MADISON, Wis. -- President George W. Bush on Wednesday signed legislation that will increase fuel efficiency to an industry average of 35 miles per gallon by 2020.

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But the key word is "average," and experts said it opens up a major loophole in the law.

The law claims to increase the fuel efficiency of cars and trucks by 40 percent. It says that by the year 2020, automakers will be required to produce vehicles with an industry average fuel efficiency of 35 miles per gallon.

But WISC-TV auto expert Matt Joseph said that the law won't actually change much.

"What it appears to say is that everything falls into that. What it says is that you could have a dozen different classes, but they have to average that," Joseph said.

Joseph said the devil is in the details and that automakers will find loopholes because of the word "average" and put their cars and trucks in different classifications to maintain that mandated average.

"Which is to say a 7-series BMW has certain attributes, which say that it's not going to get 40 miles per gallon, and essentially what this bill says is that you can go on building those cars," Joseph said.

He said that more needs to be done on improving overall fuel efficiency -- not just an average.

Joseph said he worries that until gas prices reach upwards of $6 a gallon, which he predicts will happen in the next few years, nothing productive will be done about the nation's dependence on foreign oil, or oil in general.

"We really need to look at making all cars more efficient, getting away from the idea that if you're very wealthy it's OK for you to waste energy in a bigger car than you need," Joseph said.

The bill signed Wednesday is slated to begin in 2011, and automakers have until 2020 to comply with the new standards. The measure also requires a six-fold increase in ethanol use, to 36 billion gallons a year by 2022, WISC-TV reported.

Joseph said he doesn't like the idea because it takes more energy to produce ethanol than what consumers actually get out of it. He said he would rather see more of an emphasis on other alternative fuels like hydrogen.