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can some1 please give me a good explanation on why gear ratios are important and how they relate to speed:burnout:
 

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It's more of how they match to the characteristics and abilities of the engine that is of importance.

I'm not sure of the specifics, but I think that using larger gears with smaller changes in size ("shorter" ratios?) allows the engine to stay in it's power band and thus make more power. That's why close-ratio gearboxes are good for performance (but slightly worse for NVH and fuel efficiency at cruise speeds).
 

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Alright, here's my crack at explaining this.

The gear ratios determine the ratio between engine speed and road speed. So, say your engine speed is 4000 rpm at 20 mph in first. if you double the speed of the engine, you'll double the speed of the car (ie, 40 mph @ 8000 rpm)

Each gear results in a different engine to road speed ratio. When you shift up, the engine will rev less in the new gear than in the old, which we all know, and the new engine speed will depend on the gear ratio and the current speed of the vehicle. This has a huge effect on performance, because if changing to the next gear drops you out of the powerband (as on the GTS), you will not get the best acceleration. If you shift at 8300 rpm, and you're doing 40 mph, but second gear at 40 mph means the engine turns at 5500 rpm, you have to wait until it reaches 6000 rpm until you hit lift again. During that entire time, the engine is not in its powerband, and acceleration is sluggish. Now, if you changed the gear ratio so that 40 mph in second gear was 6000 rpm or more, you would be in the powerband, and you'd accelerate more quickly.

Disclaimer: The numbers above are close to those of the GTS, but not identical.
 

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i have a feeling we'll need Chui to explain this to us. I myself don't know the exact details of it other than the shorter the gear, the higher the number, and lower top speed.

with smaller engines, you want a shorter gear to help the engine reach its powerband. with larger engines you don't need as much gear since the powerband is more broad. In my V6 the move from the stock 3.27 gear to the 3.73 was worth a nice tenth or two at the drap strip because it helped me reach my powerband quicker.
 

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Discussion Starter · #6 ·
marcus_GTS said:
Alright, here's my crack at explaining this.

The gear ratios determine the ratio between engine speed and road speed. So, say your engine speed is 4000 rpm at 20 mph in first. if you double the speed of the engine, you'll double the speed of the car (ie, 40 mph @ 8000 rpm)

Each gear results in a different engine to road speed ratio. When you shift up, the engine will rev less in the new gear than in the old, which we all know, and the new engine speed will depend on the gear ratio and the current speed of the vehicle. This has a huge effect on performance, because if changing to the next gear drops you out of the powerband (as on the GTS), you will not get the best acceleration. If you shift at 8300 rpm, and you're doing 40 mph, but second gear at 40 mph means the engine turns at 5500 rpm, you have to wait until it reaches 6000 rpm until you hit lift again. During that entire time, the engine is not in its powerband, and acceleration is sluggish. Now, if you changed the gear ratio so that 40 mph in second gear was 6000 rpm or more, you would be in the powerband, and you'd accelerate more quickly.

Disclaimer: The numbers above are close to those of the GTS, but not identical.
thanx for giving it a crack for me but i want to know about the gear ratios and speed in general. not only about the GTS.
 

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Discussion Starter · #7 ·
SlasherX said:
In my V6 the move from the stock 3.27 gear to the 3.73 was worth a nice tenth or two at the drap strip because it helped me reach my powerband quicker.
see this is what i'm talking about. I have no idea what each gear ratio mean or even the final gear ration:ugh:

EDIT: so if the final gear ratio is big, doesn't that mean it's more of a larger drop in rpms when shifting??
 

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http://www.autoshop-online.com/auto101/drive.html

That is a basic overview of Driveline.

Now, gear ratio:

A final drive ratio of 1:1 means that every time the engine rotates one full rotation, the wheels rotate once. All of the other numbers link into this basic concept of X:Y where X is the number of times the input rotates and Y is the number of times the output rotates.

eg a 2:1 ratio is 2 engine rotations for 1 tire rotation. Comparing the 1:1 to the 2:1, the 1:1 will have twice the top speed and 1/2 of the torque. The 2:1 will be high torque at the expense of top end.

in general, a higher number ratio will have more torque but less top speed. This is sometimes refered to as Long (fast) or Short (torquey) gears.

also in general, more torque will get you off the line and to your top speed much faster (more acceleration) but your top speed will be less.

I hope that helps some.
 

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Final gear ratio has no affect on the change in RPMs when shifting. Only the ratios between gears come into play in that scenario.

Celica GTS 6-speed: 3.166, 2.050, 1.481, 1.166, 0.916, 0.725, Final 4.529

Ignoring clutch slip for this entire discussion to make things easier. Someone should probably check my math on these as well since it's near time to go home on Friday and I may not be punching the calculator correctly. Numbers are not exact and reflect an imaginary world where tires are actually exactly the size they say they are.

Taking a look at the Celica 6 speed as an example only. When you are in first gear the ratio is 3.166:1 and final drive ratio is 4.529:1 for a net drive ratio of ~14.339:1. If your engine is doing 1000RPM then in first gear your tires are doing about 69.7RPM or just about exactly 5MPH. If your engine is doing 1000RPM in second gear your drive ratio is ~9.284:1 and your wheels are doing about 107.7RPM or about 7.7MPH. At the other end of the rev band at 8000 RPM in first you are doing 557.9RPM at the wheel or almost exactly 40MPH. You can continue doing the math yourself if you want, a 205/50-16 tire does 838 revolutions per mile.

So in the RPM drop side of the equations we can do this purely by using the gear ratios and ignore the final drive ratio. A shift from 1st to 2nd at 8000RPM will end up at (8000*2nd gear ratio)/1st gear ratio or about 5180RPM. In this case the drop out of the power band happens no matter what the final drive ratio is.

So...moving taking back into consideration the example of the GTS where the real power doesn't exist until 6000RPM. With the gearing the way it is the car is already moving almost 30MPH before you reach 6000 in 1st gear. If we were to utilize a shorter final drive (one that is higher numerically) like a 6.0:1 (picked at random to make math easy) we would see that you reach 6000 RPM at about 22.6MPH. In this case the car SHOULD reach it's power band sooner (time wise) than if it had the stock final drive but the trade offs include having to shift to 3rd to reach 60 and the fact that when cruising at 80MPH in 6th your engine is running at 4860RPM instead of the stock 3660. The car will also reach it's redline sooner (MPH) in every gear and have a lower top speed. Net result generally, if the final drive ratio is picked correctly, is a car that has faster acceleration but cruises at higher RPMs.

Real world example: A BMW 325i with the stock 3.15:1 ratio is almost half a second slower than the same BMW 325i with a 3.73:1 ratio to 45MPH. By 60 with your average driver things are probably going to even out a bit since the stock car will be in second and the modified car will have had to shift to 3rd.

You can create the same effect of having a shorter final drive by using a smaller diameter tire. So for instance I use a 205/50-15 tire for autocross which does 874 revolutions per mile. This is the rough equivalent of changing my final drive to 4.724:1.

Not sure if that made things clear as mud or not...www.howstuffworks.com has a pretty good discussion on how manual transmissions work that might help clear it up.
 

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TRD StreetRacing said:


thanx for giving it a crack for me but i want to know about the gear ratios and speed in general. not only about the GTS.
Well, the info applies to any car. I was using the GTS as an example only.

Again, the basic idea is that the gear ratio determines how fast the car goes at a certain rpm. If the gear ratios are made so that the speed at redline in any gear will allow the car to stay in the powerband when the next gear is selected, then you accelerate faster because you stay in the powerband of the engine.
 

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Discussion Starter · #11 ·
Phil C said:
Final gear ratio has no affect on the change in RPMs when shifting. Only the ratios between gears come into play in that scenario.

Celica GTS 6-speed: 3.166, 2.050, 1.481, 1.166, 0.916, 0.725, Final 4.529

Ignoring clutch slip for this entire discussion to make things easier. Someone should probably check my math on these as well since it's near time to go home on Friday and I may not be punching the calculator correctly. Numbers are not exact and reflect an imaginary world where tires are actually exactly the size they say they are.

Taking a look at the Celica 6 speed as an example only. When you are in first gear the ratio is 3.166:1 and final drive ratio is 4.529:1 for a net drive ratio of ~14.339:1. If your engine is doing 1000RPM then in first gear your tires are doing about 69.7RPM or just about exactly 5MPH. If your engine is doing 1000RPM in second gear your drive ratio is ~9.284:1 and your wheels are doing about 107.7RPM or about 7.7MPH. At the other end of the rev band at 8000 RPM in first you are doing 557.9RPM at the wheel or almost exactly 40MPH. You can continue doing the math yourself if you want, a 205/50-16 tire does 838 revolutions per mile.

So in the RPM drop side of the equations we can do this purely by using the gear ratios and ignore the final drive ratio. A shift from 1st to 2nd at 8000RPM will end up at (8000*2nd gear ratio)/1st gear ratio or about 5180RPM. In this case the drop out of the power band happens no matter what the final drive ratio is.

So...moving taking back into consideration the example of the GTS where the real power doesn't exist until 6000RPM. With the gearing the way it is the car is already moving almost 30MPH before you reach 6000 in 1st gear. If we were to utilize a shorter final drive (one that is higher numerically) like a 6.0:1 (picked at random to make math easy) we would see that you reach 6000 RPM at about 22.6MPH. In this case the car SHOULD reach it's power band sooner (time wise) than if it had the stock final drive but the trade offs include having to shift to 3rd to reach 60 and the fact that when cruising at 80MPH in 6th your engine is running at 4860RPM instead of the stock 3660. The car will also reach it's redline sooner (MPH) in every gear and have a lower top speed. Net result generally, if the final drive ratio is picked correctly, is a car that has faster acceleration but cruises at higher RPMs.

Real world example: A BMW 325i with the stock 3.15:1 ratio is almost half a second slower than the same BMW 325i with a 3.73:1 ratio to 45MPH. By 60 with your average driver things are probably going to even out a bit since the stock car will be in second and the modified car will have had to shift to 3rd.

You can create the same effect of having a shorter final drive by using a smaller diameter tire. So for instance I use a 205/50-15 tire for autocross which does 874 revolutions per mile. This is the rough equivalent of changing my final drive to 4.724:1.

Not sure if that made things clear as mud or not...www.howstuffworks.com has a pretty good discussion on how manual transmissions work that might help clear it up.
thanx for the insite.....but i'm going to read that another 4-5 times till i FULLY get what your saying:gap:
 

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Take a look at this thread on miataforum.com
http://www.miataforum.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=6;t=012930
and pay attention to formerdatsun510man, he knows his ****. The thread is about whether shifting at redline is the best for acceleration, and some posts go into why or why not it may best to shift at redline. I think fd510man's explaination about hp at specific rpms explains why gearing is important, and what the optimal gearing should be.
 
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