In my experience, I have only come across three prefixes. W, CM & C.
W was used on both “Vintage Wide” and “Modern Wide” boxes. The only difference being the 2nd gear ratios. Vintage Wide being 8.13 and Modern Wide being 6.74. Bottom and top gears were the same in both boxes at 13.32 and 4.63.
CM was sometimes called “Medium Close” and sometimes called “Close”. The ratios being – 1st 9.84, 2nd 6.15 and top 4.63.
C was sometimes referred to as “Vintage Close” and sometimes “Ultra Close”. The ratios being 1st 8.14, 2nd 6.12 and top 4.63.
I have the C box on my prototype Clubmans and it involve a lot of clutch slipping when in slow moving traffic or when pulling away on hills. More suited to Holland or the fens!
I have a CM box on my Brum Scott and life in traffic and hilly Surrey (I live on the North Downs) is much easier on the clutch and in my opinion is probably the best box for todays traffic.
It is possible to get other gear ratios if you have a built up layshaft by swapping the cogs around if you have a selection to choose from, but it is essential that each pair of gears adds up to a total of 43 teeth.
Dave
