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Engines are most efficient when they have low manifold vacuum. Higher gearing gives you lower manifold vacuum and greater efficiency. Most engine BSFC charts show best efficiency at lower than 3k RPM. Fewer power strokes with greater load equals better efficiency. Here is my 30-60-90-120-150 280Z. 3k in 5th @ 75 MPH. 2k at 50 1k at 25. Very close to the ZX final drive. It had a 3.45 R200 diff which I changed to a 3.90 R200.
With the 3.45 it would do 9 more MPH in top gear at 3k RPM, close to 85 @ 3k in 5th.
regards
Mech
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