Here's what myFICO.com says:
Looking for a mortgage or an auto loan may cause multiple lenders to request your credit report, even though you're only looking for one loan. To compensate for this, the score counts multiple auto or mortgage inquiries in any 14-day period as just one inquiry. In addition, the score ignores all mortgage and auto inquiries made in the 30 days prior to scoring. So, if you find a loan within 30 days, the inquiries won't affect your score while you're rate shopping.
It's hard to apply that exact science to real life situations though, as often within 14-30 days items such as payment history, credit card balances, and other items fluctuate.
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