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Thread: Calculate Idle RPMs

  1. #1
    Join Date
    Aug 2008

    Calculate Idle RPMs

    I am creating software to determine what gear I am in and display the gear for me and later a shift light for redlining. Does anyone know of a way to calculate if I am in neutral. I know how to calculate my speed based on my RPMs and Transmission gear ratio and Differential Gear Ratio. I have a VW GTI 2010 that I am going to test my software with. My formula is this 0.00595*((RPM*10.994625)/(TransGear * DifGear)) = Speed. Once I figure out the speed I compare it to my current speed and if it is in a certain bound i'm in such and such gear. So after a long winded story how would I calculate my engines Idle speed. would I need my compression ratio, temperature, etc? If I can't calculate it can someone give me some possible logic i could use in my software. I usually calculate for three gear at the same time(Lower, Last Known Current, Higher). I want to indicate gear Neutral when the user shifts out of gear but is still rolling whether its down a high or on the flats.

    Below is my gear ratios:

    Trans Gear Ratio
    1st 3.36 GVT
    2nd 2.09 GVT
    3rd 1.47 GVT
    4th 1.10 GVT
    5th 1.11 GVT
    6th 0.93 GVT
    Reverse 1.64 GVT

    Differential ratio
    Final I 3.94 GVT *for Gear 1-4, R
    Final II 3.09 GVT *for Gear 5-6

  2. #2
    Raw Wave
    Join Date
    Nov 2009
    You want to calculate - or measure - the idle speed?
    Is it EFI?

    As to neutral, it'll be any ratio that doesn't match with the above ratio bands (ie, with tolerance).
    The problem though is that then "Neutral" is displayed for clutch slip, clutch disengaged, vehicle stationary, and - if the speedo is driven off separate wheels - any wheel spin. And of course the gears will flash anytime the speed & engine RPM match that ratio.
    Hence why gear indicators are usually switches etc on the selectors.

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