Do I have to update ECU, in case of changing a wheel size ? - Ford Focus Forum, Ford Focus ST Forum, Ford Focus RS Forum
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Old 09-15-2012, 02:50 AM   #1
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Do I have to update ECU, in case of changing a wheel size ?

Is it necessary ? I'm going to change the wheel from the OEM 17" to an aftermarket 18" one.


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Old 09-15-2012, 02:51 AM   #2
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if they have the ability to do it, I would. OR, figure out what the plus size is on the tires. (so you have the same circumference of tires as stock)
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Old 09-15-2012, 03:20 AM   #3
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I go from 15" in the winter to "18 in the summer it only about 2-3 mph difference. If i'm going 68 i'm actually going about 65. It's a little peace of mind when I'm speeding. lol
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Old 09-16-2012, 08:59 PM   #4
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The key is the overall diameter of the tire not necessarily the wheel diameter. For example, 195/60-15, 205/50-16, and 205/45-17 all the same overall diameter and require no need to mess with the ECU when changing between these wheels and tires. Your goal should be to use an 18" tire with the same overall diameter as the 17" tire that came on the car.

If your car came with 215/45-17, 215/40-18 or even 225/40-18 are very close to the overall diameter of the 17's. You just need to take into consideration wheel width, tire width, and wheel offset to avoid potential rubbing and clearance issues.
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Old 09-16-2012, 09:34 PM   #5
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Yea I meant to put tire size and forgot. I go from 195/75-15 to 215/40-18.
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Old 09-17-2012, 09:33 AM   #6
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Okay, which mean i have nothing to do with ECU after all, I'd heard from my friend where it can correct mph, mpg, and even correct a gear shifting for a previous model. By the way, my car came with 215/50 -17 which mean I have to change to 235/40 -18 right ?
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