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Thread: 3.91 in A6 auto scale correct?

  1. #1
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    3.91 in A6 auto scale correct?

    Recently had a 1LE 3.91 installed on my G8 (before that I was running a 3.45 diff from a Camaro) and I loaded in the new 3.91 gear ratio on the Speedo table and hit commit.

    My 4 > 5 shift is now set for 99mph. It was 113mph with the 3.45.

    Can I really trust auto scale anymore?

    2009 G8 GT - White Hot

    226/234 601/601 112+4, Yank SS 3600, SLP Longtubes, Magnaflow exhaust, Rotofab, 3.91 Rear

  2. #2
    Tuner in Training
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    Try the bluecat trans table program, worked well for me.

  3. #3
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    Quote Originally Posted by B-_-RAD View Post
    Try the bluecat trans table program, worked well for me.
    Hm I'll take a look. Downloaded the program and will start messing around.

    Do you prefer Bluecat's over the built in HPT auto scale utility?

    2009 G8 GT - White Hot

    226/234 601/601 112+4, Yank SS 3600, SLP Longtubes, Magnaflow exhaust, Rotofab, 3.91 Rear

  4. #4
    Tuner hivoltagedriver's Avatar
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    I used auto scale when I did my gear change, but I also used the BlueCat calc. Then I did some tweaking on my own.
    2017 JKU Rubicon: Purple, long arms, tons, 40s, Banks CAI, Borla cat-back, TD2/HPT tuned, daily.
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  5. #5
    Tuner in Training
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    I haven't tried autoscale, but the bluecat tool worked well for me

  6. #6
    Senior Tuner Russ K's Avatar
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    The auto scale is correct. 3.45/3.91=.8849x113=99.99

    Russ Kemp

  7. #7
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    Quote Originally Posted by Russ K View Post
    The auto scale is correct. 3.45/3.91=.8849x113=99.99

    Russ Kemp
    I put in my actual tire size of 275/35/19 (26.6) and now it comes out to 101.

    2009 G8 GT - White Hot

    226/234 601/601 112+4, Yank SS 3600, SLP Longtubes, Magnaflow exhaust, Rotofab, 3.91 Rear

  8. #8
    Senior Tuner Russ K's Avatar
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    Quote Originally Posted by Allmachtige View Post
    I put in my actual tire size of 275/35/19 (26.6) and now it comes out to 101.
    You don't enter the actual tire dia, as the loaded tire radius is smaller. Just change the tire size by the % difference of the new/old tires. The correct value in the tire field will always be less than the actual tire dia.

    Russ Kemp

  9. #9
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    Learning a lot through Bluecat's program. Its interesting how I've been running tunes based off the stock bin for years where it would never allow a shift into 6th at WOT.

    Granted that a stock G8 GT with a 2.92 doesn't shift into 5th until 131mph.

    With the 3.91 I'm programed to shift into 6th, NOT 5th...6th, at 129mph.

    2009 G8 GT - White Hot

    226/234 601/601 112+4, Yank SS 3600, SLP Longtubes, Magnaflow exhaust, Rotofab, 3.91 Rear

  10. #10
    Tuner snowvette's Avatar
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    What happened to the 3.45 camaro diff.?
    Last edited by snowvette; 06-12-2014 at 02:48 PM.
    2007 Chevy Trailblazer SS AWD with boltons GP tuned
    2012 Chevy Cruze A6 LTZ RS Vermount Stade 0 tuned

  11. #11
    Advanced Tuner HartRod's Avatar
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    Quote Originally Posted by Russ K View Post
    You don't enter the actual tire dia, as the loaded tire radius is smaller. Just change the tire size by the % difference of the new/old tires. The correct value in the tire field will always be less than the actual tire dia.

