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Thread: anyone care to help adjust tire size?

  1. #1
    Advanced Tuner Boost's Avatar
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    anyone care to help adjust tire size?

    frank truck tuned no wheels.hpt

    I am posting the current tune, installed 305/30/26 wheels and now the speedo shows 3 mph slower than actual and I'm sure it's worse at high speeds.

    Having a tough time doing the calculation and adjustment, I think that comes to 33.20" rolling diameter but what all parameters need adjusted to actually make it accurate??

    Stock file posted.

    Thanks!!!!
    '12 Caprice PPV 6.0 L77 - daily transportation
    8.7 @ 84 (1/8 mile) bolt-ons

    '02 Silverado RCSB 5.3 L59 - regularly street driven
    8.2 @ 86 (1/8 mile) stock cam and spray
    8.6 @ 84 (1/8 mile) cam and heads no spray

    Our YOUTUBE CHANNEL featuring the Silverado

  2. #2
    Tuning Addict 5FDP's Avatar
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    Use the gear tire wizard in the edit tab. Plug in your new tire size and the same gear ratio number and have it auto scale the transmission parameters.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  3. #3
    Advanced Tuner Boost's Avatar
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    Yup that took care of it thank you. I don't know why sometimes I forget basic things the older the stupider.
    '12 Caprice PPV 6.0 L77 - daily transportation
    8.7 @ 84 (1/8 mile) bolt-ons

    '02 Silverado RCSB 5.3 L59 - regularly street driven
    8.2 @ 86 (1/8 mile) stock cam and spray
    8.6 @ 84 (1/8 mile) cam and heads no spray

    Our YOUTUBE CHANNEL featuring the Silverado

  4. #4
    Tuner in Training
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    Not sure how you are checking the speed. I assume you are using a GPS on a phone. If not make sure you validate the GPS on another car you know the speedometer is accurate. I went with a new set of 22's that were close to the old set but just plugging in the tire size in hptuners wizard tool and using that I still had to fidget with the numbers till it got the speedometer to match the GPS. New tire size worked out to be 31.6 on diameter but I had to set the hptuners wizard 30.83 to get the speedometer to match the GPS...just a suggestion....

  5. #5
    Advanced Tuner Boost's Avatar
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    I am in fact using a smartphone GPS app. It seems pretty accurate so far. Back in the days we used those flashing police traps that display your speed purposely blasting by them to get a more accurate reading at 60-70 mph lol

    Yes I have the same finding. For instance every time I switch to these 15" race tires in the video below, I have to do additional fine adjustments besides typing in the exact tire size, or else it can still be off 2-3 mph and occasionally bang into the rev limiter and cause shifting problems

    https://youtu.be/x9Kax5lGiXQ

    Thank you for the suggestions!
    '12 Caprice PPV 6.0 L77 - daily transportation
    8.7 @ 84 (1/8 mile) bolt-ons

    '02 Silverado RCSB 5.3 L59 - regularly street driven
    8.2 @ 86 (1/8 mile) stock cam and spray
    8.6 @ 84 (1/8 mile) cam and heads no spray

    Our YOUTUBE CHANNEL featuring the Silverado

  6. #6
    Tuner in Training
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    Mn
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    Subtract about an inch from the actual measurement for diameter. Gm does this from the factory. Put new tires on a truck the other day and measured side to side, then up and down with vehicle on ground. It was about one inch less due to normal tire squat/flex with weight of vehicle. Obviously cars and lighter vehicles will have less squat but it's a good starting point.

  7. #7
    Advanced Tuner Boost's Avatar
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    Quote Originally Posted by HardH20 View Post
    Subtract about an inch from the actual measurement for diameter. Gm does this from the factory. Put new tires on a truck the other day and measured side to side, then up and down with vehicle on ground. It was about one inch less due to normal tire squat/flex with weight of vehicle. Obviously cars and lighter vehicles will have less squat but it's a good starting point.
    wow great info thanks. yeah I did notice that the GM cals are not exactly the factory tire size, yet they work
    '12 Caprice PPV 6.0 L77 - daily transportation
    8.7 @ 84 (1/8 mile) bolt-ons

    '02 Silverado RCSB 5.3 L59 - regularly street driven
    8.2 @ 86 (1/8 mile) stock cam and spray
    8.6 @ 84 (1/8 mile) cam and heads no spray

    Our YOUTUBE CHANNEL featuring the Silverado