The gear ratios between the ZF S6-650 and S6-750 are identical. While the transmission is a true 6 speed unit, its forward gears are labeled L-1-2-3-4-5, distinguishing its relatively low true 1st gear. The input shafts are different on the 650 and 750 models, thus they are not interchangeable. Both transmissions utilize an integrated oil pump and external transmission cooler, a unique feature for a manual transmission that promotes adequate bearing lubrication and protection. The fully synchronized transmissions are quiet, offer smooth shifting with positive engagements, and have not been the subject of re-occurring problems.
ZF S6-650 & S6-750 Ratios
| 1st | 2nd | 3rd | 4th | 5th | 6th | R |
| 5.79 : 1 | 3.30 : 1 | 2.10 : 1 | 1.31 : 1 | 1.00 : 1 | 0.72 : 1 | 5.23 : 1 |
ZF S6-650/S6-750 Manual Transmission Specs
| Transmission | ZF S6-650 & S6-750 | |
| Type | 6 speed manual transmission | |
| Manufacturer | ZF Friedrichshafen AG | |
| Predecessor | ZF S5-47 | |
| Model Years/Applications | ZF S6-650 | 1999 - 2003 Ford Super Duty, 7.3L Power Stroke 2001 - 2006 GMC Sierra/Chevrolet Silverado, 6.6L Duramax |
| ZF S6-750 | 2003 - 2010 Ford Super Duty, 6.0L & 6.4L Power Stroke | |
| Synchronized | Synchronized in all forward gears & reverse | |
| Case Material | Aluminum | |
| Max Input Torque | ZF S6-650 | 530 lb-ft |
| ZF S6-750 | > 560 lb-ft | |
| Max GCWR | ZF S6-650 | 26,000 lbs |
| ZF S6-750 | 30,000 lbs | |
| Weight | ~ 235 lbs | |
| Oil Capacity | ZF S6-650 | 5.5 liters, 5.81 quarts |
| ZF S6-750 | 6 liters, 6.34 quarts | |
| Fluid Type/Spec | Ford calls for MERCON V ATF GM calls for TranSynd synthetic transmission fluid Recommend Amsoil Signature Series full synthetic ATF |
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| Service Intervals | Replace transmission fluid every 30,000 miles of normal operation or 15,000 miles under "Severe Duty" conditions | |
| Features | • Integrated oil lube pump circulates fluid through transmission cooler and lubricates bearings, a feature not typical on manual transmissions • ISO/SAE 6 bolt PTO provisions (S6-650 has single PTO, S6-750 has dual PTO) • 2nd and 3rd gear feature dual cone synchronizers |
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| Shift Pattern |  |
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