Originally Posted by tq23
Need help on gearing ratios for on-road.Especially the effects of you changing the pinions or spurs etc.Really appreciate any inputs.Many thanks in advance.
The primary tuning option relating to your car’s transmission is the ability to change gear ratio by using different spur gears or pinions. Gear ratios are most often quoted in the form “2.4 to 1”. This can be represented in writing as 2.4:1. This means that the motor must rotate 2.4 times for the car’s driven wheels to complete one full revolution.
Most instruction manuals should tell you the internal ratio of your car.
The formula for calculating gear ratios looks like this:
(# Teeth on spur divided by # teeth on pinion) multiplied by Internal Ratio = Gear Ratio
We’ve already seen how gear ratios can be represented as numbers (e.g. 7.8:1). The tricky part is in describing ratio changes in general. If you put a bigger pinion on the car, the ratio will change to a small number (say 7.4:1). Whilst the numerical figure has become smaller, the actual gear ratio 7.4:1 is said to be a ‘higher’ ratio than ‘7.8:1’. Similarly, moving to a smaller pinion will produce a ‘lower’ ratio (say 8.2:1). Changing the spur gear has the opposite effect. A smaller spur gear will result in a ‘higher’ gear ratio, and a larger spur gear will give you a ‘lower’ gear ratio.
Confused? Stay with me.
Gear ratio changes do a couple of things. Let’s look at both the ‘lower’ and ‘higher’ gear ratios separately to see what we find.
A lower gear ratio will mostly give you more run time and more acceleration. It’s also generally easier on your motor.
A higher gear ratio will generally give you more top speed, and less run time. It’s also tougher on your motor.
Once you get to a certain ratio point (lets call it the ‘optimum ratio’) continuing tonchange to a higher ratio will do nothing but damage. It will result in your motor overheating and being damaged, and in extreme cases, your car may actually go slower.
Look at this simple chart, which might help make things clearer.
Pinion Spur Gear Ratio Gearing Top Speed Acceleration Run Time
Bigger smaller higher up more less less
Smaller bigger lower down less more more
For help on choosing your actual gear ratio for any given motor or track, consult your car's instruction manual or check with the local fast guys.
Hope that helps!