What exactly is "final gear"?

A

Awaji Islander

39
Could somebody please explain to me what "final gear" is? I've slowly gained an understanding of most of the things you can tune in GT, but the final gear still confuses me. Is it related to the last, highest gear on a car? In a car with 5 gears, is 5th gear the final gear? That doesn't really make sense, but I can't think of what else it would be.
 
the final gear, also called "differential ratio" is a relationship between how fast the output shaft of the gearbox spins in relation to the driving wheels

imagine i have a 3.5 final drive in my car. when i accelerate, when the tailshaft turns once, the driving wheels turn 3.5 times. say i change that to a 4.3. now i accelerate, the tailshaft turns once (because the engine still revs the same and the gearbox is still the same), but the wheels turn 4.3 times, so i have a much higher acceleration

however, because the wheels are turning faster in relation to the output of the gearbox, i have limited my top speed. assume i could get 240km/h out of my 3.5 final gears, now i've probably limited that to 180km/h

if you make the final drive lower (3.5, 2.0!), the car will be slow to take off, but have a high top speed up to a point. if you make the gear too high, the engine won't be able to make enough power to fight the wind resistance at higher speeds, so for a high speed run you need to find a balance between the two

if you make the final drive higher (4.3, 5.0!) the car will accelerate quicker, but you will have to shift gear sooner, and the top speed will be limited. if the gear is too high, the engine will be putting too much power to the ground and just fry the tyres everywhere

as a rule, smaller cars tend to have higher ratios (such as 4.3) because their engines are less powerful and need the multiplication of torque gained through high gearing to be able to take off fast. cars with larger engines don't need this, and realistically don't want this, as needlessly maintaining higher rpm will use more fuel, so they use things like 3.08s

i've seen aftermarket 5.5 kits for some toyota offroad diffs, and i once read about a massive big block salt flats car that has a 1.80 final drive, that's phenomenal!

hope this helps
 
The final gear ratio refers to the gearing in the differential. In case you are confused about it, this is the way it's layed out. (in a rwd car anyway)

ENGINE/CLUTCH/TRANSMISSION/DRIVESHAFT/DIFFERENTIAL/DRIVEWHEELS

The Differential is what splits the power to both rear wheels, and the ratio of the input shaft of the diff to it's output shafts (determined by the diffs internal gearing) is refered to as the Final gear ratio. it's completely separate from the transmission and changing it actualy effects every gear. Making the final gear taller makes every gear taller, and making the final gear shorter makes every gear shorter. the final gear ratio is good if you want to keep the gears in the tranny spaced the same, but make the overall engine to wheel ratio shorter or taller. By the way. we're talking about reduction here, so if the final gear shows as 4.000 then thats a 4 to 1 underdrive ratio. which means the input has to spin four times to spin the output one time. thusly, the lower the number the taller the gear. (Taller meaning more top speed)

p.s. this is the short version. I can come back with the essay version if your still confused.
 
Super Jamie
imagine i have a 3.5 final drive in my car. when i accelerate, when the tailshaft turns once, the driving wheels turn 3.5 times. say i change that to a 4.3. now i accelerate, the tailshaft turns once (because the engine still revs the same and the gearbox is still the same), but the wheels turn 4.3 times, so i have a much higher acceleration
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Everything else you wrote is pretty much correct, but you have this a bit backwards. With a 3.5 final drive ratio the tailshaft turns 3.5 times to 1 revolution of the wheels, not vice versa. With a 4.3 FD, the tailshaft must turn 4.3 times to make the wheels go around once. This is why cars with short final drives (larger numbers) accelerate better but reach redline sooner and thus have lower top speeds.

And don't forget, of course, that the engine's speed is affected by the gear it is in, so that the engine may be turning more or less revs than the output shaft of the transmission. That's what "overdrive" means - the top gear(s) of the transmission have a ratio of greater than 1:1, so that the transmission actually turns over faster than the engine speed for comfortable cruising.
 
I put it on another post but i'll repeat myself.. think of the gears in a car to be the same as the gears on a race/mountain bike. The gears on the wheel would be the ones in your transmission, and the gears on the pedals would be the final gear..

The higher the final gear is the more "teeth" there are on the gear so the gear is "bigger" and makes it easier to accelerate but the top speed is effected
 

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