Why can't the world's leading car companies make a vehicle that gets 100 mpg?

That's a question Popular Mechanics asked and they found that many technology innovations already implemented in low-production, high performance racing vehicles and concept cars could soon be feasible in mass-market vehicles. They're not going to deliver an affordable 100 mpg but the road to 75 mpg just might be getting smoother.

Steve Lapp, a professor from Ontario, told Popular Mechanics he's managed to hit 100 mpg --occasionally-- in his 2001 Toyota Prius. How did he do it? He mounted solar panels on the car's roof to keep the batteries charged when the sun is shining.

While that kind of creative thinking is promising, more often than not consumers have been frustrated by the fact that the remarkable vehicles that often sound too good to be true, are generally priced at levels that most of us simply cannot afford.

Major advances are coming in vehicle weight reduction, says Tadge Juechter, assistant chief engineer for Chevrolet Corvette. he says that if you look at exotic vehicles they're all engineered for high performance but the same type of technologies can be applied to meet fuel-economy goals. The carbon fiber that is used extensively in motorsports and exotic cars today offers an outstanding combination of light weight and strength that he says could gain greater presence in mass market vehicles.

Other weight savings that can be gained focuses on glass. PM's report says glass is one of the heaviest components of an automotive body and is heavier per square foot than the most commonly used steel.

Additionally, polycarbonate glazing will be coming to mass production vehicles within the next 10 years, Juechter says. The material is already used to cover headlights. For windows, a plasma process will be used to deposit a very thin layer of glass on the polycarbonate and to an ice scraper or car wash it will look like glass, but it provides a 50 percent weight reduction.

PM says engineers are even looking to shave weight from the vehicle by paring down the hardware, cabling and wiring needed to support the vehicle's electronics. "Given the current state of electronics, it's possible to imagine a car without bulky hardware for audio, video and navigation. Instead, you'd have a thin screen with a port. Many cell phones already carry these functions in them, so there would be a wireless connection to the display screen," says Richard Plavetich, a technical design manager for Nissan Design America.

PM says any weight reduction plan needs to address wheels and tires, and Dymag, a British wheel manufacturer, is already on it. The company has produced a very light wheel with a magnesium center and carbon-fiber outer rim... It costs about 40 percent more than standard wheels but reduces the weight of each wheel by 11 pounds.

Where tires are concerned Pirelli's Steve Carpino says: "Imagine a tire that could sense its environment and adjust air pressure to minimize rolling resistance; essentially a tire with active pressure management. We have our group in Milan looking into technology embedded in tires. It's certainly thinkable."

While 100 mpg vehicles may be doable (in small production), there's little doubt that cars capable of that would still cost many thousands more than today's bigger and more powerful vehicles. Absent additional government regulation or far higher fuel prices driving consumer demand, there does not appear to be a clear business case for it because you'd never make up the cost premium at the pump.

However, working out the real-world requirements for a 100 mpg vehicle makes it clear that a 75 mpg vehicle may be feasible for far less money. The 100 mpg vehicle may be attainable, but if it's going to run you six figures do you really want one?