Choice of rear axle ratio

Many owners who use their Magnette regularly have changed to bigger engines from the MGA or even the MGB. But the extra power is wasted with the standard diff. I used my ZB Magnette for a short while with the standard 4.55 diff and a 1800 3brg engine. This reminded me of the times when I drove a Mini: super acceleration but no top speed.

To get the extra power on to the road you need another final drive ratio. Luckily the Magnette rear axle is a standard BMC item and was used in numerous production models with ratios to suit. (See Register's Modification Manual for list of cars and ratios).

But which one to choose? To some extent, choice of final ratio is trial and error, using road tests in various conditions to see how the car performs. However, you may want to calculate your over-all ratio as it stands, so as to approximate the desired new ratio. This will give you an indication of whether you need a higher or lower diff ratio.

The following formula shows how to calculate your road speed per 1000 revs:

S = E x W x 60
G x D

S is road speed

E is engine speed in revolutions per minute

W is the distance travelled by a fixed point on the circumference of the wheel during one revolution. If you want S in mph you must use W expressed as a fraction of a mile. For kph use W as a fraction of a kilometre. As a guideline, for a 165 R 15 wheel, W = .001239 miles or .001962 kilometres. For absolute accuracy in relation to your own car, put a chalk mark on the wheel at ground level and a similar mark on the floor. Then push the car forward until the mark is again at the bottom of the wheel, make another mark on the floor and measure the distance between the two floor marks. Normal tyre pressure and loading assumed.

G is the gearbox output ratio, e.g. the standard Magnette first gear is 3.64, top is 1.0

D is the differential ratio e.g. the standard ZB figure is 4.55, the ZA is 4.875

Here are some examples of the most common choices:

a) in miles

		@1000revs	@4000revs	@4500revs
Standard ZB	4,55	16,34 miles	65,35miles	73,52miles
Standard MGA	4,3	17,29miles	69,15miles	77,80miles
Option MGB	4,1	18,13miles	72,53miles	81,59miles
Standard MGB	3,9	19,06miles	76,25miles	85,78miles

b) in kilometres

		@1000revs	@4000revs	@4500 revs
Standard ZB	4,55	25,87 km/h	103,49km/h	116,43km/h
Standard MGA	4,3	27,38km/h	109,51km/h	123,20km/h
Option MGB	4,1	28,71km/h	114,85km/h	129,20km/h
Standard MGB	3,9	30,18km/h	120,74km/h	135,83km/h

These calculations become more interesting with an overdrive or Ford Sierra 5 speed conversion. The fifth gear has an output ratio of 0.8 (instead of 1.0 on standard gearboxes).

@1000revs

@4000revs

@4500 revs

Standard ZB
4,55

20,42 miles

81,69 miles

91,90 miles

Standard MGA
4,3

21,61miles

86,44 miles

97,25 miles

Standard MGB
3,9

23,83 miles

95,31 miles

107,22 miles

For more details, i.e. solving the problem of square splined and star splined half-shafts, and a list of BMC cars with the diff fitted as standard, I recommend the Registers Modification Manual @ £5.00 sterling (+ post & packaging depending on destination). Look on the Books page for more details or the Regalia page for how to order.