WASHINGTON (MarketWatch) — Rep. Paul Ryan says that his plan to slash government spending and reform the tax code would lead to a renaissance in the American economy. But his numbers just don’t add up.

Ryan’s budget plan promises much more in terms of job growth than it could ever deliver.

Ryan, the Wisconsin Republican who’s chairman of the House Budget Committee, says it’s essential that we drastically cut government spending and bring the national debt under control. Otherwise, he says, this once-proud nation will be reduced to the status of Greece or worse. The economy would decline and we’d be faced with “massive tax increases, sudden and disruptive cuts to vital programs, runaway inflation, or all three.” Read Paul Ryan’s budget plan.

In other words, in order to prevent “sudden and disruptive cuts to vital programs,” it’s essential that we make sudden and disruptive cuts to vital programs. The cost of such cuts — in human lives — might be justified if the benefits — in economic growth and stability — are high enough. Are they? It’s impossible to know, because we don’t have all the details of his plan, especially the all-important details about taxes.Read the latest news on the budget battle in Washington.

Let’s put the numbers we do have to the test. Based on an analysis by the Heritage Foundation, Ryan claims that his “Path to Prosperity” would boost U.S. output by a total of $1.5 trillion over the next 10 years. Instead of growing at an average 2.8% annual rate (after inflation), the economy would grow at a 3% rate. That’s significantly better, if it’s true. See the Heritage Foundation’s report on Paul Ryan’s “Path to Prosperity Plan.”

What’s more, Ryan says his plan would lower the unemployment rate — which is now 8.8% — to 4% by 2015 and to 2.8% by 2021, because marginal tax rates would be lowered to encourage more people to work and more companies to hire.

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Lowering the jobless rate to 4% by 2015 would be terrific, but that prediction is implausible. The numbers just don’t add up. Getting to 2.8% would be very nearly impossible, considering that the Federal Reserve would deliberately crash the economy if the jobless rate ever got that low, just like it did the last time, back in 1953.

But it’s not the only implausible prediction Ryan makes, as we shall see.

Magic pixie dust

How does the economy go from 8.8% unemployment to 4% in five years? Magic pixie dust. To do it, the economy would have to create 15 million jobs under the most likely assumptions about population growth and the labor-force participation rate. The economy has managed to create 15 million jobs in five years only twice: from 1976 to 1980 and from 1983 to 1988.

Economic growth in those two periods was much stronger than we expect over the next five years, averaging more than 4% per year compared with 3.1% growth forecast through 2015. In the 1970s and 1980s, women were joining the labor force by the tens of millions. That influx of cheap, willing workers is not going to happen again. Participation rates for women have topped out.

According to the Heritage Foundation’s analysis of Ryan’s plan, most of the improvement in the unemployment rate comes in 2012, when it is projected to drop all the way down to 6.4%.

It’s more economic pixie dust from Ryan, because this amazing drop in unemployment occurs before the new budget really kicks in: In 2012, the expected budget deficit will still be nearly $1 trillion, and the boost to GDP from the budget cuts and tax reforms is expected to be only 0.2% under the most favorable assumptions.

According to Heritage and Ryan, we’ll get an extra 1 million jobs added in 2012 from a 0.2% boost in GDP. That’s about five times as many jobs as would be expected from that much stimulus. Of course, most mainstream economists would say Ryan’s budget cuts in 2012 wouldn’t be a stimulus at all, but would instead be a modest drag on the economy, as federal spending declines and hundreds of thousands of government workers are fired.

We’ll see what the Congressional Budget Office has to say when it scores Ryan’s plan, if he ever proposes one that it can score.

If the 1 million jobs don’t come from demand-side stimulus, where do they come from? Pixie dust! The extra growth trumpeted by Ryan in 2010 comes entirely from an inexplicable 57% increase in investments in housing compared with the current level of home building. Ryan doesn’t explain why cutting the budget would suddenly mean we’d want to build more homes.

Ryan says companies would be much more willing to hire workers if tax rates were slashed and pro-growth policies were put in place. But we’ve been down this road before (remember the 2003 tax cuts?) and we’ve never gotten the growth we were promised when tax rates were lowered. Read my column on how trickle-down economics has failed to deliver.

The 15% solution

There’s one other prediction contained in Ryan’s budget that’s not only implausible, it’s laughable: Ryan predicts that, if his plan were followed, federal government spending would fall below 15% of GDP by 2050. Over the past 60 years, federal spending has averaged 21% of GDP. If nothing is done, the expected growth in spending on Medicare and Social Security would boost government spending to nearly 50% of GDP by mid-century.

Fifty percent isn’t sustainable in our system. That’s a big reason why almost everyone across the political spectrum thinks we’ll have to make some dramatic changes in the budget, once the economy has recovered from the recession.

But to reduce it to 15% is impossible, politically. It can’t be done. People (that is, voters) like their Social Security and their Medicare and lots of other things besides.

The last time federal spending was that small was in 1951, before Medicare, before Medicaid, before the SSI program for disabled adults, before most workers were covered by Social Security. In 1951, just 13 million Americans were over 65, or about 8% of the population; in 2050, it’ll be 87 million, or 21%.

Ryan claims that his budget won’t destroy Medicare, Medicaid or Social Security, but to meet his goal, we’d have to destroy them.

Ryan envisions a nation with untold wealth that lets its old people die in poverty without adequate health care. Such a society could be called many things, but I don’t think “prosperous” is one of them.