If the moderators feel the need to remove this post, as the vehicle is off-topic, please do so; but the example fits well with the topic of this thread.
In 1975, I was bitten by the "buy American" bug. Before that, I bought what I could find, liked, and could afford.
In 1972, I special-ordered a Toyota Corolla, with a factory 5-speed from my best friend, who was co-owner in a Toyota dealership.
As I planned to enter the car in rally events, I spent some time with the physics books and the slide rule doing some research.
Once the car arrived, modified the car with a different carburetor (duh), factory headers, a recurved distributor, American Racing wheels, and Michelin tires. The tire diameter thus the size, was determined by the aforementioned calculations.
As it was necessary for success in rallies, we calibrated both the speedometer and odometer, so we knew the accuracy of various test results.
Once the engine was sufficiently broken-in, mileage tests acquired 39.3 MPG at 70 MPH (Missouri speed limit).
In 1973, the Federal speed limit changed to a maximum of 55 MPH. We had an "interstate highway" trip planned a couple of weeks later, and was really looking forward to seeing if I could break 40 MPG !
Imagine my chagrin when I found the engine would NOT function smoothly at 55 MPH in 5th gear !!!!!
I now had to run highway speeds in FOURTH gear to keep the engine RPMs in the power curve (see post 3 in this thread by panic).
I learned a lot FOLLOWING this exercise about fuel economy.
At the time I was still working on my first degree, and had an excellent relationship with some of the physics professors (I minored in physics). It was a real life problem they could sink their teeth into. In a nutshell, any fuel economy benefit of an overdrive may be acquired with some math, some physics, and tailoring the transmission high gear ratio, rear differential ratio and tire diameter to the vehicle dynamic resistance curve (air resistance) in conjunction with the engine efficiency curve.
The real benefit of a four speed or better, a five speed, is improved applied torque (driveability) over the useful speed range of the vehicle.
I have since proved this several times on various vehicles.
Of course, shifting more than twice (I once had a sports car with 7 speeds) impresses some of your passengers
Jon
