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MINI Differential Ratio’s

MINI Final Drive/Mini Differential/Mini Diff Ratio’s
STANDARD  FACTORY  DIFFERENTIAL

    
The final drive in any Mini is held in the rear of the gearbox and is the last set of 2 gears before the driveshafts/wheels (hence final drive). Part of the final drive is the actual differential (diff), this allows the wheels to revolve at different speeds when turning corners. The inside wheel goes slower, this is essential to allow you to turn without scrubbing tyres, going straight on and developing huge biceps…


   The final drive (diff) ratio and the camshafts are the 2 components ultimately responsible for the way your Mini performs after the suspension….all 3 need due consideration when it comes to your Mini especially if you want to use it for a specific Motorsport discipline. All 3 are discussed to varying degrees in different pubs, clubs, show-grounds, and forums, some are informed, some miss-informed. As quite often the case, a little knowledge is more dangerous than none, so for those that read and understand, the following lists/charts and information, they can be used as a guide as to the various applications of the standard Mini diff ratio’s available. The two gears are the pinion (small gear on the end of the mainshaft) and the crown wheel (big gear in diff housing), to establish what ratio you have, count teeth and check the chart below or to calculate the ratio divide the driven by the driver (this applies to all gear ratios)….eg  62 divided by 18 = 3.44, this means the pinion turns 3.44 times to 1 turn of the crown wheel, assuming 4th gear and standard parts that means the wheels turn 1 revolution for 3.44 engine revs. From this it’s pretty easy to work out how fast your Mini will go at any engine revs. There is a chart on here which shows speed in mph per 1000 engine revs…It all comes down to counting the teeth and pressing the buttons on your calculator.


    Ultimately in simple terms the whole gearbox is a power lever between the engine and the wheels and the diff fixes the overall leverage, which affects performance. A low ratio diff gives a leverage advantage and allows quicker acceleration, because you have more leverage, hence you can accelerate quicker in 2nd than 4th and with a 3.9 diff than with a 2.9, it comes down to engine revs per mph. which is covered later.


   The main confusion with gearing is the terms low and high, without over complicating it, low means low speed, and high means high speed. Eg at 3500 revs  in 4th gear  a low 4.1 ratio will do 47 mph and at the same engine revs a high ratio 3.44 would be doing  56.2 mph  ….So a 2.95 ratio diff is HIGH (high top speed, reduced acceleration, increased economy),  a 3.9 ratio is LOW (low top speed, increased acceleration). Further confusion comes from the fact that a low diff ratio of say 4.1 can produce a fast Mini, this is because the engine for that specific Mini will have to rev highly which will give both quick acceleration and high top speed, (but low for the revs involved). A 2.95 ratio will give slower acceleration because the 'mph per engine rev' is greater. The torque at the wheels will be less, and the engine revs at cruising speeds will be lower. This is more suited to a driver who does more motorway driving at over 70mph, and will often get better mpg as the engine will not rev as high for any given speed.


   If on the other hand you want faster acceleration and are not worried about cruising speeds, then a 'lower' diff ratio is better. This would have a higher ratio/number, but would be lower geared (more leverage) eg 3.76 ratio would give faster 0-60 times than a 3.2, but top speed would may be limited by the fact that the engine would be revving higher when at high speed. Good for the twisty roads, and spirited town driving but not as good for motorways.


     A more powerful engine will generally rev higher than standard and can accelerate more quickly with a low ratio diff, and if you are willing and only if your engine is able to rev high then top speed will still be high, (so you are getting towards two corners of the triangle). Our 999cc Cooper S hillclimb/sprint Mini had a 4.3 diff ratio and pulled 90 mph plus in 3rd gear, but had to rev to 8500 doing it…..economy was not a consideration but could be about 27 mpg if driven steadily or about half that when driven hard, it all depends how lead footed you are. Just because you have a low geared Mini does not mean you cannot be relatively economic with the fuel, just don’t accelerate quickly, if you have a powerful engine it will be pretty efficient and this can be reflected in fuel use if you only feather the gas pedal instead of jumping on it. Looking at it from a fuel economy point you could run a very high ratio 2.76 and that will get you nearer the fuel efficient corner but even with a powerful engine you will never have quick acceleration and top speed may be disappointing because the drag is difficult to overcome because such high gearing is a poor leverage. Also the economy may be disappointing because the engine revs will be kept relatively low and outside the optimum.


     Unless you are needing a Mini with rapid acceleration or a specific top speed, you are more likely to want to use the car for town driving, B roads, A roads and occasional motorways, in which case, you will need to compromise and plumb for a good all round ratio, perhaps based on previous experience and or personal preference.


