Anonymous,

While I absolutely agree that "Cost is not the only reason to buy a Prius"; however...

1. I bet at 7000 mi/yr in my saturn I'm generating fewer pollutants than than chickadee driving about 35,000 mi/yr in his/her prius.

2. Building a new car generates pollutants. I'm not sure how much, but it needs to be factored in.

3. While cost is not the ONLY factor, it is a factor. And over the last 3 years I've spent the money I could have spend on a prius fixing up a small (800 sq/ft) brick cape in an inner ring suburb and much of that money want into insulation (walls, basement and attic), new high efficency furnance, new 40 yr roof adding attic ventilation and fan... Now I'm starting to replace all the windows.

It's all a trade-off. Here's hoping an electric car will be on the market when it's time for me to replace my saturn.

In the old days of muscle cars cars burnt lots of extra fuel. The extra fuel cooled the engine and allowed the engine to run at its most powerful fuel-air ratio. They got terrible mileage. Now with computer controlled engines, and also much better engine designs we can have efficiency and power at the same time. I read somewhere that a GM V-8 of the 1960's cost GM $25 to build. In todays dollars that would be maybe $200. I suspect that our much more complex engines of today cost more.

Variable valve timing, which flattens the torque curve, allows efficient operation at low RPM, which reduces engine friction and pumping losses. Yet these same engines when revved to 6000 or more RPM produce gobs of power. So a modern engine with a six speed transmission can drive a car at 80 MPH at less than 2000 RPM, and by shifting down and revving up can deliver 250-300 HP with no efficiency penalty. Small cars of the '80's weighed around 2000 lbs; they now weigh over 3000, except for the very small cars now on the market. The engines of the'80's would get terrible mileage pushing todays (relative) behemoths.

Those who are old enough can remember the Messerschmitts and BMW Isettas of the '50's. If they were out today with modern engine technology they would probably get 80 MPG in the city. And if you got hit by a modern sub-compact you would die.

The Insurance Institute of Highway Safety has a web site that charts the death rate per million registered vehicles of almost all cars on the road. My Odyssey has one third of the fatality rate of a Suburban, for example, so size isn't everything. But a car can get so small that it is inherently hazardous to drive it. So we will have fairly heavy cars for the foreseeable future , but modern engine-transmission combinations and hybrid technology will sure help. I think we will have $10 gas within five years. Make your plans accordingly.

For the person who won't buy a Pirus because he want his car to last a long time. Well Toyotas out last any car. My husband had a Camery and would not get rid of it as he want to surpass the 350,000 mile mark, and make it to 400,000. Fortunately it finally gave started to give out at around 375,000, so he got new car.

More correctly, torque is a measure of efficiency and horsepower is an extrapolation of torque.

If there are 2 engines of a given identical displacement burning fuel at an identical rate, the one producing more horsepower is more efficient. And that is entirely possible in a modern ICE - the computers are most of the difference between a 2007 ICE and a 1990 ICE.

What can be done is to use this increased efficiency to allow for smaller displacement engines that use less fuel to produce the same horsepower. In Europe they used to (maybe still do) have outrageous taxes on engine displacement - a much better approach than CAFE and gas guzzler taxes because it encourages efficiency. Taxes on turbo and superchargers would be a good idea as well, since they allow an engine of a given displacement to burn more fuel.

And you are absolutely right about weight. Work is a measure of mass moved over time. Torque is a measure of work. And torque in an ICE is also, to a large degree, a function of the amount of fuel burned. Less weight means less work, which means less fuel burned to cover a given distance in a given time in a car of a given weight. Weight is the hidden enemy here, as subcompact cars have pretty much doubled in mass over the last 20 years. If we went back to 1985 weights for small cars,through the use of advanced materials and manufacturing techniques, fleet fuel efficiency would be greatly enhanced.

as a physics ignoramus, I have to ask:

is it even possible to trade horsepower for efficiency? Or is that not a sensible question?

I thought that if one were making engines more efficient (that is, if engines produced more useable energy for a given amount of gasoline, less energy wasted as heat), one could have less horsepower, and greater gas mileage. Is this a completely incorrect notion?

Of course, this does not take into consideration the weight of the chassis/body/everything else. I assume if a smaller, lighter engine of greater efficiency is mated to a very light chassis/body one can get quite high gas mileage for a given horsepower..

>the frictional heating of the brakes is directly equivalent to wasted gasoline.

You're exactly right about this on normal cars. Hybrids use this energy to recharge the battery--that's why hybrids don't need to be charged conventionally.

You might be right on the acceleration, but consider this: rapid acceleration implies wide open throttle with its reduced pumping losses. Over-acceleration which would require excess braking would consume more fuel, but rapidly accelerating to a steady speed might be more fuel efficient than slower acceleration. Ideally, the way to accelerate is to get in the highest possible gear so that large throttle openings are required for adequate acceleration. Most automatics shift down under those conditions, but I have noticed that my Odyssey is reluctant to downshift unless I give it lots of throttle. And I have never gotten less than 20 MPG, even in town driving.

My approach to getting good mileage is to treat the brake pedal as forbidden territory, and to drive so as touch it as little as possible. After all the frictional heating of the brakes is directly equivalent to wasted gasoline.