Hmm, looks like no one in this thread has actually taken an I.Q. test before (or didn't remember what was in it).
I.Q. tests cover a lot more than people give them credit for, though they of course have their weaknesses. One misconception is that they only measure one type of intelligence. This is untrue. In fact, there are two main groups and four subgroups of intelligence covered by the WAIS I.Q. test. If anyone's interested, click this: http://upload.wikimedia.org/wikipedia/en/thumb/d/d2/Wechsler_Adult_Intelligence_Scale_subscores_and_subtests.png/650px-Wechsler_Adult_Intelligence_Scale_subscores_and_subtests.png. Anyway, I won't go into too much detail, but other than its inability to gauge social intellect and cultural biases in the knowledge portions of the test, the WAIS IQ can tell you a lot about how a person thinks.
Luffy... would have a lopsided score. In other words, Luffy would score highly on some parts of the test but do terribly on others. A lot of people here have said or hinted at this in one way or another, but let's break it down quickly here:
- Verbal IQ - Verbal Comprehension: He'd do poorly. Especially his vocab and information tests would be a loss (he STILL doesn't know all the Shichibukai), but he'd do alright on comprehension.
- Verbal IQ - Working Memory: His worst sub-score. Arithmetic? Letter number sequencing? Remembering someone's name for more than 3 seconds? A dog would likely score higher in this category.
- Performance IQ - Perceptual Organization: His best sub-score. His strong visual reasoning are what make him able to spot attacks and hidden objects with ease. Might have a slight problem with matrix reasoning but would score very highly overall.
- Performance IQ - Processing Speed: Also strong. Might be hard to make him understand digit-symbol coding, but once does, he'll destroy it.
Overall score: ~100 but heavily skewed, very high performance IQ, very low verbal IQ. Luffy has a very fast moving mind with very little ability to retain non-visual information (faces, not names). Tack on a social IQ test (he'd do well but wouldn't ace it, tests don't measure charisma) and you have a good portrait of a nigh-illiterate force of nature.
Put Luffy in our world (sans powers)? Holy shit this kid would be on soooo much ritalin. He'd probably end up an Olympic wrestler or an MMA fighter. Or in prison.