Results 1 to 10 of 10

Thread: Speedo Calc, LS1 Corvette

  1. #1
    Member
    Join Date
    Jun 2014
    Posts
    56

    Default Speedo Calc, LS1 Corvette

    I've read a few threads on the speedo calc but I still don't get it.

    The car is a 2000 LS1 Corvette with manual transmission and 3.42 differential (H0103).

    Stock rear tire = 275-40-18. When I plug that into the calculator it comes up with:
    H0101 = 18802 (edit: fixed previous number what was in the wrong units)
    H0102 = 11.70

    When reading the stock tune it shows:
    H0101 = 19280
    H0102 = 40

    My current dyno tune shows:
    H0101 = 19884
    H0102 = 40

    Are these just minor tweaks to fine tune the speedo? The dyno tune is off the stock tune by over 5%. I'm thinking the tuner just copied / pasted from another cars tune and missed that the other car had different size tires.

    I'm changing the rear tires to 305-35-18, the calculator indicates:
    H0101 = 18984
    H0102 = 40

    Should I just start with 18984 and adjust according to my GPS at highway speed?

    What is the calculation performed by the calculator?

    Thanks!
    Last edited by CR500; April 5th, 2015 at 01:21 AM.

  2. #2
    Joe (Moderator) joecar's Avatar
    Join Date
    Apr 2003
    Posts
    28,403

    Default

    Is the Transaxle checkbox checkmarked...?

  3. #3
    Joe (Moderator) joecar's Avatar
    Join Date
    Apr 2003
    Posts
    28,403

    Default

    Post your tune file.

  4. #4
    Lifetime Member
    Join Date
    Mar 2007
    Posts
    349

    Default

    FWIW - I've played with the speedo calculations on the full sized trucks. From what I have found, yes, the factory "tweaks" the cal to get the speedometer to read right. I did the hand calculations, and compared it to what was in the stock files from the factory. Yup, GM tweaks them to get the right .

  5. #5
    Member
    Join Date
    Jun 2014
    Posts
    56

    Default

    Joecar, yes the box is checked!

    Turbo_bu, thanks. Guess I'll go with the calculator value for the new tires and adjust according to my GPS. What is the calculation?

    Rob

  6. #6
    Lifetime Member
    Join Date
    Mar 2007
    Posts
    349

    Default

    Example of what is calculated:

    {H0101} Vehicle Speed Sensor Pulses per Kilometer 62763 Pulses/Km
    {H0102} Vehicle Speed Sensor Pulses per Revolution 40 Pulses/Rev
    {H0103} Differential ratio 3.71997 :1

    {H0101} is calculated using the following formula:
    tire size (T), H0102 (P) and H0103 (D) using the equation:
    Pulses per mile: H0101=P*D*63360/(T*pi) where T is specified in inches.
    Pulses per km: H0101=P*D*1000000/(T*pi) where T is specified in mm.

    If you start plugging in your numbers you will probably see what the "theoretical" value is ... and what the actual factory value is.

  7. #7
    Member
    Join Date
    Jun 2014
    Posts
    56

    Default

    I must be doing something wrong.

    T = Tire diameter (inches)
    305-35-18 = 26.41"
    T*pi = circumference = 82.96"

    63360 = inches / mile

    D = H0103 = Differential ratio = 3.42
    P = H0102 = Pulses per rev = 40
    H0101 = 40*3.42*63360/82.96 = ~104480 (calculator shows 18984)

  8. #8
    Lifetime Member
    Join Date
    Mar 2007
    Posts
    349

    Default

    Not exactly sure, but seem to remember that under the Speedo Calculator there is a check box for Corvettes. The pop-up info for the check box says that the pickup is on the axle and not the transmission output. Guessing that this might be part of the issue??? I have not played with corvettes yet, mainly trucks, Camaro's and swaps.

  9. #9
    Member
    Join Date
    Dec 2006
    Posts
    96

    Default

    When measuring your tire diameter it also makes a difference on the width of the rim. The 26.4" diameter for your tire is measured on a 12" wide rim. If you have hoosiers/kuhmo autocross tires on 10.5" wheels they will be a slightly larger diameter. The Corvette PCM will make the corrections based on axle rotation. I have changed tire sizes on both my Escalade and Corvette and the GPS shows the spedo to be very close to its reading based on 10 mile runs.

  10. #10
    Member
    Join Date
    Jun 2014
    Posts
    56

    Default

    Turbo_bu, Joecar had asked about the checkbox and it is checked. It looks like when it is checked the calculator uses an H0102 of 11.7 (instead of 40). When I use that and calculate by hand (spreadsheet), I get close to the same result as the calculator. Curious why it isn't exact. I'm not rounding off and I'd expect the calculator isn't either.

    I noticed that I made a mistake in post 1. I gave pulses / km from the calculator and pulses / mile from the tune. They are now both in pulses / Km and not that far off.

    OEM Tune = 19280
    Calculator = 18802
    Difference = 2.5% probably due to GM tweaking for the actual tire rather than calculated values.

    Calculated by hand = 18808, fairly close to the calculator.

    Thanks

    Rob
    Last edited by CR500; April 5th, 2015 at 01:36 AM.

Similar Threads

  1. delete corvette LS1 DBW
    By stairman in forum Conversions / Aftermarket Parts
    Replies: 2
    Last Post: February 13th, 2015, 11:30 PM
  2. Corvette C4, LS1 P1SC Procharger need tune
    By legendvette in forum Forced Induction and Nitrous Oxide (N20)
    Replies: 8
    Last Post: June 8th, 2010, 03:04 PM
  3. Problems with Speedo Calc. / PLEASE HELP
    By Boost in forum E37, E38 & E67 PFI ECM's
    Replies: 22
    Last Post: April 17th, 2008, 10:17 PM
  4. New update - speedo calc. still funky
    By Boost in forum Duramax 06 LLY / 06+ LBZ & LMM
    Replies: 10
    Last Post: February 25th, 2008, 07:14 AM
  5. 1998 Corvette LS1 VATS problem
    By Rewire in forum Gen III V8 Specific
    Replies: 6
    Last Post: September 12th, 2007, 09:36 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •