ZF S6-650 & S6-750 6 Speed Manual Transmission

The ZF S6-650 and S6-750 (also 6S-650 and 6S-750) are a series of heavy duty manual transmission employed in the 1999 to 2010 Ford Super Duty and 2001 to 2006 GMC Sierra/Chevrolet Silverado. The transmission features six forward gears with the lowest achieving a 5.79 to 1 gear reduction and sixth gear achieving a 0.72 to 1 overdrive for an overall ratio spread of 8.04 and an average gear step of 53 percent. All Ford versions of the ZF 6 speed use an internal oil pump to move lubricating oil from the transmission sump through an external transmission fluid cooler integrated into the radiator. GM versions of the transmission do not employ a transmission fluid cooler.

The transmission has a three piece aluminum case comprised of a main housing, which includes the integral bellhousing, an intermediate housing, and a rear housing. At under 2 inches long, the intermediate housing is a relatively short piece that connects the larger main and rear castings. There are several variations of the ZF 6 speed within the S6-650 and S6-750 series, including 4 wheel drive and 2 wheel drive versions. The obvious difference between the 2 and 4 wheel drive variations is in the rear housing of the transmission case, which either adapts to a transfer case or a tailshaft housing depending on the drive type.

ZF S6-650 vs S6-750

The S6-750 is an improved version of the S6-650 found in the 2003 to 2010 model year Ford Super Duty equipped with the 6.0/6.4 Power Stroke diesel. The S6-650 is found in 1999 - 2003 model year Super Duty trucks equipped with the 7.3L Power Stroke, all 6.8 V-10 engines, and 2001 to 2006 model year GM vehicles equipped with either the 8.1L Vortec (gas) or 6.6L Duramax V-8. The bellhousings are different on 7.3L and 6.0L/6.4L Power Strokes thus the S6-750 is not cross-compatible with nor will it bolt up to the 7.3L Power Stroke.

The gear ratios and most major components are identical between the S6-650 and S6-750 transmissions. The input shaft on the S6-750 is slightly larger and this remains, to the best of our knowledge, the most significant difference between the two units and is primarily responsible for the improved input torque rating. Note that there is significant differences in torque output between the 7.3L and 6.0L/6.4L Power Stroke engines. There are rumors that the S6-750 also employs a different mainshaft, however the replacement part numbers for the 7.3L and 6.0L/6.4L ZF 6 speed mainshafts appear to be identical.

ZF S6-650/S6-750 Transmission Specs

[1] - Passenger side PTO provisions appear to be more common on pickup truck models while driver side PTO provisions are more common on chassis cab models. There may also be a correlation between PTO location and drive type (2 wheel vs 4 wheel drive). In any instance, the S6-650 only has an access cover on one side of the transmission case, while all S6-750 units have access covers on both sides of the transmission case.
[2] - OEM service intervals vary by vehicle manufacturer and model year; we recommend fluid replacement at 60,000 mile intervals (or less) for all vehicles
[3] - MERCON V automatic transmission fluid specified for early Ford applications; specification changed in 2008 to Motorcraft synthetic manual transmission fluid.
[4] - TranSynd manual transmission fluid specified in GM applications; the MSDS for GM 12378515 (ACDelco 10-4029) lists the product as Castrol TranSynd.

ZF 6 Speed Features

• Helical cut gears for reduced NVH
• Molybdenum coated shift fork pads
• Countershaft driven integral lube oil pump (Ford models only)
• Hydraulic master-slave clutch actuation. GM versions employ a concentric slave cylinder mounted over the transmission input shaft. Ford versions utilize an externally mounted linear type slave cylinder
• Stiff, spring loaded reverse and low lockout detent system
• Common SAE 6 bolt PTO accommodations

Shift Pattern & Identification

The forward gear progression on Ford versions of the S6-650 and S6-750 transmission are labeled L, 1, 2, 3, 4, OD (a "D" encapsulated in a circle). GM S6-650 transmissions are labeled more traditionally in gears 1 through 6. The shift pattern features reverse to the far left and up and low to the far left and down, both on the same linear trajectory. There is a relatively strong, spring loaded detent that makes it relatively difficult to access the reverse/low gate unintentionally. Sixth gear (overdrive) is located to the far right and up, with no selection below it. Depending on the driver's build and seat position, you may find that selecting reverse and low gears requires bringing the gear shift to one's right leg.

The transmission identification tag is located above the fill plug, nearly to the top of the transmission. A "ZF" logo is also cast into the embossment for the clutch slave cylinder (excluding GM versions) on the upper segment of the bellhousing, drivers side. GM versions of the transmission will not have an external slave cylinder and the hydraulic line enters directly into the bellhousing. S6-650 and S6-750 units are easily distinguishable by their respective power take-off provisions; S6-650 units have a single 6 bolt PTO cover while S6-750 gearboxes have 6 bolt PTO covers on both the driver and passenger side. All transmission cases are cast of aluminum and comprised of three pieces with the bellhousing being an integral part of the front housing. The intermediate or center housing is only approximately 2 inches thick and small in comparison to the bell and tail housings.

ZF S6-650 & S6-750 Transmission Gear Ratios

Gear Steps (Percent Step)

Transmission Swap Compatibility

There are several variations within the S6-650 and S6-750 transmission series, which includes models with different bellhousings based on their intended application. One of the most inquired transmission conversions, or swaps, is the replacement of the ZF 5 speed units in 1988 to 1997 model year Ford F-Series with a 6 speed unit. This conversion, and any swap in general, is entirely feasible but not without its hurdles. As a very basic guideline to this procedure, the following remarks outline the most burdensome aspect of the conversion:

• S6-650 transmissions must be sourced from a 1999 to 2003 Ford Super Duty also equipped with its transfer case; ZF 5 speed transmissions have a different output shaft, transfer cases will not adapt to the 6 speed. Transmissions from GMC/Chevy trucks or Ford 6.8L engines will not adapt to the 7.3 IDI/DIT engine.
• Custom length driveshafts will be required.
• The ZF 6 speed is physically larger and some sheet metal work may be required to physically fit the transmission in the tunnel.
• Custom crossmembers will need to be fabricated.
• Transmission cooler will need to be installed and routed to the transmission.
• For 1988 to 1991 vehicles, the ZF 6 speed has no provisions for the mechanically driven speedometer cable. An aftermarket gauge and sending unit will be required to have a functional speedometer.

The practicality of the ZF 5 to ZF 6 swap is questionable, as there is actually very little to gain. While the ZF 6 speed has a lower "creeper" gear and a slightly taller 0.72 (versus 0.77 or 0.76) overdrive ratio, the ZF 5 speed is not necessarily handicapped by its ratio spread or overdrive ratio. Assuming a 4.10 rear end ratio, 33 inch tall tires, and all else being equal, the difference between a 0.77 and 0.72 overdrive ratio translates to a cruising engine speed reduction of 114 rpm at 55 mph, 136 rpm at 65 mph, and 156 rpm at 75 mph; effects on fuel economy will be marginal at best. A more feasible and practical conversion for ZF 5 speed owners is to ditch the close ratio transmissions (4.14 or 5.08 1st gear) for the wide ratio version (5.72 1st gear).

Converting from an automatic to a ZF 6 speed manual transmission is an entirely simpler endeavor if the patient and donor vehicles are of the same generation. In these instances, a new PCM/ECU calibrated for a manual transmission is typically required for maximum compatibility. Transmissions mated to the Ford 6.8 liter engines will not mate to an IDI, Power Stroke, or Duramax engine. Transmissions found in Fords will not bolt-up to GM engines and visa versa. To the best our knowledge, both the 6.6L Duramax and 8.1L gas engines utilize the same version of the S6-650. It is also noteworthy that two wheel drive and four wheel drive versions of the transmission use alternative main and output shafts, so converting a 4x4 transmission to a 2x4 transmission is not as simple as replacing the rear housings.

