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Tailored differentials
Richard Lane’s comments on high gearing in the Porsche 718 Boxster GTS’s manual gearbox struck a chord with me (Our Cars, 15 September).
This problem isn’t new and was one faced by Jaguar when it launched the E-Type back in March 1961 (it had only a four-speed gearbox but was capable of 150mph). Its solution was simple and straightforward.
Depending on the market for which a car was destined, it would be fitted with one of three rear differential ratios: 3.54:1, 3.31:1 or 3.07:1. For countries with low speed limits, such as the US, cars got the 3.54 differential (where 3.54 turns of the propshaft resulted in one turn of the rear wheels). This lower gearing gave faster acceleration but not too high a level of revs for cruising at the low national speed limit.
For markets where there were far higher limits or none at all, such as Europe, the 3.07 ratio provided higher-speed cruising capability.
The difference in real life was useful. In the US, 3000rpm would equate to around 65mph; whereas in Germany, it would equate to around 75mph. The effect was replicated through the gears, given the ratio change was in the rear differential and not the gearbox.