 235x 40 x 18 tyres and NOW the same journey at the same speed , the petrol light comes on at 500 miles.

    Surely fitting new alloys hasnt cost me 100 miles on my tank of diesel??
    The wheel and tyre height is almost the same as the 16" wheel and tyre i had on. I cant understand this but i know there must be some technical explanation for it . Anyone got the answer??

    Martin

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    maybe your new wheels have lead in them? :-)


    Andy

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    In theory you should get more miles to the gallon as your now travelling 2" further per revolution. Lol. Seriously though i'd be surprised fitting bigger wheels can knock your speedo out slightly so your going slightly faster than the dial shows but not by that much.

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    http://www.alloywheels.com/tyrecalc.asp

    have a look how far out your tyre is

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    Default mpg

    ....weather is colder....cold air affects mpg

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    Rolling resistance from 235 wide sport tyres will be much worse than 205/215 normal tyres.

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    Regular Member Neil's Avatar
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    you might need to visit your local stealers and get them to tech2 the new tyre size into the ecu.

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    Sounds right to me.

    The rolling diameters of the tyres you have fitted is virtually identical to the original 215/55/16's so don't worry about the speed - the difference at 70mph will be + 1/4mph... So unless you plan on regularly driving at 200mph plus, it's a total waste of time to change.

    What has changed is:
    1. Rolling resistance
    Just changing from one brand to another can have a huge effect, even for the same size tyre - check out the Auto Express group tyre test!
    2. Drag
    Wider tyres will increase the frontal area of your car and hence the drag. The effect is actually quite noticeable at anything above 60mph.

    Have you tried to top out the car since changing the tyres? You'll probably have lost between 5-10mph from your top speed due to the increased drag.
    So, sorry to be the bearer of bad tidings, but the change has simply made the car less fuel efficient.

    James

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    Vehicle : SRi


    Engine : Z22SE (2.2 Petrol)

    Year : 2004

    Mileage : 137000

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    They dont do a 3L cdti in the Xpack coss the wheels are 19's and due to this the car would fail its euro 4 emissions.

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    Thanks for everyones help on this...... I think i will visit the dealer and see what their service manager says. I i really have lost 100mpg then the wheels and brand new tyres will be advertised on these pages very soon!!!!

    Martin

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