If 1st gear is low enough to support this, it might be possible to modify the diff for a higher final drive ratio across the board.

Might be cheaper than a tranny.
Of course, your speedometer would need to be recalibrated as well.

Pretty flat in most of Oz, right? Higher 1st (and reverse) gear may not be a problem. Certainly would save some petrol.

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1986 300E 5-Speed 240k mi.