Originally Posted by 1983F1503004x4

Granny vs any other first gear:

A granny gear is usually a really low ratio first gear. This gear can be synchronized (helical-cut) or un-synchronized (spur-cut). A synchronized gear you can shift into while moving. An un-synchronized gear, you have to be at a dead stop to shift into.

4th gear can be direct drive (1:1 ratio) or overdrive (less than 1:1 ratio, or anywhere around 0.60 - 0.80:1). To understand direct drive and overdrive easier, understand final drive ratio.

Final drive ratio is the total gearing in the drive line of the vehicle, dependent on what gears your in.

For example, NP435 1st gear is 6.69:1, my transfer case is 2.6:1 low range, and a rear end ratio in my truck of 4.10:1.

To calculate the final drive ratio, you multiply.

6.69 x 2.6 x 4.10 = 71.31:1 crawling gear.

If I were doing normal driving:

Second FDR: 3.34 x 1:1 (transfer case high range) x 4.10 = 13.69:1

Third gear FDR: 1.76 x 1:1 x 4.10 = 7.26:1

Fourth gear FDR: 1:1 x 1:1 x 4.10 = 4.10

If it was overdrive: 0.76:1 x 1:1 x 4.10 = 3.11:1

Overdrive gears, being less than a 1:1 ratio, simulate having a higher set of rear end gears. As you can see, overdrive with 4.10 gears is the same as having 3.11 gears in the rear end with no overdrive (or, a 1:1 4th gear).

Your last gear is your highest ratio forward gear that you can drive in. For example, in a 4 speed with no overdrive, it's 4th gear. In a 4 speed WITH overdrive, it's still 4th gear. With a 5 speed it's 5, with a 6, it's 6, and so on.

Gears themselves are torque multipliers. That's their purpose.

So, say you have a motor making 250 lbs-ft of torque at a given RPM. In my last gear, FDR of my truck being 4.10, you would multiply 4.10 x 250 lbs-ft to get around 1000 lbs of torque to the ground at the given RPM.

If I were in 4 wheel low and 1st gear, 250 lbs-ft of torque, here is what you'd have:

Torque x Transmission Gear x Transfer-Case Gear x Rear-End Gear Ratio = Torque to the ground at peak torque RPM.

So, let's say peak torque is 2000 rpm, at 250 lbs-ft.

250 x 6.69:1 x 2.6:1 x 4.10:1 = 17,828 lbs-foot of torque to the ground at 2000 rpm.

Now you can see why some trucks are known as stump-pullers. It's because they can!

Originally Posted by Gary Lewis

I agree with all you said, with one minor exception: "An un-synchronized gear, you have to be at a dead stop to shift into." Some of us know how to double-clutch and can get into non-synchro'd gears w/o stopping. Otherwise, spot-on.