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Thread: MPG decrease after adding 19's

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    Regular Member paul_sri150's Avatar
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    Default MPG decrease after adding 19's

    just wondered if anyone else has noticed

    After having my first full tank burnt off after adding my 19's, i have notced i have only got 580 miles from a tank

    with my standard 17's on, i was getting around 650-700

    anyone else had this? it may be the fact that i still havent got the tyre pressure right

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    Have you had your speedo recalibrated? You probably have to get it done via Tech 2.

    An extra 2 inches on your wheels will affect your speedo reading and also your calculated MPG. As your wheels are bigger they will take longer to do a full rotation and so your mileometre will also be out. If ive got this right... Your car will "appear" to be doing less miles as a result. Your actual MPG wont be that different i wouldn't have thought though.

    Hope this helps.

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    Quote Originally Posted by riverdees View Post
    Have you had your speedo recalibrated? You probably have to get it done via Tech 2.

    An extra 2 inches on your wheels will affect your speedo reading and also your calculated MPG. As your wheels are bigger they will take longer to do a full rotation and so your mileometre will also be out. If ive got this right... Your car will "appear" to be doing less miles as a result. Your actual MPG wont be that different i wouldn't have thought though.

    Hope this helps.
    The wheels are bigger yes, but the rolling road circumferrance of the tyres is the same as before. The profile size of the tyre should only now be 35mm to compensate for the bigger rims.

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    As he said above. I consulted my wife and she said an extra 2 inches makes no difference at all. She is an expert in this area (an engineer :-).

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    Regular Member Penfold101's Avatar
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    Ok - the rolling radius of the wheel/tyre combo is ALMOST the same. There are calculators online that will tell you the exact differnce but it should only be a few mm at most, which is no different to having worn down your tyres.

    Differences could be caused by the wheels being slightly heavier, bad fuel in the tank, or a million other things. See if it gets any better after a few tanks as that may have jsut been a random coincidence...

    Also, the profile of the tyre is 35% of the tyre width, not 35mm...

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    Regular Member john_k_sri's Avatar
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    Vehicle : Vectra

    Trim : SRI NAV XP

    Engine : 1.9 CDTI 150

    Year : 2007

    Mileage : 72K

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    Quote Originally Posted by paul_sri150 View Post
    just wondered if anyone else has noticed

    After having my first full tank burnt off after adding my 19's, i have notced i have only got 580 miles from a tank

    with my standard 17's on, i was getting around 650-700

    anyone else had this? it may be the fact that i still havent got the tyre pressure right
    Under inflated tyres will not help at all. And it won't do the tyres any favours either. 39/35 front/rear is what the manual says. Don't be tempted to lower pressures to soften the ride.....it doesn't.

    The twice plus two 'rule' is a good guide IMO........at least for the fronts.

    After a little bit of experimentation I'm running 41/38 front/rear and it feels just fine for me.

    I went from a 2.0T on 17"s to an SRI CDTI 150 with 19"s, and I'm getting a minimum of 550 per tank, as opposed to 350 with the petrol, which is about right for my driving mix.
    Last edited by john_k_sri; 3rd September 2010 at 08:32.

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    I can tell you how, but I have often read in car magazines that larger wheels equals both poorer ride quality and poorer fuel efficiency. I can understand the ride issue, but unless the tyres are wider on the bigger diameter wheel, I am at a loss to explain the loss in fuel efficiency.

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    Regular Member john_k_sri's Avatar
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    Vehicle : Vectra

    Trim : SRI NAV XP

    Engine : 1.9 CDTI 150

    Year : 2007

    Mileage : 72K

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    The difference in fuel economy as with CO2 emissions will be minimal. But as said under-inflated tyres will cause more of a difference.

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    Regular Member MalcGSI's Avatar
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    Quote Originally Posted by WilliamLX View Post
    I can tell you how, but I have often read in car magazines that larger wheels equals both poorer ride quality and poorer fuel efficiency. I can understand the ride issue, but unless the tyres are wider on the bigger diameter wheel, I am at a loss to explain the loss in fuel efficiency.
    well 235/35/19 is wider than 215/50/17

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    215/50/17 vs 235/35/19 gives a 0.05% difference which is 0.3mm in diameter and a 0.94mm difference in circumference. not enough to mechanically throw it out out of kilter.

    The main thing that'll impact the MPG is the rolling resistance. Whether it's properly inflated or not there's an extra 20mm x 100mm (100mm approx for distortion on contact with road) patch on each tyre in contact with the road.

    More contact means more resistance therefore more juice required to keep pushing you along.

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