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Mileage Conversion

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Me89'Jimmy, Jul 16, 2003.

  1. Me89'Jimmy

    Me89'Jimmy 1/2 ton status

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    Does anyone know the formula used to get the correct mileage readiing off the odometer after the gears have been changed? the gears are 4.88's thanks
     
  2. JustHorsinArownd

    JustHorsinArownd Registered Member

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    What's been the easiest for me is to take the truck to what it thinks is sixty miles an hour and drive at a very constant sixty for a few road miles all the while running a stop watch. Remember that at sixty it takes exactly a minute to go a mile. Since you're most prolly goin to be goin faster than sixty it'll take you less than a minute to go the mile. Once you figure out how long it takes to go a mile do this, divide 60 by however many seconds it took you to go the mile and this will give you how many percent your speedo is off. Sorry for the novel but kinda takes a while to explain the whole thing. Hope this makes any sorta sense. Good luck!

    Allan
     
  3. cjblais

    cjblais Registered Member

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    I got a buddy with a nice gps. They give mph figures so I used it to figure out where 35 and 65 were on my speedometer. No tickets so far
     
  4. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    What gears were there before the swap? Have you made any other changes to tire size from stock (I assume you have)...

    What I would do is start with the "factory" setup and calculate the number of tire rotations per mile based on the stock tire diameter and stock gearing. Then do the same calc with the new gearing and tire setup.

    The difference between those two values should allow you to come up with a "correction factor" to use when you check your odometer mileage.
     
  5. rampage

    rampage 1/2 ton status

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    Get a friend to drive with you down the freeway with him doing 60. See what your speedometer reads. If your speedo says 50 then it's slow by 20%

    (60-50)/50=.2 or 20%
    Your actual speed = 50 + (50*.2)

    Same goes with the odometer. If your speedo is slow by 20% then your odometer will also be slow by 20%. You need to know what the milage reading was on the odometer before you changed gears/tires. Subtract that from what the odometer reading is now and multiply that by the percent you're off.

    ((current odomter reading) - (orig. odometer reading)) * (% diff) + (current odometer reading)= actual milage

    or

    ((current odomter reading) - (orig. odometer reading)) * ((friend's speed - your speed)/your speed ) + (current odometer reading)= actual milage

    This is assuming your speedo is reading slower than it should be. Hope this makes sense.
     
  6. Me89'Jimmy

    Me89'Jimmy 1/2 ton status

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    as far as i know they were 3.73 gears with 31's, now its 4.88's with 35's.
     
  7. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    3.73 w/ 31's = 2628RPM @ 65
    3.73 w/ 31's = 3032RPM @ 75

    4.88 w/ 35's = 3042RPM @ 65
    4.88 w/ 35's = 3514RPM @ 75

    Assuming you have a tach.....when you reach ~3000RPM in 3rd (not overdrive) you are NOW at an ACTUAL 65 MPH.....compare that to the "indicated" speed at that RPM and you'll have a good idea which way (and by how much) your speedo is wrong.


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