Great idea and excellent write up. I drive a 1991 bmw 318i that came equipped from the factory with a 4.11 LSD differential which runs about 3300rpm at 70mph.
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Not only was the high RPMs on the highway annoying but first gear was virtually useless for a daily driver. I had to shift to second almost immediately after starting from a light. I found myself using 4th and 5th gear on city streets more often than I was using 3rd. Eventually I replaced the differential with an open 3.64 which made a world of difference. Fuel economy improved by about 10% in all driving situations and the cabin is a much quieter place to be.
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On the E30 BMW offered virtually any diff ratios between 2.79 to 4.44 while they kept the gear box ratios the same.