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Thread: Tire Calc ?

  1. #1

    Tire Calc ?

    Hey Guys,
    I have a question about the tire calc in the speedo section.
    If you look at the entered value in my tune which is the same as stock, it shows a value of 24.29. I have change tire size on my car 235/45/17 and the Nitto website says that the tire height is 25.51". I went to Pirelli's website and looked up my factory tire size 205/50/17 and it shows 25.01".
    Why would GM have entered the above 24.29 for the tire calc ?
    I want to change this to get my speedo reading correctly, at 60mph on the speedo compared to my gps 62-63 mph it is 2-3mph slow.
    If I am correct on my thinking that entering 25.51 would make the speedo off by more than what it is now, so I have not done this yet.
    Hoping to get some good info from the pros on here before I mess with it.
    What do you think ?
    Thanks in advance for any input on this subjet.
    2007 Pontiac G5 GT.
    Attachment 29356

  2. #2

  3. #3
    Thank You, I'll give it a try.

  4. #4
    Advanced Tuner
    Join Date
    Sep 2009
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    Angola NY, (Buffalo)
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    IIRC there is a calculator in one of the pull down tabs.
    It was a while ago since i used it, can't completely remember.
    HIH
    The Regal has been laid to rest..... rust
    Now have an 06 Lucerne with the 3.8 and wishing HPT would cover that car
    Loved my 2001 Buick Regal GS.
    With more than 334,000K, 3.4 & 3.6 pulley, Change stats and pulleys with the seasons, Upgraded grounds & power wire, Volt booster, Caspers timing commander(AKA happy knob), Spectra intake-fender wall, PLog, 3" DP & 2.5 exhaust, HPTunner 12" Impala duel piston front brakes, New trans,GMPP suspension... yada, yada...

  5. #5
    Potential Tuner
    Join Date
    Mar 2011
    Posts
    5
    Granted I am refering to a trailblazer 05 i6...
    But when i get to the tire / calc tab there is nothing listed..
    Now i dont know if that is locked or is going to be added later, cause there are alot of us trailvoy's who would like to get the speedometer and the ecu back on the same page.
    I have tried that spreadsheet and it does not work for me.

  6. #6
    Well I really don't know what I'm doing here, but this is what I tried tonight on way home from work. Obviously wrong ! I took the 25.51" and divided it by 25.1 = 1.01633466. See original post.
    So I took the tire height in stock tune 24.29 and X it by the 1.01633466 =24.68.
    I entered that in the tune and took off for a drive, lol, at 30mph on my gps, the speedo read 9 mph. The question is after I made that change 2 of the "final drive ratio" columns changed from what stock is. The stock tune is on the left and the changed tune is on the right pictured below. The stock says in the Ratio column marked with red line says 1.00 and the Err column says 3.60, after the tire size change the Ratio and Err both says 3.29 ?
    Should I change those numbers back to what stock tune says ? I did go out a few ago and measured the new tire height and it appears to be just under 25".
    I did try the tire size calculator in the tools section and it says tire height is 25.3" for a 235/45/17.
    I will try either one of these ideas tomorrow morning, but I'd just like to hear from you guys about the 1.00 in the Ratio column and the 3.60 in the Err column.
    Thanks for looking and maybe an answer to this.
    Attachment 29438

  7. #7
    Well tonight on the way home from work I tried changing the FINAL DRIVE RATIO
    back to what it was stock (see above post attachment).
    Ratio- back to 1.00
    Err- back to 3.60
    seems like that worked, I only got to go 40mph,
    but the GPS and the speedo were both reading the same.
    I 'll try up to a 100mph this weekend.
    All shift points seem to be the same, no log, so I think I'm on the right track here.

  8. #8
    Potential Tuner Monte83's Avatar
    Join Date
    Apr 2019
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    4
    Did you ever figure this out?