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The Highs and Lows of Differential Gear Ratios

How do you determine what gear ratio you have? Sages will tell you to look for numbers on the case, which is great if:
a.    you can find them, and
b.    the gear set hasn’t been changed by a nefarious past owner.

The easiest way to determine the gear ratio is to jack up the car (safely) and block one rear wheel so it can’t rotate. Most British cars have “open” differentials, where a spider gear causes the wheels to rotate opposite one another. If you block one, the other wheel spins twice as quickly. When driving, this allows the wheel on the inside of a turn to rotate slower than the one on the outside of the turn. With the transmission in neutral, rotate the unblocked wheel two full turns and note how many times the pinion flange of the differential rotates. Marking the wheel and flange with a white marker helps keep track of the rotations. The number of turns is the gear ratio, which is called out as number of turns of the differential pinion to one turn of the axle.

So why is this number important?

The lower the number, the faster the car will go with the same number of engine revolutions. The higher the number the better the car will accelerate, but at the expense of high speed cruising.

Now for the confusing part of the story. A high numerical gear ratio is called a low gear or low rear end, and vice versa. Low gears give fast acceleration, high gears give better cruising.

A smaller engine will need a lower rear end to give adequate acceleration. A more powerful engine can use a higher rear end to give relaxed cruising and a higher top speed. So if you install a bigger, more powerful engine you probably want to change to a higher gear ratio from stock.

Shorter tires require a higher gearing to travel the same distance as a taller tire, so as tire sizes got smaller and engines produced more power, the gear ratios tended to get higher. That is why for instance an MGB with 14″ wheels was fitted with a 3.90 gear ratio and the earlier MGA with 15″ wheels had a lower gear ratio.

Here are some typical ratio numbers with the gear teeth counts (Pinnion/Crown Gear):
Big Healey:     4.10 10/41    3.90 11/43    3.545 11/39
MGT:    5.125 8/41    4.875 8/39
MGA:    4.30 10/43    4.10 10/41
MGB:    3.90 11/43
MGC:    3.07 14/43    3.30 13/43    3.70 10/37
Spridget:    4.22 9/38    3.90 11/43    3.73 11/41
TR2-6:    3.45 38/11    3.7 10/37    4.1 10/41

By Kelvin Dodd

Comments (2)

  • Gilbert

    Thank you very much! My question was clearly answered in this site.

    Reply
  • MASOUD

    Hi, I have 1995 Mercedes-Benz which has problems with the Differential Gear of Gear Ratio 3.92. I can’t find a replacement. So is there an Equivalent Ratio Number such as 3.67?

    Any help is appreciated.

    THANK YOU

    Reply

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