The Lambretta gearbox is almost identical between models following the LD models. Most Series 1-3 gearboxes will fit in other model bikes making a huge range a varied gearbox ratios available.
Below is an overview of a typical Lambretta gearbox.
The Primary Drive
This is where the Lambretta gearbox is more flexible then the Vespa gearbox. Both the clutch bell and the crank sprocket can be changed independently of each other. A special chain will have to be used if you vary from the 47/18 GP200 set up or the 46/15 of all the other models. Companies like WCLW sell chains with extra links in them to accommodate almost any variation of front and rear sprockets. They also sell aftermarket front sprockets ranging from 13 to 22 teeth.
The Standard Gearbox
Each model came with a very specific group of four free gears and a single piece main cluster gear. Although these may be shared by different models they are always installed in pairs to be sure that each gear has a meshing counterpart. There are exceptions to this rule which we will add to the gearing calculator once we are sure the information we have is correct.
This is where the fun (or the headache) begins. All these items can be switched out with other model parts or aftermarket parts to achieve a huge range of gears. To make things easier my brother and I created a gear calculator which includes all the standard gearboxes and aftermarket crank sprockets. This information was found in the Lambretta Manual of Performance and Conversions by Ken Herlingshaw