Posted by on 03 October 2012 12:29 AM
A Gear Ratio is a numerical value that shows the relationship between two gears (a spur and pinion gear). A gear ratio can also refer to the relationship between the FIRST gear and the LAST gear in a gear train, or 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, these numbers are expressed as such: A 72 Tooth Spur divided by a 20 Tooth Pinion = 3.60 to 1 or 3.60:1
What this number refers to is how much reduction happens as a result of that gear set. Simply take the RPM of 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. If you want to go faster, then you need more RPM's going into the Tranny, 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. If you want more acceleration, then you need fewer RPM's going into the Tranny; 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.
To go Faster/Max Speed - Lower numbered gear ratio.
To Accelerate Quicker - Raise the gear ratio.
Key points to remember:
A Lower numbered Ratio is actually a "Higher Ratio" and = Higher Top End, Slower Acceleration.
A Higher numbered Ratio is actually a "Lower Ratio" and = Quicker Acceleration, Lower Top end.
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.