Everyone across the political spectrum admits that the white-black test score gap is a major social problem, but nobody is thinking about the white-Hispanic test score gap, even though we have much more influence through immigration policy over whether Hispanics will be a large or huge proportion of the American population in the future. Fortunately, the facts are available, but they take a lot of digging to uncover.
Here's the best estimate I've yet seen: A 2001 meta-analysis of 39 studies covering a total 5,696,519 individuals in America (aged 14 and above) came up with an overall difference of 0.72 standard deviations in g (the "general factor" in cognitive ability) between "Anglo" whites and Hispanics. The 95% confidence range of the studies ran from .60 to .88 standard deviations, so there's not a huge amount of disagreement among the studies.
One standard deviation equals 15 IQ points, so that's a gap of 10.8 IQ points, or an IQ of 89 on the Lynn-Vanhanen scale where white Americans equal 100. That would imply the average Hispanic would fall at the 24th percentile of the white IQ distribution. This inequality gets worse at higher IQs Assuming a normal distribution, 4.8% of whites would fall above 125 IQ versus only 0.9% of Hispanics, which explains why Hispanics are given ethnic preferences in prestige college admissions.
In contrast, 105 studies of 6,246,729 individuals found an overall white-black gap of 1.10 standard deviations, or 16.5 points. (I typically round this down to 1.0 standard deviation and 15 points). So, the white-Hispanic gap appears to be about 65% as large as the notoriously depressing white-black gap. (Warning: this 65% number does not come from a perfect apples to apples comparison because more studies are used in calculating the white-black difference than the white-Hispanic difference.)
Source: Roth, P. L., Bevier, C. A., Bobko, P., Switzer III, F. S. & Tyler, P. (2001) " Ethnic group differences in cognitive ability in employment and educational settings: a meta-analysis." Personnel Psychology 54, 297–330.
This fits well with lots of other data. For example, Hispanics generally do almost as badly on the National Assessment of Educational Progress school achievement tests as blacks, but that average is dragged down by immigrant kids who have problems adjusting to English. The last time the NAEP asked about where the child was born was 1992, and Dr. Stefan Thernstrom of Harvard kindly provided me with the data from that examination. For foreign-born Hispanics, the typical gap versus non-Hispanic whites was 1.14 times as large as the black-white gap. But for American-born Hispanics, the gap between non-Hispanic whites and American-born Hispanics was 0.67 times as large as the gap between non-Hispanic whites and blacks, very similar to the 0.65 difference seen in the meta-analysis of IQs.