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Tire Size and Speed Question

Discussion in 'The Garage' started by MEPR, Jan 16, 2003.

  1. MEPR

    MEPR 1/2 ton status

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    Alright this may be a bit small for you guys but here it goes. I had 25" tires stock on my luv but now it has 27.5" tires on it, what is the mph differance? Or can you guys redirect me to a calculator. Thanks
     
  2. Thunder

    Thunder 3/4 ton status

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  3. jeffro

    jeffro 1/2 ton status

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    That's right at a 10% difference taller. So if your speedo indicates 65MPH you add 10% and you're actually going 71.5MPH.
     
  4. MEPR

    MEPR 1/2 ton status

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    well that explains why i got stopped today, was going just under 60mph, trooper said i was going 65+. OOOPS! /forums/images/graemlins/blush.gif
     
  5. vansterdam88

    vansterdam88 Newbie

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  6. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    A speedo will be off by different percentages at different speeds as they work on a sliding scale so to speak. They are magnetic driven and this is why they work on a sliding scale. The odometer on the other hand is gear driven so it will be off by the percentage difference unlike the speedo. This is of course assuming we are dealing with a mechanical type speedo and not an electric speedo.
     
  7. fordeater

    fordeater 1/2 ton status Premium Member

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    wow, glad we dug up a 4 year old post for a new calculator
     
  8. 70jimmy

    70jimmy 3/4 ton status Premium Member GMOTM Winner

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    On most mechanical speedo's both the speedo and odometer would be affected by tire size change. One variable is that tire diameters are not usually very accurate. The calculators put you in the ball park but you would want to physically measure the diameter of you new and old tire on the vehicle at whatever pressure you run and use those figures, not just what it says on the side of the tire.

    A better way to measure your vehicle would be to go a set speed for one mile using mile markers for the distance since your odometer may be off. Keep your vehicle at a constant speed and time how many seconds it takes to travel that one mile. Here are some figures to tell you how fast you are going. There is a formula to figure it out but I won't bore you with it.

    Time Speed
    45.5 79
    46 78
    46.7 77
    47 76
    48 75
    48.5 74
    49 73
    50 72
    50.5 71
    51.5 70
    52 69
    53 68
    53.5 67
    54.4 66
    55 65
    56 64
    57 63
    58 62
    59 61
    60 60 DUH
    61 59
    62 58
    63 57
    64 56
    65.5 55
    66.5 54
    68 53
    69 52

    Another thing many people don't realize is that old speedos were based on the distance traveled in one rotation so when the diameter of the tire or circumference of the tire is changed that is why the speedo error happens. Also as you increase speed the # of miles per hour that your speedo is off changes. So if you are 5 mph off at 30 the error increases as you go faster speeds.

    All those in favor of no more speeding tickets please raise your right foot!
     

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