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  •  » final gear ratio

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    10/09/2020 5:40 AM  #1

    val fulesday
    I FYI 24-7

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    Registered: 9/23/2013
    Posts: 403

    final gear ratio

    I am in the process of buying a 8" post  with some kind of a posi unit with 3;80 gears, and going to finally put in my t5 . i'd like to know what the formula is for figuring out the final gear ratio. so with the assumption that 2000 rpm is where you'd get the best gas millage, I am trying to figure out what would be the best gear ratio to accomplish that, yes I am aware that the tire height plays into this 215 70 15 is the tire size.
     

     

    10/09/2020 8:24 AM  #2

    RV6
    Senior 24-7 FYI Addict

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    From: Pine Junction, CO
    Registered: 3/01/2016
    Posts: 1,249

    Re: final gear ratio

    Try this calculator:
    https://www.tremec.com/calculadora.php
    With 3.55 gears 2000 rpm would give you around 71 MPH

    Gary Zilik - Pine Junction, Colorado - 67 Coupe, 289-4V, T5
     

    10/10/2020 10:41 AM  #3

    Nos681
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    From: Atlantic Mine, Michigan
    Registered: 7/14/2016
    Posts: 3,336

    Re: final gear ratio

    I had factory stock 5.0/T5 (3.35 first gear)/3.73 (8.8” rear)/ 245/45/17 tires in ‘92.

    First gear was pretty much a granny gear.

    Shifting into second after/through intersection depending on throttle input.

    Fifth was great for highway.

     

    10/10/2020 3:25 PM  #4

    val fulesday
    I FYI 24-7

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    Registered: 9/23/2013
    Posts: 403

    Re: final gear ratio

    I thought the gear in my falcon was a 3.25. a friend of mine after riding in the car says it's steeper than that.
     

       •Thread Starter
     

    10/10/2020 5:43 PM  #5

    TKOPerformance
    Hyper FYIFORD ADDICT

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    From: Newark, DE
    Registered: 12/15/2014
    Posts: 7,502

    Re: final gear ratio

    The RPM at which you get the best fuel mileage at 2,000RPM is not all that well founded.  Its really just an arbitrary RPM people shoot for, because its not got the engine screaming going down the highway.  To truly know the RPM at which the engine gets the best fuel mileage you would have to do considerable real world testing, and the number of potential variables make it a daunting test. 

    To determine cruise RPM at a particular MPH the formula is :

    RPM = MPH * Gear Ratio * 336 / Tire Diameter

    Gear ratio is the 5th gear ratio times the final drive (0.68 * 3.50 = 2.38 for example).  So in this example, RPM = 70MPH * 2.38 *336 / 26, so RPM at 70MPH would be 2,153. 

    IMO a 3.80 gear is too tall for a typical street driven Mustang with a T5.  Most guys will tell you a 3.25-3.50 ratio is ideal.  I have a 3.50 and have run the car on several longer trips (1hr or more each way) of mostly interstate driving, and the car easily kept up with traffic running 75-80MPH.  RPM was reasonable, and hell of a lot better than it was with the same gears and the C4. 

     

    10/10/2020 6:09 PM  #6

    Toploader
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    Registered: 4/15/2013
    Posts: 1,640

    Re: final gear ratio

    I know mine is a 4 speed, but I have a 3.00 ratio and on the highway it is very nice. It's pretty decent taking off from a set of lights too. Sometimes I wonder if a 2.80 ratio would be better, but I will probably keep what I have.

    1964-1/2 D Code Coupe - 289 V8, 4 Speed Toploader, 3.00 ratio rear, Autolite 4100 Carb, 15" tires, Pertronix ignition
     

     

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