XK150 five speed gearbox ratio selection
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The ratio selection process depends on the rear axle ratio in the car and the tyre sizes.

Which T5 fifth gear ratio did I select?

For my car, with its near-standard tyre rolling diameter (185VR16 radials) and the 4.09:1 (45:11) rear axle standard for an overdrive car, I opted for the 0.63 fifth gear ratio. For an XK150 with standard tyres and a 3.54 rear axle, the 0.63 fifth gear ratio would be too tall. A fifth gear ratio of not less than 0.73 would be preferable for a car with a 3.54 rear axle. 

Set out below is a comparison of the individual gear ratios and the RPM at highway speed:

Rear axle ratio 4.09 4.09
Moss+OD  T5
First 3.38 2.95
2nd 1.98 1.94
3rd 1.37 1.34
4th 1.00 1.00
OD/5th 0.78 0.63
RPM at 100 km/h 2,730 2,205
RPM at 60 MPH 2,641 2,133
Based on Jaguar XK150 owner handbook


Based on all types of highway and country road driving since the installation of the T5 in my car, the 0.63 fifth gear ratio was a good choice. There are not any taller fifth gear ratios available in the T5, but even if there were, it would not have been wise to "go taller".

The 0.63 fifth gear ratio would have been too tall for typical UK highways in the 1950s, and for the majority of US highways at that time. (Construction of the first US Interstate highways started in 1956 with sections opening soon after. The first section of motorway in UK opened in 1958.)

With the T5, the change in ratios for 2nd and 3rd vs the Moss original, is negligible. The 1st gear higher ratio of the T5 is a real bonus, as the standard 1st gear ratio in a car with the 4.09 rear axle is too low. The T5 first gear ratio of 2.95 brings the overall 1st gear ratio back to the equivalent of a car having the 3.54 rear axle ratio and a Moss box.

For completeness, here are the teeth counts for the T5:

Driver

Driven

Ratio of pair

Overall ratio

Input driver

24

31

1.2917

-

First on cluster

14

32

2.2857

2.9524

Second on cluster

22

33

1.5000

1.9375

Third on cluster

29

30

1.0345

1.3362

Fourth

-

-

1.0000

1.0000

Fifth pair

51

25

0.4902

0.6332


There is no hunting tooth in the first gear pair. This is nomally considered poor design practice. Fortunately, first gear is the least used forward gear in any normal car. Second gear is not much better, as the same teeth mesh every second revolution of the driven second gear.

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Other ratios

As an aside, there is a wide range of gear ratios available for the T5. First there is the choice for the 1st, 2nd and 3rd gears, inherent in which is the ratio of the input gear to the driven (front) gear on the cluster gear. Because the fifth gear pair is a separate pair of gears, mounted on the layshaft extension and the mainshaft behind the rear bearings, the choice of the fifth gear ratio can be varied, but it is affected by the ratio of the input gear/front cluster gear ratio.

Be aware that there are different gear material strengths available.

t5-sectioned

 
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