Rear end swap, messed up speedo.

Discussion in 'Drivetrain' started by chris86uk, Apr 16, 2015.

  1. chris86uk

    chris86uk Member

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    Hi everyone.

    Changed my rear end over the winter as the old one was noisy. My car is a 96 4.6 GT Manual, and the replacement axle came off a 95 5.0 Auto.

    Just got the car back on the road tonight and found my speedo is reading 10mph BELOW the speed i'm actually travelling at. I thought they were the same ratio but obviously they aren't.

    Just wondering how I correct the speedo and also does that mean that my gears are longer or shorter than they used to be?

    Thanks

    Chris
     
  2. blackdemon007

    blackdemon007 Active Member

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    If stock auto rear end will always have lower ratio than manual, because there is one less gear to work with (at least in our years).
    Anyway you need to buy a speedo gear for the transmission to correct the speedometer reading.

    Or you can swap out the gears back to what you were at.
     
  3. lutter94

    lutter94 Well-Known Member

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    Or you can buy a speedcal, best thing ever.
    [MENTION=18797]blackdemon007[/MENTION], you can't get 2.73's with a Auto Mustang? Mine came with 3.27's.

    The difference between those two ratios is close to 10 mph around 1800 rpm in 5th gear.....
     
  4. ElrodKTPQ_89

    ElrodKTPQ_89 Legend

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    My car had an AODE with 2.73's. My friends 98 GT has a 4R70W with 3.08's.
     
  5. the5.ohh

    the5.ohh Legend

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    Mine was an aode, 2.73s aswell.
     
  6. CC'S95GT

    CC'S95GT Legend

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    The plastic gear on the VSS should be all you'll need unless you go over 3:55 ratio. But you need to find out what's in the old and new rear end.
     
  7. chris86uk

    chris86uk Member

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    Thanks for the replies everyone.

    Right, so I think I've worked out what's going on. My old diff tag looks like it says "3L27" at the start of the bottom line. My door jamb says "Axle - E". So by my reckoning, my old gears were 3:27's?

    The new axle from the 95 5.0 Auto, says "2L73", so 2:73's right?

    I know you guys all like the shorter gears but the car feels plenty quick to me (my daily car is a 1.6 Fiesta!) and with fuel prices over here in England (about $9 per gallon) I'm thinking I might appreciate the better economy!

    Does anyone know what speedo gear I need to correct it based on the info I've just given? Also, am I going to struggle to find the right gear because I'm now using an Auto ratio axle with a T45 manual?



    Anyway, as things stand, here's what I'm showing on speedo vs sat nav.

    [Speedo] [Actual (Satnav)]
    30mph - 36mph
    40mph - 48mph
    50mph - 59mph
     
  8. chris86uk

    chris86uk Member

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    Brilliant, thanks for that lutter94, really appreciate it!