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speedometer question

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by GONZO, Dec 9, 2000.

  1. GONZO

    GONZO Registered Member

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    i just got my 33's installed on the 88 blazer. they would have fit great if i didn't have running boards. so i had to trim the edge of the running boards stightly. anyway on to the question. anyone know the differance 33's will have on the speedo reading? original tires were 235/75 (approx 27" diameter) new tires are a true 33" diameter. i'd hate to get pulled over for speeding when i think i'm doing the limit.

    <font color=blue>85 k5 Blazer 4x4
    99 Trans am 30th anniversary
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  2. Thunder

    Thunder 3/4 ton status

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    33's gave me about a 10% error. When my spedo read 60 mph. I was really doing 66 mph. I went from 235x 15 to 33" tall tire.
     
  3. GONZO

    GONZO Registered Member

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    thanks
    i was guessing about 5mph off, but was not sure.

    <font color=blue>85 k5 Blazer 4x4
    99 Trans am 30th anniversary
    79 Trans am [​IMG]
     
  4. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    You could use the mile-markers on the highway too. Your odometer will be off exactly as much as the speedo is. So if you pass 10 mile markers but you odometer says you went 9 miles then you would know your speedo is reading 10% slow.

    Rene


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  5. 90K5

    90K5 1/2 ton status

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    Just ask a cop to clock you. Tell him the situation, and I'm sure he will help you. Mine was off by about 6% from 31s to 33s. On an '89 or anything older, all you have to do is to replace the speedometer gear which is in the speedo cable connected to the transfer case.

    90K5
     
  6. geckojack

    geckojack Registered Member

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    I did a tire swap just like yours... I clocked the new tires against a friends speedo and read about 1/5th higher... When mine read 50, the other car was doing 60. It really makes a differnece ther faster you go.
     
  7. Gandolf

    Gandolf 1/2 ton status

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  8. Hobbes68

    Hobbes68 Registered Member

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    I clocked my Jimmy with a radar gun from work and found the speedo / odo to read 10% less than actual speed / distance, ie - 30 mph = 33 mph, 50 = 55, etc... The original tires were 31's and I now have 35" BFG AT's.

    BTW, the tire size calculator mentioned in the post above is neat, but keep in mind that not all tires are created equal. Most tires are not true to size / height. My BFG's are not a true 35" tall (even when they were new). I've found this to be the case with many mfg.'s. The rim width also plays a part in the actual height you end up with.
     

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