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Discussion Starter · #1 ·
Does our Ecu need to be programmed to exact tire size? I know the rim size effects the speedometer. I head tire size/rolling distance calculations are important to abs.


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Can it be? Yes
Does it need to be? No

Even slapping on 225/40 18s on stock wheels, you're technically messing up how accurate your speedometer is. But it's only off by 1mph around the 80mph mark, that's easy enough to live with and won't have any noticeable impact on ABS. It's when you're swapping from 36" mud tires to 40" that things need to be updated.

Not sure how wheel size alone will affect your speedometer, it's the overall diameter of wheel and tire that matters.
 

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Discussion Starter · #3 ·
It's the rolling diameter. A 215/50/17 vs 215/45/17 has a 5% difference in sidewall height, therefore abs is thinking the wheel should/should not be turning when it is/isn't


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Interesting... I didn't know that the ABS took that into account, I thought it was just a matter of preventing the discs from completely locking up upon rapid braking.
 

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I wouldn't think abs relies based on rolling diameter. No matter the wheel/rim size, the disk will always rotate at the same rate at a given speed.
 

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It's the rolling diameter. A 215/50/17 vs 215/45/17 has a 5% difference in sidewall height, therefore abs is thinking the wheel should/should not be turning when it is/isn't


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What...?
If all of the tires are the same, and on an
AWD, they should be, How Would It KNOW?

ABS is looking at the differential of rotation
between all four tires. Like Volvo said, as long
as they're all the same, your ABS will be fine.
 

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How Would It KNOW?
It's not about how it would know, it's that it's preventing all four corners from locking up. But if you're running larger tires in the back, those brake discs are rotating slower than the fronts. So maybe that's where it throws it off a bit and runs the risk of locking up the rears.

Come to think of it... I'm not running the same size tires front to back and I have had ABS failure. Hmm... I don't care, I like sliding and learned how to drive on cars without ABS. But that at least solves that mystery!
 

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It's not about how it would know, it's that it's preventing all four corners from locking up. But if you're running larger tires in the back, those brake discs are rotating slower than the fronts. So maybe that's where it throws it off a bit and runs the risk of locking up the rears.
it's a comparative system. if you suddenly locked up all 4 of them (not really possible because of the brake bias) and they all read the same speed the ABS computer would not take action. it functions by comparing the wheel speed sensors outputs to one another and to the VSS. That way it knows the individual wheel speeds. When one of the wheel speeds doesn't match the others it compares it to the VSS, and then figures out which wheels are not at the right rotational speed.
 
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