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Taller tires

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by rh88k5, Sep 11, 2002.

  1. rh88k5

    rh88k5 Registered Member

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    I just bought an 88 k5, it has 265/75 r 15's, I want to put 33x10.5's on the stock 15x7 wheels, my question is how much effect will this have 0n my speedometer and will I need to change the speedo gear in the tranny to compensate?
     
  2. Suffergauge

    Suffergauge 1/2 ton status

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  3. MudbogAD

    MudbogAD 1/2 ton status

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    I wouldn't change a speedo gear because your speedo will hardly be off.
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    Difference at 2000RPM is going to be about 4MPH, and at 60 it will be 5MPH. (3.73 and 1:1 assumed axle/tranny ratios, and assumed as advertised tire diameter)

    If you want an accurate speedo, you can either change gears, or find some "adapters" GM used on some trucks and cars, which change the "output" of the speedo driven gear, so you don't have to change the driven gear at all. Not to mention, the 265's you have on there now already have changed the speedo, (235x75R15 or 31" were both "stock" or optional sizes) so it will be even further off when you go to 33's than what my example was.
     
  5. muddysub

    muddysub 1 ton suburban status Staff Member Moderator GMOTM Winner

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    with only 33s i wouldn't bother, i'm running 36s and i haven't changed anything for my speedometer.
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    Yeah you are only off by 11MPH@2000RPM if your stock tire was 235x75R15, 1:1 tranny and gears are 3.73, plenty to get a speeding ticket on the freeway in some states.
     
  7. BigBadCad

    BigBadCad 1/2 ton status

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    You could also try running the cable directly into the T-case without the speedo gear. Mine ended up dead on (verified by GPS).
     
  8. MNorby

    MNorby 3/4 ton status Premium Member GMOTM Winner

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    I ran 265's for a while and it wasn't a big difference. Ran 35's w/ 4.10s and it pretty much equalled to being correct too. Then I put a TH400 and lost the speedo all together, just go with the flow of trafic /forums/images/icons/smile.gif
     

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