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FAQ - Gears
#1
Gears

The driveline is made of multiple gears changing the rotation ratio of the different parts; In a typical RWD car, the engine goes to the transmission, where a first ratio is selected then through the driveshaft and then a set of gears change the ratio again before going to the wheels.

The gears are expressed under the form xxx:1, for example 2.73:1, 4.11:1, etc

Gears that are lower numerically are called 'Taller' gears, the higher numerically are called 'Shorter' gears.

What it means is that for xxx turns on the input side, it results in one turn on the output side;for example, let's take an engine turning at 2500 rpm, going through 3rd gear on a T-56 transmission (ratio 1.35 for the late models) on a car equipped with 4.11 gears:

it means that the output of the transmission will turn at 2500/1.35 = 1851 rpm and the tires will turn at 450.5 rpm; now the same car with 2.73 gears would have tires rotating at 678.3 rpm so, what does that mean ? A taller gear (lower number) allows to go faster (higher tire rpms) than a shorter (higher numerically) gear. As the engine has a limited rpm range, it is obvious that tall gears allow a better top speed, that short gears.
On the other hand, you need to understand torque multiplication; I'm going to skip the math and provide a very simple explanation: the engine produce a certain amount of energy per rotation, if you have a tall gear set, the engine turns less for one output turn, so there is less energy put through. For example with 2.73 gears, you have 2.73 engine turns worth of energy, for one complete rotation of the wheels, while for 4.11, you have 4.11 turns worth of energy for one complete rotation of the wheels.

So, the output torque varies greatly, shorter gears deliver more power to the ground.

in essence, it becomes:

taller gears = better top speed
shorter gears = better acceleration

the gearing varies extensively depending on the platform. A car with lots of torque at low rpm, like a V8 will more likely come from the factory with tall gears; a smaller displacement engine that makes its power in the mid-high rpm range will need shorter gears. Trucks will have short gears because they need to carry weight, and moving from a stop may require lots of energy.

Now, the gear selection must be done carefully for multiple reasons:

if you want top speed, the tallest gears you can find may make your car accelerate very slowly and when you go fast enough that the air resistance really matters, there may not be enough torque multiplication to get the car to go and faster

if you want to accelerate quick, short gears are a quick way to do it, but that can translate into traction problems from too much torque and you may not have enough gears on the transmission as you'll shift gears much faster; Also, while crusing the engine will have to run at a higher rpm. Many people over geared their cars and have to shift in overdrive before the end of the 1/4

Another problem to consider is the engine's powerband: short gears make you go through the rpm range quicker, but if you have a narrow powerband it can be a problem because you'll spend more time out of the powerband, while taller gears may make you stay longer in there. A gear set must match your head/cam/tire size combo to be efficient.

Shorter gears are weaker than taller gears because they have more teeths for the same volume, this is very important for drag racing where gears go through lots of strain. When racing you want to remove all the slack from the driveline by moving the car with the brakes on so that when you start parts don't hit each others; it's usually what breaks gears.

Most cars pick the vehicule speed at the transmission; if you change the gear ratio, for a given rpm at the output of the transmission, the speed will be different and your speedometer will not be calibrated anymore; you may need to reprogram your computer; this is especially important for electronically controlled automatic transmissions.

Gears have to be perfectly aligned to mesh properly, I don't recommend doing this at home, but rather go through a shop to do it. If a gear set is whinning, it's usually either a bearing, or the angle / shimming on the gears that is bad; If it is the case (and not the bearing), don't reuse them, but get a new set.

On f-bodies, the ideal gears are:
Manual transmission: 4.11 if power is close to stock, 3.73 if more power (nos, supercharger or head/cam)
Automatic transmission: 3.42


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