Rear end swap, messed up speedo.

chris86uk · Apr 16, 2015

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 · Apr 16, 2015

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 · Apr 16, 2015

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.....

ElrodKTPQ_89 · Apr 16, 2015

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

the5.ohh · Apr 17, 2015

ElrodKTPQ_89 said:
My car had an AODE with 2.73's. My friends 98 GT has a 4R70W with 3.08's.

Mine was an aode, 2.73s aswell.

CC'S95GT · Apr 17, 2015

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.

chris86uk · Apr 17, 2015

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