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do i have too?

This is a discussion on do i have too? within the Wheels and Tires forums, part of the General Help category; I am getting new rims and tires with size 245/45/17 on the front and 284/40/18 on the rear. Do i ...

  1. #1
    Senior Member clg82's Avatar
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    do i have too?

    I am getting new rims and tires with size 245/45/17 on the front and 284/40/18 on the rear. Do i need to adjust my pcm because of these tire sizes?

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    Detailing + Design third_shift|studios's Avatar
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    check the stickies in the tire/wheel section

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    The only reason you would need to change it is if your new rear tires are going to be taller than the stock ones.

  4. #4
    98T/Aformula
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    Yeah if your rims are going to be bigger then the ones that you have now you will notice you will be be doing 10 over the speed limit and you speedomiter will read that your doing the speed limit haha just need to let the computer know whats going on if there is a change in size...

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    Member tpilews's Avatar
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    Quote Originally Posted by clg82 View Post
    I am getting new rims and tires with size 245/45/17 on the front and 284/40/18 on the rear. Do i need to adjust my pcm because of these tire sizes?
    Like Kirbbs said, you only need to make an adjustment if the tire height is different than stock.

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    Senior Member mrr23's Avatar
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    Quote Originally Posted by clg82 View Post
    I am getting new rims and tires with size 245/45/17 on the front and 284/40/18 on the rear. Do i need to adjust my pcm because of these tire sizes?
    yes. the 285/40-18 are 27.1" tall. the 245/45-17 are 25.7" tall. this will cause the ABS light to come on and affect braking as well. for correct front to rear diameters, you need 285/35-18. they are 25.7" tall. doing it this way will keep the ABS system from freaking out and do not need to adjust the speedo calibration in the PCM.

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    Senior Member clg82's Avatar
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    Quote Originally Posted by mrr23 View Post
    yes. the 285/40-18 are 27.1" tall. the 245/45-17 are 25.7" tall. this will cause the ABS light to come on and affect braking as well. for correct front to rear diameters, you need 285/35-18. they are 25.7" tall. doing it this way will keep the ABS system from freaking out and do not need to adjust the speedo calibration in the PCM.
    So I won't be able to drive? Because of the ABS system?
    Last edited by clg82; 06-07-2007 at 11:12 PM.
    2000 Arctic White trans am, ws6 hood, ws6 air box, slp lid, slp smooth bellows, slp loudmouth exhaust,shift kit, rebuilt trans, LS6 intake, 3.73 gears, Shaner S3 p/p throttle body, and a tune . SFC. C6 ZO6 rims 18" in back 17" in front. Drilled and slotted rotors.

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    Senior Member clg82's Avatar
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    Well I installed the new rims and tires this evening and have no problem with the ABS light or the speedo being off. So I am not sure where you received the info about that but hopefully this helped.

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    Funny, I didn't seem to have that ABS and speedometer problem either. That .6 differance must not be enough to affect anything...

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    Crazy Canuck ! Eugenio_SS's Avatar
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    you should be getting 275/35-18 for the rear... or 285/35-18... not 40 ratio.
    The 40 ratio makes your rear tire taller by a full inch compared to the front and stock size.
    Eugenio_SS
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    Senior Member mrr23's Avatar
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    Quote Originally Posted by clg82 View Post
    So I won't be able to drive? Because of the ABS system?
    didn't say that. said it would kick off the ABS light, which, in turn affects your braking. the PCM sees the front tires turning faster than the rear and assumes the rears are sliding. when you hit your brakes, the ABS tries to compensate for that.

    Quote Originally Posted by clg82 View Post
    Well I installed the new rims and tires this evening and have no problem with the ABS light or the speedo being off. So I am not sure where you received the info about that but hopefully this helped.
    how do you know the speedo isn't off? i received my info from me being in the tire and wheel business for many years. that and my personal experience with my race tires. fronts are 25.5" and rears are 27". going over 50 mph, the ABS light comes on.

    Quote Originally Posted by ZR28YELLOWJACKET View Post
    Funny, I didn't seem to have that ABS and speedometer problem either. That .6 differance must not be enough to affect anything...
    check your math. 27.1 - 25.7 = 1.4" difference. again, how do you know your speedo isn't off? speedo is calibrated based on gear ratio and overall tire diameter.
    Last edited by mrr23; 06-08-2007 at 03:41 PM.

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    Senior Member mrr23's Avatar
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    My wheel width dilemma
    here's thread in this forum where a member has this same setup and has issues with it.

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    Senior Member clg82's Avatar
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    Quote Originally Posted by mrr23 View Post
    didn't say that. said it would kick off the ABS light, which, in turn affects your braking. the PCM sees the front tires turning faster than the rear and assumes the rears are sliding. when you hit your brakes, the ABS tries to compensate for that.



    how do you know the speedo isn't off? i received my info from me being in the tire and wheel business for many years. that and my personal experience with my race tires. fronts are 25.5" and rears are 27". going over 50 mph, the ABS light comes on.



    check your math. 27.1 - 25.7 = 1.4" difference. again, how do you know your speedo isn't off? speedo is calibrated based on gear ratio and overall tire diameter.
    Driving next to my wife we were reading the same on our spedos and also went 105 mph today and no abs light.

  14. #14
    Senior Member mrr23's Avatar
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    well, consider yourself a marine. one of the few, so far.

    and this is the tire sizes you have correct?
    245/45/17 on the front and 285/40/18 on the rear

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    Senior Member clg82's Avatar
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    That tire size is correct!

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