Usually, before looking at alternate gears ratios, you need to see what gearing your engine would need or what best suits it.
In an ideal world, the difference between each ratio would be selected such that - for example - as the engine reaches Peak HP in 2nd, then shifting in to 3rd, would put you right on Peak Torque in the Engine's Power Band, same for 3rd to 4th. Same would apply to 1st gear, but it is usually a little lower than this ideal.
In a Road going engine, these two points in the Rev Band are much wider apart than is the case on a Race going engine. This is why on a Race Car, they run closer ratios.
In the Mini Drivetrain, there are several sets or groups of gears;-
Transfer or Drop Gears
Gears (in the gearbox), and
Diff or Final Drive Gears
Each of these sets are available as in straight cut types, and fitting one group, doesn't mean that it's necessary to change other, ie, you can for example just fit straight cut transfer gears if you like.
There seems to be several suppliers / manufacturers of these at present, but I will say, you get what you pay for, while true of most parts, this is more acute with gear sets.
I'll just throw in, contrary to some popular belief - gear for gear - straight cut gears are weaker than helical cut gears.
In regards to the transfer gear sets, there are a few different ratios available, from 1:1 (as most helical sets already are) to a reduction or under-drive sets. They tend to be very noisy, mainly because they are not all held together within a single casing (like the gearbox is), but between 2 or 3 depending on which gear you are looking at so alignment is never 100%, also the transfer case webbing which holds the gear bearings (half of them) is comparatively thin so it tends to amplify rather than dampen the noise from these gears.
In regards to the Gear Sets in the Gearbox itself, there are quite a number of different sets available, most are very close ratio, ie little ratio difference between gears. These are suited to very 'Cammy' engines that only have a narrow power band and don't suit street engine too well at all. First gear is quite high which suits race engines as they run low Final Drive ratios. On some sets, First Gear is almost the same as Second on an ordinary Helical set, so when driving around for a while, to give you an idea of what that would be like, don't use First. Your clutch won't last as well as it otherwise would.
Race Cars 'get around' this by running Diff Ratios in the 4:1 to 5:1 ratio, by contrast, most of our street Minis, run in the 2.9:1 to 3.4:1 ratio. Those low suit race cars again with cammy engines as they don't work too well below 4000 RPM.
With the Diff Gears, I haven't looked to be 100% sure, but I think most suppliers these days only do 'semi-straight cut' (which are in reality a reduced angle helical cut) only have ratios from around the 3.9:1 to 5:1 available. Not really suited to a street car.
For a street car, I have tried SC Transfer Gears and also the Gearbox Sets (original Special Tuning ones). I would recommend you stick with Helicals, if you are after something 'sportier' and depending on the spec of your engine, then I might suggest the Gear Sets fitted to the Cooper S, these I found are nice without going overboard.
Edited by Moke Spider, 10 July 2015 - 07:16 AM.
) version, with Thrust Roller (there's that word again!) Bearings too. Was a huge pile of work to do, I got tired just looking and thinking about it! 
