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Does tire OD have an effect on a tune?

Discussion in '2011-2014 V6 Mustang Tech' started by Feyd, Feb 3, 2016.

  1. Feyd

    Feyd Member

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    I have only done one event so far but am scheduled for 6 days of road racing this year through HOD and plan to do this regularly. I have many plans but have to do them a bit here and there, the next thing I want to do is a 3.73 rear end (I sometimes vacillate toward a 3.55). This will require a tune so I am planning to get the Bama. My daily driver tires are 255/45R18 which is a 27" OD but I want a dedicated track set at 275/35R18 which drops OD to 25.6 (for further torque gains). So here's the question: If I put a tune on the car will it have negative consequences when swapping the wheels and tires back and forth? If its only the speedo difference I don't care. If it is a big deal I will wait til my current 255s wear out and replace with a smaller set to match the track set.
     
  2. Phil II

    Phil II Cone Destroyer Staff Member

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    You're correct regarding the speedometer. The difference between a 26" and 27" tall tire will not amount to significant changes in the speedometer, nor will it affect engine load, which is also tune related.

    What rear ratio do you have now? I suggest using the tremec gear ratio calculator to see which ratios with whichever transmission you use will do. I also strongly recommend driving another track day or two, especially at the one you will frequent. This way you could anticipate if higher gear ratios are inappropriate and leave you busy rowing gears instead of driving. I'm talking about choosing between riding the limiter in 3rd or going to 4th for 3 seconds before a brake zone.

    Skip the tune until you need to correct major changes. Shop around, don't press the easy button by default.
     
  3. Feyd

    Feyd Member

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    stock is 2.73 The thread from your other post addressed how changing the rear would further complicate the weak driveshaft issue and would be in trouble at 108mph. I'm already hitting the governor 2/3rds of the way down the straight. I talked to the tranny guy and If I change the shaft now I will have to change it again when I swap the tranny. It seems I have to do all the drive-train stuff at once. Which is obviously ideal but expensive. It's likely to be more than a year before I can make that happen.