While what you describe involves a comparison, it would not involve two samples of at least 30 observations.  There is...

While what you describe involves a comparison, it would not involve two samples of at least 30 observations.  There is data on all cars by category.  I think if you choose the category with the most types of cars in it you could do a study like the average mpg of US cars vs foreign brands.  You might have to use two model years to get at least 30.