OK I am putting all the politics and BS aside and asking a technical questions to all the engineer and mechanical types out there. Shouldn't horse power be in the mix? I thought I read somewhere that if you tune an engine down X% in HP you could get a Y% gain in MPG. If this is true does every car need 250+ HP? Do we all need to go 0-60 in 5.0 sec? Can't a car be safe, affordable and get better MPG if it is slower than all they are now? The average car has much more HP now than most did back in the day. Are we spending our tech advancements to get more power instead of more MPG?
Just give me technically correct answer. Not "I have the right to blah blah, or "people will always want more blah blah" . Just form an engineering point of view can lower HP give you better MPG for a given engine?
I am no engineer just an aircraft mechanic, but I have been studying this type of information in my spare time. I eventually would like to build my own car from scratch and a series hybrid at that, just for fun; if I ever had the money of course.
The answer to your question is difficult. There are more aspects to fuel efficiency than engine size and power. Basically, it works out to a common equation. It takes X amount of power to get Y amount of resistance(drag, weight, etc) to Z amount of speed in a certain amount of time. But, the slower the acceleration doesn't necessarily save you gas. And at the same token it takes X amount of power to maintain Z amount of speed with Y amount of resistance.
So, basically you will need
so much power no matter what you want
. And, in order to get said needed power you will need to burn so much gasoline. Each gallon of gas has about 124,000 BTU's of energy. It takes so many of those BTU's to convert into mechanical energy to create the amount of power you need to get up to speed and maintain it. An engines potential
power does not determine it's fuel efficiency, how you use the engines power does.
Now, on with the weight and drag issue's. These are reasons for the efficiency problems that even the small trucks have. The are like brick's in the air and they weigh a lot. A truck or SUV needs to push through the wind while cars can sort of slice their way through it. Weight has become an issue for all cars and trucks today. There are many regulations that require extra materials to strengthen the cabin of a vehicle which adds more weight which means the rest of the vehicle has to be beefed up to handle 200,000 miles of abuse with all this weight to lug around which adds more weight(vicious circle really, and it gets worse), then people want their 8 way power seats and triple zone a/c with the four 25 lb speakers and 75lb wheels (I am guilty of this, but I can afford the gas). This is the other part of the equation. The more weight you add the worse the mileage. Then you throw in the aerodynamics of a parachute. Well, you get the picture.
Now we have to look at the efficiency of an internal combustion engine (ICE). They are only about 30% efficient and these are the best ICE made today. This means that a 250hp engine is actually putting out more than 750 hp, but we are only able to extract 250hp into mechanical power. The rest is lost through heat, pumping losses, and friction. We also have to look at the advantages for every type and size of engine. Some engines work better than others in low rpms like big V-8's and diesels, while others are better at revving high like smaller 6's and I-4's. V-8's and V-6's have an advantage over I-4's in bigger cars and SUV/Trucks. The V-8 or V-6 can almost idle a large vehicle at highway speeds giving the engine less heat and friction to overcome, while a I-4 would be high revving. But, I-4's are great around town because they don't use much gas to get you to speed because they use less gas to rev up. This can be compared easily with a Mazda CX-7 and Saturn Vue. The Mazda is a Turbo 4 while the Vue is a 3.6L V-6. The Mazda gets 1mpg better in the city while the Vue gets 1 mpg better on the highway. On the other spectrum an I-4 is better than a big V-8 in a smaller car because the car doesn't require as much torque to maintain speed.
I guess it's all about balance. But, asking for more than a material can physically deliver is an impossible request. That is where alternative energy sources com into play. Hybrid electric cars and such, but they are still being powered by the same inefficient ICE. The only things a hybrid does is take the wasted energy (braking and coasting etc.) and store it to supplement the inefficient ICE at certain moments. It also, and more importantly, turns off the engine when it is not needed; no gas burned equals the best mileage you will ever get. This does work, but it is still only so much of an improvement and doesn't fix the inefficiency issue of the ICE.
In order for an ICE engine to become more fuel efficient new technology that has never been thought of before will have to arise. Hence the comments about engineers and scientists having to work overtime because the ignorant public demands something. Something that they them selves cannot deliver or even contemplate on considering to possibly think about studying to understand a solution that could maybe one day lead to the eventual replacement of the fossil fuels they so direly want to be rid of.
So, in short, to answer your question. Yes and no. A 430hp Corvette will beat out a 170hp Solstice in fuel efficiency when cruising, but put these guys on a track and the Solstice won't have to fill up on his way home; even if he lost the race. The application of the power delivered by certain engines in certain vehicles will determine the mileage of said vehicle.