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Knowing the overdrive ratio without knowing the first gear ratio is only half the story, as your launching (first) gear ratio will affect how tall of a final drive gear you can install and maintain some semblance of taking off or maneuvering at low speeds.
In a perfect world of picking and choosing parts, I would pick the transmission with the widest spread between first and overdrive and then adjust the actual engine to road ratios by selecting final drive gears and tire diameter.
If you are willing to think way outside the ballpark…. You can get a GM NV4500 manual with a first of 6.34 and fifth of 0.732. Take an older GM car set up for a Big Block (like a corvette), swap in a Diesel Isuzu 4BD1, NV4500 Manual, 2.56 Axle, Tall Tires and you would have an efficiency machine.
Back in the real world, look a gear spread, final drive ratio, and tire diameter to get the full story on a given car’s gearing. Be aware of what parts are direct swaps from other models, i.e. the axle or gear from a similar model or the same model overseas to get a better final drive ratio. Also look at where a given engine makes its torque, and if you can get ahold of one look at a BSFC chart.
Or just buy a car and be happy with it. As long as you aren’t picking a car mainly based on the color, I don’t think you will make a bad decision.
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