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I never calculate with gear ratios and final gear drives and tires. Too much math.
I see how much RPM per MPH, and base the result on my experience with other vehicles.
Depending on the engine size, a 1.4 liter turbo engine or 1.8liter NA engine really can't take much lower than running at 2,5k RPM at 80MPH; while this is easy for any 1.6l turbo, 2.0 liter NA or more.
Those are my preferred, and most optimal gear ratios.
2.5k RPM at 60MPH for a 1.6 liter turbo hatchback or sedan results in ~40MPG avg, 50 hwy.
I once ran a my Honda Rebel 250 motorcycle at 3k RPM at 45MPH just fine. Down from 4.5k RPM. That's a drop of 33%. For such a tiny engine it ran quite low in RPM.
If I could have, I would have ran it at a 35% lower than stock ratio, but hardware wise, I wasn't able to modify the bike to go any lower in RPM. Otherwise I'd have done it.
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