m46 or 3.54 rearaxis

The difference between M47 5th gear and M46 overdrive is so small as to be not worth thinking about. The 3.54 ratio would make a noticeable difference. (The last manual transmission 240s over here had 3.31 axles.)

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I've never changed a rear axle, but a 5% higher ratio (lower number) has to reduce torque available to the wheels by 5% at a given road speed. That said, you gain something back because you hold each lower gear 5% longer.