I'm a car enthusiast, and for as long as I can remember I've followed developments in the auto industry -- particularly developments in high-performance cars and American car companies -- very closely.

Back in 1990, I was all excited by the news that GM (NYSE:GM) would produce a special, super-limited-edition Corvette with an exotic engine that would allow it to hit 180 mph -- hot stuff in those days. This Vette, the ZR-1, had everything to make well-heeled car guys like me drool.

The ZR-1 had a sticker price of just under $60,000, but the first ones went for well over that -- six figures in some cases. GM only made about 3,000 the first year, and many were snapped up by collectors.

A year or two later, I met one of those collectors. He had paid close to $100,000 for his 1990 ZR-1 and had put it in storage without driving it, the sticker still on the window, and the plastic still on the seats.

"That's my retirement fund," he said when he showed it to me. "That car will be worth a million bucks someday."

Out of gas
Fast-forward to 1997. I was almost 30, my girlfriend and I were talking about marriage and a house in the suburbs, and I had just gotten a big promotion. I decided to buy a nice sports car while I still could. After looking at a bunch of different cars, I stopped at a small dealership that specialized in used Corvettes.

I never expected they'd have a ZR-1. More to the point, I never expected they'd have a nice ZR-1 that I could afford. But there it was. Ruby red, a 1990 like the one I'd seen years ago, clean as a whistle, one owner, 21,000 miles, ready to go. It had been bought new by an NFL player for a sizable premium over sticker.

I bought it for a lot less than a million bucks -- $28,000, to be exact. You can find nice ones today on eBay (NASDAQ:EBAY) for well under $20,000. And Vette experts warn folks to stay away from the ones that were "put away" because they typically need expensive reconditioning before they can be driven. Those cars, I hear, sell for peanuts.

So much for that guy's retirement fund.

Check the attic
Not every hot collectible -- or even every hot new collectible car -- works out like the ZR-1, of course. There are some old Vettes that sold for $5,000 new and are now worth half a million dollars or more.

Plenty of similar stories abound, about art, antiques, jewelry, coins, stamps, and more. And interest is zooming: The popularity of PBS's Antiques Road Show has thousands of people wondering whether Grandma's old oak dresser might be worth a fortune. It has also driven prices up across the board, some antiques experts say, and many corners of the art, coin, and old-car markets are at or near all-time highs -- as is stock in Sotheby's (NYSE:BID), the only major collectibles auction house still public.

If you're lucky enough to have a Picasso gracing your wall, or to find Uncle Louie's old Ferrari race car in the back of the family barn, that's great news. But as lead analyst Robert Brokamp points out in this month's issue of Motley Fool Rule Your Retirement newsletter, available online at 4 p.m. ET today, collecting your way to a secure retirement is a very risky strategy.

Viewed dispassionately, problems with such strategies are clear: Markets often aren't terribly liquid, and valuations are subject to whims and fads and fluctuate wildly. Unless you're a professional car broker, numismatist, art dealer, or similar expert, it's difficult to know the true value of a particular item, and predicting the direction of the market can be a crapshoot even for experts. Valuing a painting or that old Ferrari in the barn is a trickier, and less precise, discipline than valuing a stock. As a cornerstone of your retirement portfolio, collectibles are usually a poor choice.

Finding the middle ground
All that said, it is possible to make money from a hobby, and it's reasonable to think of a collection or a work of art as part of your overall portfolio. Brokamp stresses that collectibles tend to be poorly correlated with the stock market, which makes them a useful way to diversify risk.

But thinking of your collectibles as investments, rather than as knickknacks, prized decorations, family heirlooms, or fun to drive on weekends, requires a different mind-set, and Brokamp's article holds several suggestions that merit serious consideration.

And by the way, I did enjoy driving my ZR-1, without any illusions about it funding my retirement.

You can read the entire article and access our Rule Your Retirement library -- with back issues, discussion boards, and a unique set of planning tools and calculators -- by clicking here for a complimentary 30-day trial. There is no obligation to subscribe.

Fool contributor John Rosevear sold that ZR-1 years ago. He misses driving it, but he doesn't miss the unending repairs. He does not own any of the stocks mentioned in this article. eBay is a Motley Fool Stock Advisor recommendation. The Motley Fool's disclosure policy has a huge collection of Beanie Babies and can't wait to cash in and head for Vegas, baby.