You must be hypermilling or something, your prius is rated at 41mpg
https://www.fueleconomy.gov/feg/noframes/16705.shtml yet you get twice that? And that metro xfi was rated at 51mpg yet again you found someone that gets almost twice that?
I don't own a Prius.
My father-in-law drives his Prius like an asshole and still averages around 50. When I've borrowed it, I've seen the gauge climb as high as 70-80mpg while taking it easy on country roads (but doing nothing special, just driving in a straight line at a steady speed).
Though my Honda Insight was rated for 70mpg from the factory, if you take it out on a warm day and drive in a straight line at a steady speed, with zero modifications, this is what you get:
How to read the gauge: 63mph presently, 2300RPM, hybrid system neither charging nor assisting, currently getting 100mpg, averaging 94mpg over 35.7 miles.
How to read the gauge: 44mph presently, 1700RPM, hybrid system neither charging nor assisting, currently getting 110mpg, averaging 118.4mpg over 9.3 miles.
The XFi was rated for 51, but like these two cars, if you drive it in a straight line on a flat road on a warm day, you'll see 60+ or even 70+mpg.
Although I'll grant you that there are technical advantages to a hybrid such as being able to capture energy when braking and shutting the engine off at stop lights (you don't even need a hybrid for this anymore), 95% of the fuel economy savings come from cutting weight, tweaking the body shape, using noisy and generally poorer handling LRR tires, downsizing the engine and gearing it to keep RPMs low on the highway. You'll notice that none of these have anything at all to do with the hybrid systems, which exist (in my mind) for two main purposes:
1) To make it so that people feel better about buying an economy car. Suddenly, that underpowered, noisy, bumpy econobox is cool, and supposed to be that way, and
2) To make driving that econobox slightly more bearable. Although I understand the transmission in a Prius can't technically run without the hybrid system functioning, if it could, you'd be pretty miserable driving around town without electric assist making that 1.5-1.8L Atkinson cycle engine feel bigger than it is.
Honda's Insight CAN be driven without the hybrid system functioning, because all the hybrid system is, is an electric motor where the flywheel used to be, that either assists or drags on the engine. With the hybrid system disabled, it delivers exactly the same fuel economy, but you lose 75% of the torque at low RPM, so you're tempted to rev it up and lose economy.
The XFi was in essence the same thing as a Honda Insight without the electric assist. It takes the better part of 14 seconds to get to 60, has no sound insulation and uses tiny tires, and for these reasons it gets phenomenal fuel economy.
EDIT: And this is exactly what Elio is doing. They're taking a Geo Metro and cutting the frontal area by half, tweaking the body shape, cutting weight, modernizing the interior, and getting it to pass safety standards by avoiding them. As a result, you have an 84mpg car that accelerates about 40% better than a Geo Metro because, surprise, it weighs 40% less. It does this using a very simple engine design which lacks variable timing or cam profiles and doesn't use DI. It's essentially a 20 year old Metro engine with a timing chain.