The 20% more gas (at 120kph instead of 100kph) is not per amount of time but per distance travelled. This is mostly caused by air and road resistance, but also by engine/transmission efficiency.
So lets say that at 100kph your fuel economy is 10 liters/100 km (23.5 mpg), at 120kph your fuel efficiency would be around 12 liters/100 km (19 mpg). You travelled the same 100 kilometers in 20% more time, but saved 20% on gas/emissions.
Your Treehugger homework for this week (previous homeworks can be found here and here) will seem obvious, but looking at public roads, it is self-evident that it is not that obvious to most people (once again, common sense is not so common). So your task is - if you have a car - to follow the speed limit (particularly on the highway) and to explain the benefits of doing so to friends and family (most don't realize just how big a difference it can make). The whole point is to trade time for gasoline. When you drive at 120kph/75mph, you are burning about 20% more fuel than at 100kph/60mph (and even more than 20% if you are in a particularly non-aerodynamic vehicle like a SUV). You are also travelling 20% slower, so it will take you a bit more time to reach your destination, but unless you are a doctor answering a life or death call, a few extra minutes are probably fairly easy to find and could make you save up to 1/5th on gas (which means up to 20% less CO2 and smog-forming emissions in the air). Make sure that your tires are properly inflated for even more savings. Bonus: Nobody likes speeding tickets and high speed accidents.Update: To avoid confusion, I'll copy here what I said in the comments: