You have lot of factors in vehicle economy, so you don't have "one shoe fits all" situation here.
Each vehicle is having its "sweet spot" and for example where my Ford Truck makes huge mpg at 40 mph, my Mercedes at this speed becomes gas guzzler as it makes way better mpg at 60 mph.
(I can find a speed where 8000 lb diesel pickup and 3500 lb gasoline car will make exactly the same mpg. )
As I mention, low numerical gearing will always give you best mpg, but with the cost of lesser convenience of more gear shifting, so it is always about sacrificing one feature for other.
Than if your transmission gives you 2000 rpm between the highest gear and one below, that sure brings additional issues.
Electric vehicles don't have those problems, so let's wait for electric pickups.
