While simultaneously, those with higher gears should show an improvement in mpg by being able to keep the rpm lower across a greater amount of cruise speeds, than those with fewer gears.
Ultimately, the more gears a transmission has, the closer it comes to being a CVT. And ultimately, a CVT is the ultimate transmission. However, I don't believe they are as robust as a manual transmission, and they've been controlled to act like a X speed automatic transmission which is just pure stupidity IMO. The first Prius I drove (while an engineer at Toyota) seemed to have a CVT that worked exactly how a CVT should work. But admittedly, it was weird driving being so different than any other vehicle I've ever driven. Controls have been added to make them act somewhat like a traditional x-speed transmission. Because feedback to the OEMs was vast and mostly negative, with customers thinking something was wrong with the transmission (CVT) ... because like I said, they did seem very weird when they operate as they were initially intended. Clarification: The CVT seemed weird but it was actually the correct way to operate.