SN95 3.73 Gear Change For Automatic

[LoganCT]

1994 GT Automatic Transmission - I know that information is scattered around, but have been unable to find a complete guide on the steps for swapping stock gears out for 3.73s on an automatic transmission.

I know that when changing gears you have to get the speedometer adjusted and shift points adjusted due to the auto trans, but looking for info specific to the 94 auto. Do I need a speedo gear or can it be done with tuner? Will something like an SCT chip adjust the auto trans shift points? Apologies for the noob question!

 

[General karthief]

Any other mods to the car?
Oh andd welcome aboard

 

[LoganCT]

Any other mods to the car?

Nope. Not yet. I plan on installing CAI and throttle body in the near future though

And thanks

 

[General karthief]

All I got for ya is a bad joke, maybe @jrichker can clue you in or fetch someone who knows more than me, which is just about everybody.

 

[jrichker]

The speedo problem is fixed by swapping the speedo drive gears. There are some limitations with auto trans cars since he speedo drive gear is made into the transmission output shaft and is not replaceable.

Late Model Restoration (http://www.latemodelrestoration.com/products/79-93-Mustang-Transmission-Speedometer-Correction) has a complete range of replacement speedometer gear sets.
See https://lmr.com/products/Mustang-Speedometer-Gear-Calculator for help in choosing the correct speedometer gear.

If the above link doesn't have the right combination for your gear/tire size choice, there are other alternatives.
Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy within 1%-3%. The drawback is that it isn't cheap. Cost is $95-$120 from www.gaugeguys.com

See http://www.gaugeguys.com/ratio-adapters.html for more info.

See http://www.davidsonsales.com/docs_pdf/Stewart_Warner_HD_Catalog.pdf for more information on the Stewart-Warner speedometer gearboxes.

 

[LoganCT]

The speedo problem is fixed by swapping the speedo drive gears. There are some limitations with auto trans cars since he speedo drive gear is made into the transmission output shaft and is not replaceable.

Late Model Restoration (http://www.latemodelrestoration.com/products/79-93-Mustang-Transmission-Speedometer-Correction) has a complete range of replacement speedometer gear sets.

If the above link doesn't have the right combination for your gear/tire size choice, there are other alternatives.
Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy within 1%-3%. The drawback is that it isn't cheap. Cost is $95-$120 from www.gaugeguys.com

See http://www.gaugeguys.com/ratio-adapters.html for more info.

See http://www.davidsonsales.com/docs_pdf/Stewart_Warner_HD_Catalog.pdf for more information on the Stewart-Warner speedometer gearboxes.

Thanks for the info! Sorry for being thick, but does what you just mention apply to the 1994 stang? (the link is for 79-93). And any thoughts on the auto trans shift point adjustment? That's what I'm really struggling with, as I don't want to mess up the transmission with a gear swap.

 

[jrichker]

Thanks for the info! Sorry for being thick, but does what you just mention apply to the 1994 stang? (the link is for 79-93). And any thoughts on the auto trans shift point adjustment? That's what I'm really struggling with, as I don't want to mess up the transmission with a gear swap.

The 94-95 speedometer gears are the same as the earlier Mustangs. The electronic speedometers didn't come until 96 and later Mustangs.

 

[Mustang5L5]

Use this calculator as it takes rear tire size into consideration. 94-95 uses an 8 tooth drive gear.

https://lmr.com/products/Mustang-Speedometer-Gear-Calculator


If having a really accurate speedo is a concern, you'll have to adjust tire size slightly as well. You'll notice that the optimum gear might be "19.6 teeth" but unfortunately speedo gears don't work that way, so by adjusting the tire size you can get the calculation close to a full number of teeth like 19, 20, 21.