Lubricant Choices, Requirements, & Specifications

ZF, General Motors, and Ford Motor Company each offer unique oil specifications for the ZF 6 speed transmission.. Ford specified MERCON V automatic transmission fluid through the 2007 model year. For 2008, the spec was switched to Ford XT-M5 manual transmission fluid, a full synthetic 75W-90 oil. This occurred when the 6.4L Power Stroke was introduced, which utilizes a single mass flywheel and thus the change could simply be the result of Ford attempting to mitigate excessive NVH without switching back to the less reliable dual mass flywheel. Unfortunately, there is not much information regarding the change in fluid spec for the S6-750.

GM specifies its TranSynd synthetic manual transmission fluid (GM 12378515, ACDelco 10-4029), which is a re-branded version of Castrol TranSynd 668 according to the product MSDS. Although GM's version is advertised as a "manual transmission fluid", Castrol calls TranSynd 668 a fully synthetic "powershift and automatic transmission fluid". At any rate, the properties of this lubricant is in within that of the realm of automatic transmission fluids.

ZF, the transmission manufacturer, identifies in lubricant document TE-ML 02 that it recommends its EcoFluid M 75W-80 manual transmission fluid. As of 2025, this product was not readily available from any local or online distributor with inventory in-stock in the United States, suggesting that this product does not have a strong domestic presence.

When the four lubricants are compared, the physical properties of MERCON V and TranSynd are quite similar (refer to table below). Ford's later specification, XT-M5-QS manual transmission fluid, stands out as a significantly more viscous fluid. The ZF EcoFluid lands somewhere between the ATF and manual transmission fluid categories. Note that both Ford options have the highest viscosity index, a favorable characteristic that means their viscosities are more stable over a wider temperature range.

It is important to note that all of these lubricant options are fully synthetic and that any aftermarket alternatives that may be used must also meet the full synthetic requirement. Conventional lubricants will not bear the same resilience to thermal breakdown and the intense heat generated within the transmission during operation will cause them to degrade more rapidly.

Popular aftermarket fluid alternatives for the ZF 6 speed transmission include Amsoil 75W-90 manual transmission lubricant, Amsoil Synchromesh Manual Transmission Fluid, and Redline 75W-80 Manual Transmission Lubricant; all of which are fully synthetic oils.

Redline MTL and Amsoil Synchromesh are very similar to ZF EcoFluid, but both have a higher viscosity index. Amsoil Manual Transmission Lubricant is a very close match to Ford XT-M5-QS, which are both more viscous oils than the ZF EcoFluid. In reality, any lubricant mentioned in this section is compatible with the ZF 6 speed manual transmission, regardless of application. Some personal experimentation with different transmission fluids may be the best way of determining the best solution for your individual needs.

After Ford introduced the XT-M5 fluid specification, which is much more viscous than ATF, we began experimenting with various manual transmission fluids. Amsoil Manual Transmission Lubricant and Ford XT-M5-QS have become our two favorite transmission oils for the ZF 6 speeds as they seem to provide excellent shift quality.

Gear Rollover Noise

Some noise from the transmission when it is in the neutral position or while driving in the lowest gear at idle is entirely normal and to be expected from applications that utilize a single mass flywheel. In dual mass flywheel applications, the transmissions are generally much quieter. While the dual mass flywheel is preferred for comfort, i.e. a quieter cabin for the driver and passengers, single mass flywheels are preferred for reliability because they are a much simpler design with less potential modes of failure.

All 7.3L Power Stroke and 6.8L Ford V10 applications equipped with a ZF 6 speed manual transmission employ a single mass flywheel. F-450 and F-550 trucks equipped with the 6.0L Power Stroke engine also relied on a single mass flywheel, but F-250 and F-350 models utilized a dual mass flywheel until the end of the 2006 model year. 6.4L Power Stroke equipped Ford Super Duty trucks all used a single mass flywheel. The Chevy Silverado and GMC Sierra employed the ZF 6 speed with a dual mass flywheel.

