History of the Tremec TR3650
by Stan Peace, www.Pro-ForcePerformance.com

This article is a culmination of research begun in 2011; it is periodically updated with new information and clarifications.  The author does NOT give permission to copy and re-post this on other websites without providing credit to the author and a functioning link back to this document.    TR3650 History Article  
Questions, or requests for updates may be made by contacting the author through his website. 

Overview
The Tremec TR3650 first appeared around February of 2001 which would be mid-production of the 2001 Mustangs. Mustangs produced before this date were equipped with the T-45 and a 10.5 inch clutch. After this date they were built with the TR3650 and an 11 inch clutch. The build plate on the car will show a "K" under the "TR" category to reflect that it was built with a TR3650 transmission. 

We think of the TR3650 as being exclusive to the Mustang, but it was used in Australia in some of the BA Falcon and Tornado cars as well as in the UK in the MG-ZT. 

The TR3650 employs a synchronizer design that is different from the Tremec's previous design, the TR3550/TKO, but retains a 3-rail shifting mechanism that is somewhat functionally similar. 

One might say: Wait a minute, the T-45 was the predecessor of the TR3650.  From a Mustang standpoint this is true, but the T-45 and T-56 were Borg Warner designs that were later acquired by Tremec. 

2001-2004 vs 2005-2010
The TR3650 can be divided into two designs. The early units were for 2001-2004 Mustangs and the later units were for the 2005-2010 models. While the insides of these two styles are almost identical, the exterior case pieces and shift rail mechanisms are not. The three main differences are in the shifter mechanism, the throwout bearing construct, and the output shaft connection.  There was no difference in a TR3650 for a Mustang GT and one for a COBRA.  

Throwout Bearing
The 01-04 TR3650 uses the same throwout bearing and cable-operated clutch fork that appeared in the 96 Mustangs equipped with a T-45. 

The 2005-2010 Mustangs were equipped with a hydraulic operated throwout bearing that fits around the input shaft.

Shifter Assembly
The 01-04 TR3650 uses a shifter base and handle that appears to be identical to the T-45, but the shifter base is a different size, so they are not interchangeable. One shifting improvement in the TR3650 is that positive, non-adjustable shift stops were incorporated in the design eliminating the need for any troublesome aftermarket adjustable shift stops as were commonly added to the T-5, T-45 and TR3550/TKO transmissions.

The 05-10 TR3650 uses an external shifter mechanism that manipulates a shaft that protrudes from the upper-rear of the transmission. Performance enthusiasts dislike this design because twisting in the drive train on a hard launch causes the shifter, which is anchored to the body, to tend to mis-index the 2-3 shift.  There are aftermarket shifters that address this, but the most effective "fix" has been to "upgrade" to Tremec's T56 Magnum-XL 6-speed transmission which reverts to the previous concept of the shifter being bolted directly to the transmission housing.

Output Shaft Connection
Beginning with the 2005 model, Ford eliminated the slip-yoke on the tail shaft of their Mustang manual transmissions replacing it with a fixed flange similar to what is seen on the front of a differential. Likewise, the drive shaft design had to change to provide the necessary "slip."

Broken Tail Housings
Both the 2001-2004 TR3650 as well as the 2005-2010 TR3650 have a tendency to crack the Extension Housings allowing the transmission fluid to run out on the road or track. To date, there is no improved housing design, but in most cases, the issue has been traced to excessive driveshaft vibrations. Many have solved the problem by replacing the driveshaft with an all aluminum racing shaft.  In custom applications it can also be due to having an incorrect "Pinion Angle", which is the angle of the output shaft versus the pinion shaft in the differential.      

Gear Sets
There is only one set of production gear ratios for gears one through four, but there are two gear sets used in the TR3650. The ratios are all 3.38, 2.00, 1.32, 1.00.  The three possible ratios for fifth gear are described below.

The reason for the change in design is unclear, but the newer style gears first appeared in the 2003 models. Only the second, third, fifth, and input gears are compatible with both gear sets. All other gears including reverse are unique to the gear set being used. 

To identify the early vs late design gear sets there is a small, but visible "step" in the counter shaft between the teeth for first and reverse that does not appear on the early design. 

The input shafts are compatible between the two sets, but the shaft LENGTH is different between the 2001-2004 units and the 2005-2010 units. The 2005 input shaft is approximately 1/2 inch longer than the 01-04 shaft.

Fifth Gear
There were only three ratios made for fifth gear on a TR3650, In general, the fifth gear ratios were as follows: 2001 = 0.68, 2002-2004 = 0.62, 2005-2010 = 0.68. The third ratio is 0.74 and only appeared in the MG-ZT version of the TR3650. 
For the benefit of anyone reading this and becoming excited about the 0.74 fifth gear ratio I will add that Tremec reports that they have discontinued these as replacement parts, so, finding a pair of these gears is nearly impossible. A custom road race fifth gear pair is also available with a ratio of 0.81.

