Originally Posted by
HarryB
You are combining two different matters, final drive selection and differential selection.
3.9, 4.1 and 4.3 refer to the gear ratio on your final drive (ring and pinion), and these are interchangeable. What you need/want depends on your gearbox and the use of the car. 5 speed cars had either 4.1 or 4.3, 3.9 is too tall for an N/A car. 3.9 were used only on the 6-speed gearboxes, due to the gearbox itself having different ratios, thus 3.9 was needed to compensate. If you are not interested in highways I would suggest to go with the 4.3, or even with a shortest one. FYI Miata diff rings are 7" in diameter, so there are some options out there, the most popular here coming from KIA Sportages.
The actual diff units are known as T1 and T2. In general, T2s are more forgiving than T1s during normal driving, due to their different lockup characteristics.