Hi Ken and welcome to the forum. Thanks for taking the time out to drop in and share more about your incredible Fiat with all of us.
It is not a surprise to me that the 633cc engine was utilized out of the 3 engine options that were available at the time. The intake system and the carb was also optimized which would make sense.
I think what people do not quite understand here is that your car while from the exterior is still 100% Fiat Under the hood was a much different story. While the car achieved 304mpg at low speeds my educated guess is at speeds averaging between 25-30mph this is still quite remarkable and could never have been achieved without the laundry list of system modifications. This Fiat was a highly modified and tuned car that had 1 mission and that is high mpg.
Now to put this into perspective we are talking about a car that from 1959 and had a curb weight of 1,290 lbs, a 633cc gas engine, CD estimate of .5, and was rear engine rear wheel drive. There was no carbon fiber composite, no light weight alloy or magnesium wheels, no aerodynamic detail such as wheel well covers, boat tail etc, and no hybrid components what so ever only ICE!
Now lets fast forward and look at a modern family of cars that have achieved verified 1 Liter fuel consumption. I am talking about the VW 1 Liter cars. Initially the prototype was a 299cc 1 cylinder turbo diesel and has sense with its updated version moved to a 2 cylinder electric hybrid drive train. All 3 iterations were reported as achieving 1 Liter fuel economy with greatly reduced emissions the latter being the most potentially fuel efficient at a combined 310mpge and extremely low emissions output of 24g/km.
Why I dropped the VW family of cars into the mix is so we can see the progression from 1959 to now 2013. As you can see the original Fiat 600 hypermiled car while being much more basic still needed to be heavily modified and take advantage of reduced curb weight and small engine displacement to help achieve its 304mpg. All the VW cars share these same traits low weight and highly modified.
What is still a bit unclear is at what speeds did these cars achieve there 1 Liter performance?
Based on books I have read on the Eco Marathons during 1959 and later it is my educated opinion the Fiat was drove at average speeds between 25 and 30mph.
Now the question I have is with the VW family of cars. With there advanced hybrid design and other modern advanced technology were they able to achieve 1 liter fuel economy at modern real world speeds between 40 and 60 mph and under real world conditions not simply driving around a test track counting laps?
This is one of the many questions I hope to answer here in this thread as we dive deeper into the world of 1 Liter performance
GH..