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All you need to know about "Low ratio Gearboxes"

Low ratio Gearboxes Guide

Many drivers overlook the importance of the gear ratio in their tuning project. 

The manufacturers spend a great deal of time matching each individual gear ratio and final drive ratio to the power bands of the engine.  Car tuners should do likewise.

 Years ago manufacturers used to provide an overdrive gear which effectively changed the final drive ratio creating an additional set of gears.  This was primarily intended to improve the economy of the car.

Acceleration times can be greatly improved when the gear ratios are closer together.   Therefore six speed gearboxes are becoming popular options on high performance models. 

When I rebuilt my gear box which had broken for the second time I was determined to get it right.  Strength is important and you really cannot afford to skimp using cheaper components.

I stuck to a five speed gearbox and used a closer ratio first and second gear, which gave me faster acceleration.  Third gear was only very slightly lower than normal, fourth gear was unchanged and fifth gear was much higher. 

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I chose this configuration because I rarely use the fifth gear when on the track, but cruising on a motorway I really wanted the economy.  So this represented the best choice for me and was perfect for my requirements. If the car was exclusively for competition I would probably have chosen a gearbox with lower ratios througout and used the full power band in all gears.

A gear box can be built from scratch using custom cog sizes and configurations, but this can be very expensive and time consuming.  It also requires specialist knowledge of gearing and engine power bands. 

The second option is to strip down your gear box and replace the main gear Cogs with those from another car which match your performance requirements. For example if you have a 2.0 engine and there is a 1.6 hot hatch version, you can cannabilise this box for suitable shorter gears.

Dog tooth gears have better performance as power is not lost in the twist as gears mesh but this can make gear changes a little more crunchy around town.

The simplest way of getting a low ratio gearbox is merely fitting the gear box from a smaller engined version of your car.  Smaller engines have a narrow power bands, and therefore the gear ratios are closer together. 

Do not expect this option to be reliable as you will be putting much more power through the gear box than it was originally designed for, but it can be great fun and will show the benefits of a lower ration gearbox.

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