Updates to the TR3650
Numerous Service Bulletins have been issued for the early TR3650's. I won't attempt to cover these in detail, but will outline the parts that have undergone design changes. While no longer available, Ford installed "update" kits in a number of 2001-2003 units for owners who complained of shifting problems while their cars were still under warranty. These parts focused on the second gear area. Most of these updates appeared in the production 2004 units. 

Updated Gears
Second gear was redesigned. The engagement teeth on the edge of the second gear that contact the synchronizer were changed to have a more aggressive engagement angle.  

Updated Synchronizers and Rings
The synchronizer rings seem to be an issue with the TR3650, but the only change to these rings has been to the engagement teeth on the edge of the outer 1-2 rings. These rings are a 3-piece design with an inner and outer "cone" ring and a double-sided lined ring positioned in between. The redesigned rings have sections of the engagement teeth removed to allow a more aggressive engagement.

Due to "wear issues" with the rings it is highly recommended that all of the rings be replaced any time a TR3650 is serviced. It is also important to stay with the recommended fluid, or one that has been proven to not harm the rings. 

The second gear side of the 1-2 synchronizer hub assembly was redesigned to incorporate engagement teeth of staggered sizes where the original type had uniform teeth all the way around. 

Updated Shift Forks
There are three factory variations of the 1-2 shift fork for the 01-04 TR3650.
Each version contains more aluminum on the forks making them stronger.
The third variation of the 1-2 fork appeared in the "update" kits, but not in any production units. This fork is sometimes called the "Gorilla" fork because it is so massive.

The shift forks for the 2005-2010 TR3650 are different from the earlier units and are not interchangeable. The 1-2 forks beginning in 2005 are similar to the "Gorilla" forks.

Custom Billet aftermarket forks that are even stronger are now available for the TR3650.

Two additional weaknesses of the shift forks that were never addressed by Tremec are that the roll pins that attach the forks (1-2 and 3-4) to the rails tend to fatigue and shear off, and the plastic fork pads tend to crumble allowing the plastic bits to collect in the front counter shaft bearing race. Some builders use stronger roll pins and billet bronze fork pads to address these weaknesses. This applies to all models of the TR3650.

Fluids and Fluid Level
The fluid level in a ANY TR3650 should be filled to 3.2 quarts from a full drain.  This results in a fill approximately 3/4 inch below the fill plug or a 2001-2004. This was a change published in a service bulletin to help eliminate scraping while shifting when the transmission is cold. The 2005-2010 case was redesigned to locate the fill plug in a lower position and again latter in an even lower position.  Discussions on fluid level have continued to be controversial.  The only "Official" statements from Ford or Tremec is 3.0 Liters of fluid regardless of where it comes to relative to the fill plug. In US measure this at 3.2 quarts from a dull drain.   

The actual fluid choice for a TR3650 is also controversial. It is reported that the TR3650 was originally "Qualified" by Tremec using Mobil-1 Synthetic Multi-Vehicle ATF, but the official fluid specified is Ford's Mercon-V which is a quality synthetic blend. The Mobil-1 meets the Mercon-V specification, but is also fully synthetic. DO NOT CONFUSE MERCON-V ATF WITH MERCON-LV ATF.  

Another fluid that has shown good results with the TR3650 is GM Synchromesh; however, since this is older technology fluid, it will need to need to be changed more often . Other specialty fluids have shown that they are too "slick" for the TR3650 synchro rings and will damage the synchros.  PLEASE stick with one of the ATF fluids mentioned above.     

I repeat: Certain very popular synthetic fluids have detergents in them that can damage the linings of the synchro rings and should be avoided. The theory on this is that these specialty fluids are too slick for the synchro rings.

Inherent Strength
The TR3650 is a rather strong transmission but its weak spot in a high-torque application is the 10-spline input shaft. These stock shafts tend to shear off when launching a 500 HP car at the drag strip. To remedy this, builders have been modifying original input shafts by grafting a new 26-spline front onto the gear. A better solution is the availability of a custom aftermarket shaft which puts the TR3650 at an even higher estimated torque rating of about 525 ft/lbs and somewhere around 600 HP. 

Aftermarket Gear Sets
To date, there has been one custom aftermarket gear set available for the TR3650.  It is a 2.90 first gear, close-ratio set that brings the TR3650 to a rating of about 700 ft/lbs of flywheel torque.  Ratios are 2.90, 1.91, 1.37, 1.00.  On the negative side of this, the gears can whine a bit more than stock.  The fifth gear ratios are limited to 0.53 and 0.58 which are undesirable for track applications.  Custom ratios of 0.63 and 0.69 do exist.  Unfortunately, these gears were discontinued in late 2023.

Racing Applications - Scatter Shield
The integral aluminum bell housing of the TR3650 restricts the use of this transmission where SFI requirements are specified. A solution is to replace the front housing with a custom adapter plate that will allow it to bolt to a traditional bell housing that does meet SFI specifications, but the cost of this all-out racing TR3650 rivals that of the T56 Magnum. 

This article was last updated on 06/09/2024 .