Cars that can get at least 100mpg have already existed in the past. The Peel P50 for example was developed by the British Peel Manufacturing Company back in the 1960's for the small micro-car market. Sold mostly in England the car didn't really take off as a practical vehicle. The car was so small, that it only could accommodate the driver and nothing else. It was powered with a simple 49cc motor that was able of getting at least 100mpg. The problem with the Peel P50 however was that it didn't go very fast, you couldn't carry passengers or anything else for that matter, and it was slightly dangerous on the road alongside other larger cars and trucks.
The fact of the matter is that any vehicle, powered by a light 50cc engine, could achieve 100mpg. This however in not practical for contemporary use as we require larger and heavier vehicle to appeal to our current drivability standards. In order to achieve this it could be possible to have a light car (perhaps a Smart Car) powered by an electric motor that is recharged through energy converted by the brakes and a very light supplementary 50cc engine. It could also be possible to have the 50cc engine changed to be used as a direct drive instead. Essentially allowing the electric motor to take the car to a certain speed and then switching over to become powered by the gas engine once a desired cruising speed had been reached. In this way you can achieve very high fuel efficiencies.
You can also reach higher fuel efficiencies with any car by simple adjusting your driving habits. I have read that some people can get almost 1/3 better mileage with their vehicles by simply changing the way they drive. Essentially what this means is that you try to avoid doing any hard braking, choosing instead to maintain speed that give you proper distance between cars; avoiding traffic at all costs is one of the best ways you can maintain high efficiency; turn the car off and go into a restaurant instead of going through the drive through; coasting as much as possible on the road; gradually accelerate; changing routes to avoid steep hills, and so on. By doing this with a good hybrid-electric car, it should be relatively simple to get 100mpg.
The true fact of the matter is that consumers need to demand more fuel efficient cars before big automakers take it seriously. When this happens we will begin to see cars getting competitively better mileage.