Gear Ratio 101: Learn the basics of gear ratio
Definition: gear ratio n.
The ratio of the speed of rotation of the powered gear of a gear train to that of the final or driven gear.
Gear Ratio's can be difficult to understand, but with a fairly brief explanation I will attempt to make this often misunderstood equation VERY easy to grasp. Simply stated, a gear ratio is a numerical value that describes the relationship between two gears (a spur and pinion gear for our discussion). A gear ratio can also describe the relationship between the FIRST gear and the LAST gear in a gear train (or a transmission). You take the number of teeth on the DRIVEN (Spur) gear and divide it by the number of teeth on the DRIVE (Pinion) gear.........the numbers are expressed as such: A 72 Tooth Spur divided by a 20 Tooth Pinion = 3.60 to 1 or 3.60:1
Here is the Big Secret. What this number indicates is how much reduction happens as a result of that gear set! Simply take the RPM's at the DRIVE (pinion) gear and divide them by the Gear Ratio. So for an Engine turning a 20 tooth pinion at 30,000RPM with a 72 tooth Spur Gear (3.60:1 ratio) makes the Spur shaft (Tranny Input) turn at 8,333RPM, simple right! Want to go FASTER (Higher Top End)? Then you need more RPM's going into the Tranny right?
So you would look for a gear ratio with a lower number. 30,000 divided by a 2.9:1 ratio gets you 10,345 RPM's into the Tranny. Want to drop the hammer faster (maybe Wheelie)? Then you need FEWER RPM's going into the Tranny, right? (Because you're packing MORE engine RPM's into every RPM of Tranny input).
So you would look for a gear ratio with a higher number. 30,000 divided by a 5.1:1 ratio gets you 5,882 RPM's into the Tranny.
The Tranny Also has a Gear Ratio associated with it, take the Traxxas TMaxx tranny for example:
First Gear is: 6.69:1
Second Gear is: 4.56:1
Paying attention? So how would you figure out how fast the wheels will be turning at 30,000 RPM? 30,000RPM divided by the Spur/Pinion Ratio divided by the Tranny Ratio 30,000/3.6/6.69 = 1,246 WHEEL/RPM's. The trade off is this, wheelies, ripping holeshots, super quick acceleration kills your top speed. High Top Speed robs from the bottom end and will leave you with weaker acceleration.
In a Nutshell:
To go FASTER  You want a lower numbered gear ratio.
Your wheels will make more rotations for every engine rotation. You will sacrifice low end "punch" (acceleration) and you won't jump off the line as fast.
To Jump off the line quicker (wheelie)  Raise the gear ratio.
Your wheels will maker fewer rotations for every engine rotation. If you want to scream off the line then rasie the Gear Ratio, but you will sacrifice your top end speed.
Spur Gear Size 
Clutch Bell (Pinion Gear) Size  

14  15  16  17  18  19  20  21  22  23  24  


62  4.4  4.1  3.9  3.6  3.4  3.3  3.1  3.0  2.8  2.7  2.6 
64  4.6  4.3  4.0  3.8  3.6  3.4  3.2  3.0  2.9  2.8  2.7 
66  4.7  4.4  4.1  3.9  3.7  3.5  3.3  3.1  3.0  2.9  2.8 
68  4.9  4.5  4.3  4.0  3.8  3.6  3.4  3.2  3.1  3.0  2.8 
70  5.0  4.7  4.4  4.1  3.9  3.7  3.5  3.3  3.2  3.0  2.9 
72  5.1  4.8  4.5  4.2  4.0  3.8  3.6  3.4  3.3  3.1  3.0 
74  5.3  4.9  4.6  4.4  4.1  3.9  3.7  3.5  3.4  3.2  3.1 
76  5.4  5.1  4.8  4.5  4.2  4.0  3.8  3.6  3.5  3.3  3.2 
78  5.6  5.2  4.9  4.6  4.3  4.1  3.9  3.7  3.5  3.4  3.3 
Key points to remember:
A Lower numbered Ratio is actually a "Higher Ratio" and = Higher Top End, Slower Acceleration.
 Why? Because the wheels turn at a higher rate the lower the ratio.
 Higher Top Speed  Less chance of wheelies.
A Higher numbered Ratio is actually a "Lower Ratio" and = Quicker Acceleration, Lower Top end.
 Why? Because the wheels turn at a lower rate the higher the ratio.
 Lower Top Speed  More chance of wheelies, better low speed power.
Notice that if you make an uneducated purchase and decide to go with a 17 tooth clutch bell and a 62 tooth spur gear you end up with the same gear ratio (3.6 and 3.6 is stock). So your truck's performance is not going to change at all. It pays to do a little homework. Also notice also that changing the Spur has much less effect than changing the Clutch Bell.
Parts Availability:
Racer's Edge makes clutch bells that from 14 to 22 teeth that can be purchased at Stormer Hobbies.
Robinson Racing makes them in 19, 20, 21 and 23 teeth and Robins Racing Products makes some trick spurs all the way up to 76 teeth which can also be picked up at Stormer Hobbies.
Kimbrough makes Spur gears that run from 64 to 72 teeth that are available at Tower Hobbies.