You might try plausible. From NOAD:
plausible |ˈplôzəbəl| adjective (of an argument or statement) seeming reasonable or probable : a plausible explanation | it seems plausible that one of two things may happen. See note at believable.
Or look at the usage notes under believable for a synonym:
THE RIGHT WORD
Believable is the most general of these terms, used to describe anything we accept as true, even in the absence of absolute proof (: a believable story about why she was late).
Credible also means worthy of belief or confidence and is often used interchangeably with believable, but it goes one step further: a credible excuse is one that is supported by known facts.
Creditable, often confused with credible, at one time meant worthy of belief but nowadays is used to mean respectable or decent, deserving of honor, reputation, or esteem (: leading a creditable life).
Something that is convincing is believable because it overcomes doubts or opposition (: a convincing performance), while something that is plausible may appear to be convincing or believable on the surface, but may not be so upon closer examination.
Valid means legally sound, just, or authoritative; a valid criticism seldom provokes opposition.
Cogent, on the other hand, means having the power to convince; a cogent argument is believable because of its clear, forceful, or incisive presentation.
