Rear end swap, messed up speedo.

chris86uk

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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
 

blackdemon007

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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.
 

lutter94

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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.....
 

CC'S95GT

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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.
 
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chris86uk

chris86uk

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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
 

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