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  1. #1
    Registered Member One Ring
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    Oct 15 2009
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    Inflation pressure for 225/45R18

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    My winter setup is a set of 225/45R18 Dunlop SP Winter Sport M3 XLs on OEM 18x8 wheels. Since this is not an OEM setup, there is no inflation pressure guidance in the manual or on the door pillar. A call to Tire Rack, where I bought the tires, did not inspire confidence. The rep reasoned that since the air volume is the same as the 245/40R18s, the inflation pressure should be the same, but I don't understand how the 225s and 245s (with roughly the same revs-per-mile) have the same volume.

    Can anyone provide input on what the proper inflation pressure is for this setup?

    Thanks,
    jdphog

  2. #2
    Veteran Member Three Rings LI Audi's Avatar
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    Mar 13 2009
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    39836
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    Long Island NY

    Re: Inflation pressure for 225/45R18

    Quote Originally Posted by jdphog View Post
    My winter setup is a set of 225/45R18 Dunlop SP Winter Sport M3 XLs on OEM 18x8 wheels. Since this is not an OEM setup, there is no inflation pressure guidance in the manual or on the door pillar. A call to Tire Rack, where I bought the tires, did not inspire confidence. The rep reasoned that since the air volume is the same as the 245/40R18s, the inflation pressure should be the same, but I don't understand how the 225s and 245s (with roughly the same revs-per-mile) have the same volume.

    Can anyone provide input on what the proper inflation pressure is for this setup?

    Thanks,
    jdphog
    PM Doc@ tirerack and get an experts opinion
    2009 A4 premium Plus Quartz/Black/Wood
    2.0 quattro, Tiptronic ,NAV, 50% tint, H&R OE sport springs, 18"S4 reps


    1969 Datsun Roadster

  3. #3
    Veteran Member Four Rings L0U's Avatar
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    Sep 03 2009
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    47271
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    13 S8, 15 Macan Turbo, 04 Ducati SS1000DS
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    Re: Inflation pressure for 225/45R18

    Volume has nothing to do with it. Pounds per square inch does...and a 245 has more area supporting the weight than a 225 does. The S4 is a bit of a heavyweight...wearing 245s, and 255s. The 225s fit, but will be near the max load rating. You will want at least 40psi or even as much as 45psi. The tire is likely rated for 50psi max.

    I run 42psi in my 235 winters.
    2013 Daytona S8, B&O, Bk alcantara, Solar, 360 camera, adaptive cruise,
    2015 Rhodium Macan Turbo, BK alcantara, Loaded.

  4. #4
    Established Member Two Rings trigoe's Avatar
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    Oct 22 2009
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    49612
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    river edge NJ

    Re: Inflation pressure for 225/45R18

    my 245/40r18 winter dunlops are inflated to 41PSI with nitrogen.

    The ride and feel are amazing. The tires are incredible. So sticky.
    2010 S4/Prestige/S-Tronic/Brilliant Black/Black-Brown Silk Napa/Black Optic Exclusive/Sport Diff/Carbon Inlays/Sunshades

  5. #5
    Registered Member One Ring
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    Oct 15 2009
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    Re: Inflation pressure for 225/45R18

    I think you have to take the tire's load index into account. The load index is an indication of the tire's load bearing capacity when inflated to 36psi. The S4 manual lists the following recommended tire pressures for normal loads:

    39 (front) & 35 (rear), for 245/40-18s with a load index of 93
    36 (front) & 33 (rear), for 255/35-19s with a load index of 96

    Load indexes 93 and 96 indicate a load bearing capacity of 1,433lbs and 1,565lbs, respectively, at a pressure of 36psi. So, a 93 load index tire requires relatively more pressure to handle the same load as a 96 load index tire. This is why the 245's require more pressure to reach the same load bearing capacity as the 255s.

    Now, my Winter Sport M3s have a load index of 95 (1,521lbs @36psi), so that would indicate that they would require an inflation pressure somewhere between that required by the 255s and 245s.

    Based on this, I'm planning to run 37-38psi front and 34 rear. Does this make sense?

  6. #6
    Veteran Member Four Rings L0U's Avatar
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    Sep 03 2009
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    Re: Inflation pressure for 225/45R18

    Quote Originally Posted by jdphog View Post
    I think you have to take the tire's load index into account. The load index is an indication of the tire's load bearing capacity when inflated to 36psi. The S4 manual lists the following recommended tire pressures for normal loads:

    39 (front) & 35 (rear), for 245/40-18s with a load index of 93
    36 (front) & 33 (rear), for 255/35-19s with a load index of 96

    Load indexes 93 and 96 indicate a load bearing capacity of 1,433lbs and 1,565lbs, respectively, at a pressure of 36psi. So, a 93 load index tire requires relatively more pressure to handle the same load as a 96 load index tire. This is why the 245's require more pressure to reach the same load bearing capacity as the 255s.

    Now, my Winter Sport M3s have a load index of 95 (1,521lbs @36psi), so that would indicate that they would require an inflation pressure somewhere between that required by the 255s and 245s.

    Based on this, I'm planning to run 37-38psi front and 34 rear. Does this make sense?
    strangely my door has 41 front and 41 rear for my 96 load rated 255s. Canadian car with Michelin ps2's. have the sport diff and 7-speed, so it weighs a bit more than the stocker.

    my car is 4275lbs with me driving at 210. As per the waste/dump scales. A 30mm narrower tire in a 225, sounds a bit low to me at 38,34. and a load index 1 less at 95.

    Is your S4 a b8?
    2013 Daytona S8, B&O, Bk alcantara, Solar, 360 camera, adaptive cruise,
    2015 Rhodium Macan Turbo, BK alcantara, Loaded.

  7. #7
    Registered Member One Ring
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    Re: Inflation pressure for 225/45R18

    Quote Originally Posted by L0U View Post
    strangely my door has 41 front and 41 rear for my 96 load rated 255s. Canadian car with Michelin ps2's. have the sport diff and 7-speed, so it weighs a bit more than the stocker.

    my car is 4275lbs with me driving at 210. As per the waste/dump scales. A 30mm narrower tire in a 225, sounds a bit low to me at 38,34. and a load index 1 less at 95.

    Is your S4 a b8?
    You're right. The door sticker says 41 front and rear for the 255s. However, the manual has a table (page 278 in my manual) that lists different inflation pressures for "normal load condition (up to 3 occupants)" and "full load condition." The numbers I gave above are for the "normal" load. According to the table, full load calls for 41psi front and rear and for both the 245-18s and 255-19s. So the door sticker doesn't distinguish between normal and full and just gives full load number, which is probably the low-liability default. But I'm not sure that is what I want in a car that transports only me back and forth to work 90% of the time.

    Regardless of the normal/full choice, do you agree that the load index is a better determinant of proper inflation pressure than a rule of thumb that narrower tires need more pressure?

    My car is a US B8, with Prestige (includes 255/35-19s), S-tronic, and sport diff.

    Thanks,
    jdphog

  8. #8
    Veteran Member Four Rings L0U's Avatar
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    Sep 03 2009
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    13 S8, 15 Macan Turbo, 04 Ducati SS1000DS
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    Re: Inflation pressure for 225/45R18

    for sure, load index is the best bet, when you know what it is.
    2013 Daytona S8, B&O, Bk alcantara, Solar, 360 camera, adaptive cruise,
    2015 Rhodium Macan Turbo, BK alcantara, Loaded.



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