(MONEY Magazine) – If you gave yourself kudos for checking your credit report recently, you may have to take it back--unless you checked all three.

A little background: The three major U.S. credit bureaus--Equifax, Experian and TransUnion--pull together data from public records and creditors, and use it to create credit reports for some 200 million Americans. All three bureaus have independently contracted with a fourth company called Fair Isaac, which turns that data into a credit score. (Scores range from 350 to 800 points; scores of 620 and above are considered good.) The bureaus then sell the report/score combo back to creditors, insurers, landlords and employers, who use the information to decide whether to do business with you and on what terms. Because some creditors opt to report to only one or two bureaus, you have three different credit reports. And from those three different reports, you have three different credit scores.

Just how different, we didn't know--until now. The Consumer Federation of America and the National Credit Reporting Agency have compared the three scores of half a million Americans. They found an average range of 41 points per person. The difference between the highest and lowest score was more than 50 points in 29% of cases and more than 100 points in 4%.

Mortgage lenders generally buy all three scores. But, says Fair Isaac's Craig Watts, they don't take an average. Some use the middle score, but those trying to boost business take the high, while those trying to limit their risk take the low--and that translates into dramatically different interest rates. On a 30-year fixed-rate mortgage, individuals with scores between 700 and 719 were paying an average 5.94% in early January. Those with scores between 620 and 674 were paying 7.63%.

Other lenders and insurers typically access only one score. Again, which score they use could cost you. "In some cases, a 50-point difference would have no effect on your auto insurance premium," says Bernie Birnbaum of the Center for Economic Justice, who has studied credit scores. "In other cases, it could double it."

The smart response is to buy all three of your credit scores and compare them three months before you apply for a loan. Unfortunately, that can't be done. You can buy your scores from Equifax (equifax.com, $12.95) and TransUnion (transunion.com, $9). By the time you read this, you'll also be able to buy both from Fair Isaac (myfico.com, $12.95 each). But Experian doesn't sell its scores to consumers. So your best move is to check the underlying credit reports (you can buy all three plus the Equifax score from Equifax or Fair Isaac for $39.95). If there's an error, send a letter or e-mail to get it corrected.

Personally, I don't think you should have to pay $40 to ensure the accuracy of information that's yours to begin with. Eloan CEO Chris Larsen agrees. He's agitating for more transparency, by which he means that every loan quote should indicate the credit score it's based on, "so consumers can really compare apples to apples." He and other lobbyists succeeded in making this happen in California. It's time the rest of the country caught up.