What would you do if a used-car salesman told you it's against the law to check the odometer before buying? You'd shake your head and walk straight out. Similarly, investors stalked out of AmeriCredit ( ACF) stock Wednesday after the auto lender failed to offer important data in its fiscal fourth-quarter earnings statement. Improbably in this disclosure-obsessed era, the company claimed it wouldn't share the information because of regulatory guidance. The most notable omission was AmeriCredit's figure for the effective size of its bad loan reserve. The reserve number is critical for investors attempting to gauge the amount of financial cushion a lender has against bad loans. It's especially important when the lender, like AmeriCredit, is experiencing a sharp rise in problem loans. The omission could imply that AmeriCredit, which targets borrowers with tainted or incomplete credit histories, has failed to sufficiently reserve against bad loans. Meanwhile, cash continued to circle the drain in the latest quarter and credit quality deteriorated, pointing toward a possible squeeze on the company's liquidity. AmeriCredit's already-battered stock lost a quarter of its value Wednesday, falling $4.05 to $12, a 52-week low.