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  1. #1
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    Diablo Tuning wrong tire size?

    Hey All!

    Out of curiosity lately, since I'm getting higher in the mileage department, I checked my odometer via interstate mile markers and found I'm FASTER than I should be! I gain 0.1 mile for every 5 driven!

    When I had my various trucks, I saw the opposite, since they always ended up with larger tires. My last truck had 35" tires. So I always had to correct the tire size by programmers. And even the last truck with measured height instead of just a number input, I was off some, but never fast.

    I used this Diablo i3 and programmed 245/45R20 tires, STOCK size. And I also set my ratio to STOCK, 3.06 Gear.

    Anyone with factory 3.06 gearing and 20" wheels have this same issue?

    Should I just input the actual measured height? (did that with my P2 tuner and I felt like I was going too slow?)

    Appreciate the help! I need to change this soon....
    2013 Charger R/T
    Road & Track Package
    FUN Additions via: TranZformer GEN II, DiabloSport (HEMIFEVER) Tuning, Sultans Of Spark Coils, Plus a 6.4L SRT intake! Minus some resonators EQUALS Loads of Dodge Bad-Assery Built In!!



  2. #2
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    Measure the roll out and calculate using that.


    Sent from my iPhone using Scat Pack Forums
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  3. #3
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    Yeah, ok. Would you think DiabloSport should have their calculations setup properly though? Seems if their programmer is adding the option to change sizes, say for those of us LX folks that go to the 275/40R20 or even larger, then their calculated tire sizes would be wrong as well?

    I wasn't sure if the combination of factory tire size entered in conjunction with the 3.06 gears made it change. Saying if I had the 2.65 ratio, would it be right then?

    Just trying to avoid calling DiabloSport if I don't have to.

  4. #4
    Join Date
    May 2014
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    Do you have a GPS? If you do then check your speedometer with it. Adjust your tire size accordingly. I have 245/45/20 on my MRT and 28" seems to work for me. That's as close as I can get with the tuner. It only adjusts in .5" increments. Not sure which tuner you have or if Diablo has put closer increments in.
    2005 Cool Vanilla Magnum R/T
    20" 2008 SRT Alcoa wheels
    Front SRT shocks/springs
    245/45/20 Firestone Wide Oval, Front Brembo Conversion
    K&N CAI, Diablosport InTune
    MSD Blaster Coil Kit, 6 Disc RAQ Radio
    Thanks MopaR/T thanked for this post

  5. #5
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    Quote Originally Posted by classicr View Post
    Do you have a GPS? If you do then check your speedometer with it. Adjust your tire size accordingly. I have 245/45/20 on my MRT and 28" seems to work for me. That's as close as I can get with the tuner. It only adjusts in .5" increments. Not sure which tuner you have or if Diablo has put closer increments in.
    Thanks!

    I put a GPS app on my phone and was showing 1 mph less on GPS, so speedo was fast. Earlier today I checked with Diablo i3 tuner I have and it showed 28” tires. I had used a straight edge and tape measure to get that height on car. And, those tires were worn.

    I checked specs of my tires and used a used a tire calculator site to find a 245/45R20 is 28.7” tall. So I entered that but it changed to 28.5 tall. It was after this I drove with GPS app.

    So a bit ago I changed it to 29” tall. I’ll find out tomorrow on the way to work using GPS and mike markers to see if it’s better or worse.

    The Diablo still showed saved gear ratio of 3.06. I don’t think that’ll change anything but speed, right?

  6. #6
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    Oh, I should add I do have navigation. And it displays speed as well. But I don’t know if that’s satellite/GPS speed indicated or vehicle indicated speed? It does show the speed limit for the particular area I’m driving, which is usually in red, since I may be going faster than posted speed? But that indicated speed on the Nav is the same as EVIC speed.

  7. #7
    Join Date
    May 2014
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    If the GPS shows red you are speeding. Also, remember the GPS is delayed. It shows what you were going about 2-3 seconds ago. So you should take that into account.
    I used another car with my GPS to check the speed. Speedo and GPS were the same. Then I checked the GPS with my Magnum. I then adjusted the tire size until it matched as close as it could. It was just trial and error.
    Don't be afraid to experiment with tire sizes. It won't hurt anything if it is wrong. When I have to take my car to the dealer I return it to stock just in case they do something to the computer. My speedo is then about 3 mph off. Car doesn't even notice.
    Now, the wrong gear ratio on the other hand may throw a code or it might hurt the car, I really don't know. I have never changed it.

  8. #8
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    Quote Originally Posted by classicr View Post
    If the GPS shows red you are speeding. Also, remember the GPS is delayed. It shows what you were going about 2-3 seconds ago. So you should take that into account.
    I used another car with my GPS to check the speed. Speedo and GPS were the same. Then I checked the GPS with my Magnum. I then adjusted the tire size until it matched as close as it could. It was just trial and error.
    Don't be afraid to experiment with tire sizes. It won't hurt anything if it is wrong. When I have to take my car to the dealer I return it to stock just in case they do something to the computer. My speedo is then about 3 mph off. Car doesn't even notice.
    Now, the wrong gear ratio on the other hand may throw a code or it might hurt the car, I really don't know. I have never changed it.
    Yes! I am usually speeding, if only 1 mph faster, because that number on the NAV is always red!

    Anyway, I think I finally got it.

    Honestly I don't know geometry/physics, or any other than basic math....?! Because I should have LOWERED the tire size in the Diablo to lower the speedo/odometer displays. I had raised it last night to 29" and once I got on the interstate I reset Trip A to 0.0 just as my front bumper passed a yardstick on the interstate. Half mile later I already knew it was wrong. So I stopped on the shoulder of an off ramp and reset to 29.5" tire size. NOPE! Worse.

    I pulled into a rest area and reset it once more, but this time to 27.5, two inches lower than I was and I'm pretty much 100% dead nuts on odometer.

    The irritation is I've had this set wrong for a couple of months now...!! So, that means my mileage, which is nearing 125k now, isn't REALLY that high. I drive about 450+ miles a week commuting to work. So in about 10 weeks, I've lost what, 45 miles?

    Oh well. Gonna have this car till it's dead.

    Thanks for reading and the help folks!

  9. #9
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    The mile markers are not measured. They are approximate. Every once in a while you may see a sign that says something like "measured mile ahead." I used to see them.

  10. #10
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    Yeah, I know those areas. There’s a “Speedometer Check Course” here on a 5 mile stretch of US 31 that could use. It’s measured by 1 mile signs for that section. But I’m pretty happy with the setting now. Next time I drive that road I’ll check it again.



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