    Russ Kemp
    How much different is the loaded tire diameter than the unloaded tire diameter?
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  12. #12
    Tuner hivoltagedriver's Avatar
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    With the 3.27's, mine shifts 4>5 at 121, and 5>6 at 166. I was told that if you leave a little torque management in it, WOT into 6th is OK.
    2017 JKU Rubicon: Purple, long arms, tons, 40s, Banks CAI, Borla cat-back, TD2/HPT tuned, daily.
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  13. #13
    Advanced Tuner HartRod's Avatar
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    I looked at a stock 2014 Camaro SS tune and compared the tire diameter in the tune (27.8") to the actually tire diameter on the car (28.7" (275/40/20)) and it looks to be about a 3% difference.
    69 Suburban Bagged & Blown: http://67-72chevytrucks.com/vboard/s...d.php?t=200387
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  14. #14
    Senior Tuner Russ K's Avatar
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    Quote Originally Posted by HartRod View Post
    I looked at a stock 2014 Camaro SS tune and compared the tire diameter in the tune (27.8") to the actually tire diameter on the car (28.7" (275/40/20)) and it looks to be about a 3% difference.
    Sounds about right, but the best way is change the tire size by the % difference of the new/old tires. That insures that the speedo will be correct.

    Russ Kemp

  15. #15
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    Quote Originally Posted by snowvette View Post
    What happened to the 3.45 camaro diff.?
    The ring and pinion are sitting on my kitchen table. I just had the gears swapped by a shop.

    Ok Russ, so this is my question about the whole new/old % difference.

    A stock bin file from a G8 always lists the tire size as 26.22, which is way off as a stock tire on the 19" rim is a 245/40/19 and that comes out to 26.7. My tire is a 275/35/19 on the rears which comes out to 26.6 (per manufacture spec).

    So what should I actually be putting into HPT for tire size? Do I adjust the % difference from the 26.22 that's listed on a stock bin or from the actual stock tire size of 26.7?

    2009 G8 GT - White Hot

    226/234 601/601 112+4, Yank SS 3600, SLP Longtubes, Magnaflow exhaust, Rotofab, 3.91 Rear

  16. #16
    Senior Tuner Russ K's Avatar
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    Quote Originally Posted by Allmachtige View Post
    Ok Russ, so this is my question about the whole new/old % difference.

    A stock bin file from a G8 always lists the tire size as 26.22, which is way off as a stock tire on the 19" rim is a 245/40/19 and that comes out to 26.7. My tire is a 275/35/19 on the rears which comes out to 26.6 (per manufacture spec).

    So what should I actually be putting into HPT for tire size? Do I adjust the % difference from the 26.22 that's listed on a stock bin or from the actual stock tire size of 26.7?
    New/old tire size*current tire size field. So 26.6/26.7*26.22=26.12

    Russ Kemp

  17. #17
    Tuner hivoltagedriver's Avatar
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    I only play with the tire size if I find that the speedo is off a little vs. my GPS. I haven't had to change it on the G8 yet, and I am running a 285/35r19 (stock CTS-V size). I didn't have to change it when I ran the 275s either.

    I did have to play with it a little on my Yukon.
    2017 JKU Rubicon: Purple, long arms, tons, 40s, Banks CAI, Borla cat-back, TD2/HPT tuned, daily.
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  18. #18
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    Quote Originally Posted by Russ K View Post
    New/old tire size*current tire size field. So 26.6/26.7*26.22=26.12

    Russ Kemp
    Ok I think I gotcha now.

    My speedo is just maybe 1mph faster than my GPS, so this makes sense because I put 26.6 into HPT. Oops, I'll fix that.

    2009 G8 GT - White Hot

    226/234 601/601 112+4, Yank SS 3600, SLP Longtubes, Magnaflow exhaust, Rotofab, 3.91 Rear

  19. #19
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    Don't have much to add... Nice combo. What shift points are you using with your setup?

    My 2010 Camaro SS has 3200 + 3.91's and the usual stuff but no cam / head as of yet. I'm still figuring out the best shift points but I've been 12.06 before the gear and 12.14 since but in worse air.
    Last edited by Pro Stock John; 08-07-2014 at 01:24 PM.
    I like LS stuff...
    2010 Camaro SS | 12.06@113 SI

  20. #20
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    Sorry if I am wrong but I am also running 275/35/19 on the rear and 255/35/19 on the front. I set mine up at 26.6 also and mine was off by 1 MPH also. After some thought I decided to use the front tire size of 26.0 since the speed sensors are on the front hubs. Now it's spot on with my GPS .