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well in theory, if you were to have the BSFC map for your engine you would be able to see how wide, in rpm, the "island" is where you acheive your lowest BSFC. on my Ford Focus zetec it is from 1500 to 3000rpm.
So the minimum ideal closeness is gear spacing would be where when you upshift at 3000 you dont drop below 1500 for the next gear. Of course the closer your ratios, you can stay closer the centre of that sweet spot.
as a rule of thumb i think more gears are better. That why there are 7, 8, and 9 gear transmisison coming out, so you can always be in the BSFC sweet spot.
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