A dual mass flywheel dampens harmonic frequencies that would otherwise resonate through the internal workings of the transmission. These harmonics produce a rattling sound that can be described as a dry roller bearing that is being shaken around in one's hand. It is generally most notable in neutral and/or during low speed, low gear operation; sometimes simply increasing the engine RPM slightly makes the sound disappear.

Dissipating engine harmonics is accomplished by two decoupled masses connected by a network of springs; as the rotational energy is transfered from one mass to the other, the springs absorb a certain range of harmonic frequencies. The largest downfall of the dual mass flywheel is in its reliability. Whereas a single mass flywheel is quite literally a single piece of machined steel, a dual mass flywheel has at least two large components, several springs, and a central bearing that allows a limited range of rotational differentiation between the two masses. Because the systems are complex, they are more prone to failure.

It is often said that a dual mass flywheel prolongs transmission life because is reduces engine harmonics. With regard to these vehicles, however, we have yet to observe any such correlation and would argue that there are high mileage examples of both setups. That said, it may very well be that certain applications must employ a dual mass flywheel in order to eliminate damaging harmonics, but this does not seem to be the case for the ZF 6 speed. Dual mass flywheels are extremely common, but we believe this is because less noise results in fewer customer complaints and fallacious warranty claims stemming from perfectly normal audible resonance through the transmission geartrain.

Maximum Input Torque Rating Discrepancies

It appears as though, at least for most transmission models, the three to four number suffix designates the maximum input torque rating in N-m. This convention would dictate that the S6-650 and S6-750 had maximum input torque ratings of 650 and 750 N-m (480 and 553 lb-ft), respectively. However, ZF lists the input torque capacity of the S6-650 at 520 lb-ft (720 N-m) and the S6-750 at 560+ lb-ft (760 N-m). It is most likely that the figures provided in the transmission nomenclature are a simple generality for the approximate design limitation of the transmission, but that the transmissions are configurable from ZF with modified ratings. This scenario is corroborated by the fact that ZF specifies input torque ranges for many of its products in lieu of designating a single max input torque for what is otherwise an entire series of products.

ZF's maximum input torque for the S6-650 happens to correspond with the maximum rated torque output of all its Ford and GM applications - 520 lb-ft. Even the 6.6L Duramax LLY was limited to 520 lb-ft when equipped with the ZF manual transmission; automatic versions were offered at up to 605 lb-ft at its peak. The 560 "+" covers both the 6.0L and 6.4L Power Stroke engines; the 2003 and 2004 model year 6.0 liter was rated at a peak 560 lb-ft and the later 6.4L Power Stroke peaked at 650 lb-ft of torque

There is a significant and fundamental difference between the maximum input torque rating and the breaking point of a transmission, but these lines tend to get blurred in the realm of performance potential. Manufacturers must essentially certify their products to some degree, guaranteeing that they will operate problem free under certain conditions for at least some predetermined length of time. The input torque ratings they apply therefore take into account the maximum continuous stresses and loads that the transmission can handle without sacrificing longevity. The actual breaking point of the transmission should, in theory, prove to be significantly higher than the manufacturer's maximum rated input torque capacity.

Service Intervals

GM does not specify a maintenance interval for the ZF 6 speed manual transmission. In fact, the maintenance schedule in the Duramax diesel supplement claims "manual transmission fluid doesn't require change", although certain model year maintenance schedules call for the master-slave clutch hydraulic fluid to be replaced every 25,000 miles or 24 months. We recommend GM owners adhere to, at minimal, the Ford maintenance recommendations to maximize transmission life. Ford transmissions employ an external oil cooler to aid in heat dissipation while GM vehicles do not; transmission fluid in GM applications is therefore likely to breakdown faster.

Ford calls for the transmission fluid to be replaced every 60,000 miles, regardless of driving conditions, for all 7.3L and 6.0L Power Stroke applications. When equipped with a 6.4L Power Stroke engine, this interval is modified to 100,000 miles under normal driving conditions and 60,000 miles under severe service conditions. To maintain shift quality and maximize transmission life, we highly recommend all ZF 6 speed units are serviced at least every 60,000 miles.