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What would my actual MPH be after gear AND tire change?

Discussion in 'The Garage' started by y5mgisi, Jul 14, 2006.

  1. y5mgisi

    y5mgisi 1 ton status

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    I went from 3.08's to 4.10's and from 31's to 36's anyone know how to figure out my actuall MPH?
     
  2. smokkey1

    smokkey1 1/2 ton status

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  3. goldwing2000

    goldwing2000 Guest

    Man... that's some crazy damn chart. :eek1:

    Just take the truck on the freeway and count off milemarker posts. Write down your odometer reading at one post, drive for 10 miles, record the reading at post #10. Divide the number of miles your odo clicked off by ten and divide the mph on the speedo by the result. That eliminates any effects of torque converter slippage or minor differences in tire diameter.

    It sounds complicated but it's really easy.

    For instance, when I had 3.42 axles and 35" tires, my odo clicked off 7.9 miles for every 10, which gave me a factor of .79 for every mph on the speedo. I made a chart to stick on the dash so I would know how fast I was going.

    7.9/10=.79

    20/.79=25.31
    25/.79=31.65
    etc.

    Speedo Correction
    IndicatedActual
    20=25
    25=32
    30=38
    35=44
    40=51
    45=57
    50=63
    55=70
    60=76
    65=82
    70=89
    75=95
    80=101
    85=108
     
  4. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    I don't think you can calculate like that since the odometer is a fixed and not a variable, what i mean is the speedo cable has 1000 turns per measured mile whereas the speedo works on a magnetic force which varies by cable rotation speedo.
     
  5. 1985_K5_Silverado

    1985_K5_Silverado 1/2 ton status

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  6. goldwing2000

    goldwing2000 Guest

    Right. The odometer is fixed but it's still based on revolutions of the speedo cable and therefore revolutions of the transmission output, not revolutions of the engine.
    I never said to use the speedometer for calculations, only for indications. Same as if you're driving.

    Trust me. It works. I've checked it against other people's speedos.
     
  7. goldwing2000

    goldwing2000 Guest

    That still uses engine RPM and theoretical tire size (a 35x12.50-15 tire is not exactly 35" in diameter, especially if it's installed on a narrow wheel) and doesn't account for variable T/C slippage. It also does not take factors like different speedometer gears into account and doesn't tell you what your speedometer error is.

    Short of asking a cop to hit you with his radar, comparing your measured miles to actual miles is the most accurate way of figuring your correct speed.
     
  8. Chaddy

    Chaddy 1/2 ton status GMOTM Winner

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    I went from 3.08's to 4.10's and from 32's to 35's and my speedo reads 4 mph over.
     
  9. 1985_K5_Silverado

    1985_K5_Silverado 1/2 ton status

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    If you have a GPS unit (they are pretty cheap these days), you could use one of those to read your speed. I think a few forum members use them for that purpose, in additon to navigation.
     
  10. BranndonC

    BranndonC 3/4 ton status

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  11. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    Mine was under 100 bucks , and is USB into my laptop . I am 5 mph off at 55 with 3.73's and 33's in 4th on a 700r4 .
     
  12. y5mgisi

    y5mgisi 1 ton status

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    good idea.
     
  13. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    since speed is determined by ratio, i would find it hard to believe that you would only be off by 4 mph over the whole range of your driving speed. as an example, take a look at goldwing's first post in this thread. the error starts out relatively small, but as his speed increases, the error increases much more.
     
  14. 88Silverado

    88Silverado 1/2 ton status

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  15. goldwing2000

    goldwing2000 Guest

    Yep. You're always off by a percentage, not just a number. Unless your speedo is computer-controlled, that is. The speedo in my 2004 Mazda reads consistantly 2-3 mph high, throughout the range. That's a "safety margin" that the manufacturers like to put in to keep you from speeding. Can't do it with a cable, though.
     
  16. thezentree

    thezentree 3/4 ton status

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  17. Chaddy

    Chaddy 1/2 ton status GMOTM Winner

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    ha..I forgot to add going 45mph:wink1:
     

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