This is a '90 five-speed with a 3.35 First gear rebuilt with a stronger 2.95 gearset.
To determine any car's overall First-gear ratio, multiply the transmission's First-gear ratio by its rear-gear ratio. If your Mustang has a 3.35 First gear and a 3.55 rear gear, the overall First-gear ratio is 3.35 x 3.55 = 11.89. Anything greater than 9.5 could be considered overkill for the street. Ever driven a Mustang with a 3.35 First-gear trans and a 3.73 rear? First gear is virtually useless, since the overall ratio is 12.49. This is why a 2.95 First-gear T5 or the Tremec 600 2.87 trans is far more desirable if your plans include deep rear gears.
To determine the Overdrive ratio, simply multiply the rear-gear ratio by the Overdrive. For a 3.73 rear gear and a 0.72 Overdrive, that's the equivalent of a 2.68 rear gear in Overdrive. If you're considering the Tremec 600 trans, a 3.73 with a 0.64 Fifth gear equals 2.39. This is a great way to lower the highway cruise rpm. Just remember, this combination will create high driveshaft speeds if you attempt to do a top-speed run; the driveshaft and the rear gears are spinning 36 percent faster than the engine!
The strength of a manual transmission is related to four main design functions consisting of cluster-to-mainshaft center-to-center distance, First-gear ratio, gear-tooth pitch, and the alloy of the steel used to make the gears. The center-to-center distance is the dimension between the centerlines of the input shaft and cluster gear. A greater distance between these two gears increases gear diameter and strength. Note on the chart that the T5 has the shortest center distance while the much stronger T56 six-speed is more than 10 percent larger.
First-gear ratio is determined by dividing the tooth count of the driven (cluster) gear by the tooth count of the drive (input) gear. As First gear becomes deeper (higher numerically, as in 3.35 versus 2.95), the input-shaft gear-tooth count becomes lower, making the gear smaller, while the cluster gear-tooth count and size become larger. In the case of our 3.35-geared T5, several input-shaft teeth broke, which destroyed both the input and the cluster.
Tooth pitch is also a consideration. A straight cut or spur gear is the strongest but is also extremely noisy. As the pitch or angle of the gear tooth changes, it becomes quieter, but at the price of reduced strength. Another way to increase strength is to improve the quality of the steel used to make the gears. Since changing the center distance would require designing a new transmission, the T5's strength updates are limited to ratios and metallurgy improvements. Factory gears have evolved from 8620 to 4620 and 4615 alloys to improve strength. If you want serious strength, G-Force makes a helical-cut 2.94 low gearset for the T5 out of 9310 nickel alloy with a 26-spline input shaft, but it's pricey.
The bottom line on production-based T5s is that as long as you don't ham-fist the box with repeated power shifts, it will probably live a long and useful life. Abuse it, and count on becoming proficient at R&R-ing transmissions.
The weak, 3.35 First-gear T5 we started with suffered an input-shaft-gear failure. While d
A wider center distance between input-shaft (A) and cluster-shaft (B) centerlines improves