    Eventually in the late 1990’s Minis could be had with a 5 speed gearbox from the showroom. The move to a 5 speed Mini  should have been done in the early 1980’s ( in the Metro too) instead Rover went for ridiculously high geared diffs (2.76 or 2.95) which give poor acceleration, if Rover had used an overdriven 5th gear with a mid-range diff ratio (3.44 or 3.76) they would have had better high speed cruising without losing acceleration, and created a more useful all-round gearbox that would have improved the Mini….Instead Rover were obsessed with low fuel consumption and noise reduction, (personal opinion, bollocks)….yes the original Alec Issigonis conception was to be frugal on fuel, but when a  Mini  loses acceleration and top speed, the gearing is wrong, and if you have a frugal quiet noise obsession, don’t buy a Mini, spend it on a psychologist, more sound deadening or earplugs. Did Rover ever have a problem selling a performance Mini…?, and which models are the most desirable now ?

Rough guide to ratios for road cars…
    

Choosing a diff ratio is like many things in life….opinion and personal choice gained from experience… Choosing the ‘ultimate’ in many things is like trying to reach all 3 corners of a triangle at once….you will have to compromise, hopefully not too much.  For mostly  town driving with some A road / motorway, 3.44 gives a good overall performance on smaller-bore engines, and 10 inch wheels,  3.211 being  better for a more powerful or bigger cc engines over 1275cc. If you live on A roads / Motorways 3.1 will be good… 3.44 is a good overall ratio for a 1275, personally I would go for 3.64 as I like the twisty roads and to make the engines rev.  Rover went for a 3.105 ratio diff to do all the A+ engined Minis, with the odd change for ‘Specials’ - like the 2.95 in the 998 E (Economy). It was Ok in the 1275 engined cars, but killed 998cc for fun. Starting in 1996 Minis the 2.76 ratio was the normal diff, this was to reduce noise levels, and give better economy for motorway runs…but it really killed engine performance on the road especially with 13 inch wheels….with both slower acceleration and a 6 mph lower top speed  than  12 inch wheeled cars and is a long way from ‘compromise’ with poor acceleration, the engines barely have enough power to pull such a high ratio. Bear in mind when deciding , both the 970 and 1071 Cooper S Minis had the standard  ratio of 3.76 and alternatives of  3.9, 4.1 and 4.2 and the Mk 1 1275 cars were 3.44 as standard with the alternatives 3.76, 3.9 and 4.1…low gearing = performance, high gearing = economy, mid-range gearing = compromise… and never forget Mini’s should be fun.
  

Factory Final Drive Ratios  

                   Teeth  Count          A+  Part No.             Pre A+ Part No.        A+ C/Wheel          
Ratio
LOW      C/wheel    Pinion    C/wheel    Pinion    C/wheel Pinion          Casting No.


4.333        65            15    DAM3645    DAM3647    22G443    22G99      DAM3546
4.267        64            15                                                2G370     22G99    
4.133        62            15                                                 2G101     22G99    
3.938        63            16    DAM3216    DAM3218    22G340     22G338    DAM3217
3.765        64            17    DAM4779    DAM4131    22A401      22A399     DAM4780
3.647        62            17    DAM4162    DAM4137    22G940      22A399    DAM4163
3.444        62            18    DAM2677    DAM2679    22A411      22A413     DAM2678
3.211        61            19    DAM2806    DAM2808                                           DAM2807
3.105        59            19    DAM6327    DAM2808                                           DAM6243
2.95          59            20    DAM5925    DAM5927                                           DAM5926
2.76          58            21    TCB10004    TCC10001                                      TCB10005

HIGH  


 NOTES:

    Some gears have the same tooth count for different ratios, they are not interchangeable - hence the need for different part numbers…. ONLY use in known pairs
      A+ and pre A+ gears are not interchangeable as they have a different tooth profile.
      A+ pinions have flat surfaces on both sides, pre A+ have a small shoulder on one side.
      A+ crown wheel casting number stamped in is 1 number different to the part number.
    If fitting a 2.76, 2.9 or 3.1 ratio diff in a pre A+ gearbox or an A+ gearboxes with a  cast mainshaft bearing retainer it will have to be modified , so will some later sintered ones, they  need filing out to clear. This is because the pinions are larger outside diameter. Always carefully check for clearance.
If you are not sure, ask a second opinion, assume nothing, it’s not worth getting it wrong and can spoil the enjoyment of your Mini, as we say here it ‘costs nowt to ask’….use the contact page….

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