A Ford 9-inch “diff head” or third member with a 3.89 gear ratio provides good off-the-line acceleration for street and track use.
This ratio can lead to higher engine RPMs at cruising speeds compared to numerically lower ratios.
The 9-inch is a popular axle due to its strength and ease of service, though high-horsepower applications benefit from aftermarket nodular cases and stronger axles.Â
Complete 9-inch third members with 3.89 ratio
These assemblies include the differential, ring and pinion gears, case, and yoke, and are built and set up by experienced technicians.