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My experience with pickups is that they're typically geared assuming they will be towing.
While piston speed does, in theory, have some relevance, in practice increasing load is going to have a much bigger impact on economy. The tallest gearing you can get is likely still going to be short of what will maximize fuel economy.
Take the Mazda MX-5: In the US, the older models had a 4.30 ratio. In Europe and Australasia, a 3.636 was standard.
With the Honda Accord, a 4.4 was standard, whereas the Acura variant got a 4.7. In Europe, the equivalent of the Acura received a 4.05, and real-world fuel economy was much higher.
In both cases, the European variants also had smaller displacement engines as options to pair with the taller gearing, which delivered even better economy.
In old 90's Civics, final drive ratios ranged from 2.95, 3.25, 3.72, 3.89, 4.06 and 4.25. Fuel economy improved almost in a linear fashion with taller gearing.
With the Insight, in 5th gear, the car will not do more than ~75mph with the pedal flat on the floor. On the highway, load is always close to 100%. And, not coincidentally, this is the highest fuel economy vehicle ever sold, approaching 100mpg at highway speeds.
My advice: Don't overthink it. Choose the tallest gearing you can tolerate.
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