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Math problemo.........tire size vs stock

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by BurbinOR, Aug 19, 2004.

  1. BurbinOR

    BurbinOR 3/4 ton status

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    Say a factory tire is supposed to be 29" and you're running 35's............what is the formula to calculate the % odometer/speedometer difference for gas mileage calculations? /forums/images/graemlins/confused.gif

    Probably simple but I just cracked open a bottle of Yukon to cut it with lime juice (mmmmmmmmm Snakebite) and *had* to drink the two shots out of the bottle to fit the right amount of lime juice in. /forums/images/graemlins/smile.gif
     
  2. jekquistk5

    jekquistk5 Weld nekid Premium Member GMOTM Winner

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    Easy just devide 35/29 you get 1.2! Whatever your spedo reads times that by 1.2 and you have your speed!

    Whoa just realized you want something else, need to put down the JD and go to bed

    Good luck
     
  3. backcountry

    backcountry 1/2 ton status

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  4. dyeager535

    dyeager535 1 ton status Premium Member

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    Best idea is to figure out how far off %-wise your speedo is reading (GPS, speedometer check station/stopwatch) and calculate from there.

    Actual tire size, growth, actual gear ratio can all play into what you are actually off. Perhaps try both methods (timed distance and calculator), but the ratios GM used for stock speedometer gears/tires are almost never what the online calculators (or sites) tell you should have been used. Might get you close, but can't be as close as timing your vehicle for a given distance with YOUR tires. We all know advertised and actual tire size is almost never right.

    Stolen from someone that has dealt with this more than I... "so change gear ratio of the speedo gears by (example) 15% to slow them down (add to driven, reduce drive)"
     
  5. spearchucker

    spearchucker 1/2 ton status

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    [ QUOTE ]
    Whoa just realized you want something else, need to put down the JD and go to bed

    [/ QUOTE ] You probably should put down the JD because you answered it correctly.

    Theoretically, since you increased the tires size by 20% your speedo should read slow by 20%. If you don't have GPS then the simplest thing to do it follow a friend on the freeway who is doing exactly 60mph and see what your speedo reads. That'll tell you the percentage it's off by.
     

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