There are basically 2 different Spitfire gearboxes. The 3 rail box and the single rail box. The ultimate development of the 3 rail box is the GT6 version and the ultimate development of the single rail box is the TR7 version.
The following is a table of the various ratios:
| Spitfire MKI-MKIII (3 Rail) | Spitfire IV (3 Rail) | GT6 (3 Rail) | Spitfire 1500 (Single Rail) | TR7 (Single Rail) |
|---|---|---|---|---|
| 3.75 to 1 | 3.50 to 1 | 2.65 to 1 | 3.50 to 1 | 2.65 to 1 |
| 2.16 to 1 | 2.16 to 1 | 1.78 to 1 | 2.16 to 1 | 1.78 to 1 |
| 1.40 to 1 | 1.30 to 1 | 1.25 to 1 | 1.39 to 1 | 1.25 to 1 |
| 1.00 to 1 | 1.00 to 1 | 1.00 to 1 | 1.00 to 1 | 1.00 to 1 |
To fit the GT6 or TR7 versions to your Spitty it is a matter of bolting on the appropriate bell housing and having the input shaft appropriately modified, which is not difficult as there are places around that specialise in just such modifications and the same with the tailshaft.
I could go on and on about all the other versions of these gearboxes and there interchangeability, but it is bordering on useless information.
The important thing is that if you have a purpose built Spitfire race car there’s a great advantage in using a GT6 or TR7 box with the TR7 perhaps being the stronger.