Any idea how they come up with ratings? I'm pretty sure I've heard of instances where they had something rated for X but slashed that number (without any actual changes) to get into a different vehicle class for tax reasons or something. I bet low ratings are due to some strenuous test... stopping distance from 60mph and max brake temp or something.
SAE Standard J2807: It's a 30 page engineering document for anything under 14k lbs GVWR. (And as Engineers get more creative about passing the test, it gets revised to be more stringent).
Summarized:
Test requires a 150lb driver, 150lb front seat passenger, any option that adds weight found in 30% or more of vehicle i.e. if full size spare, power windows and a/c are found in >30% of vehicles, test vehicles need to have them. And there are some standards for hitches - no un-obtainum here. And there are standards for the trailers (Originally, they were Wells Cargo Utility trailers or equivalent) I think they allowed as light as 10% tongue weight
Acceleration: On level ground, 0 to 30 in <12 seconds, to 60 in <30 and 40 to 60 in <18 seconds. Launch up a 12% grade both forward and reverse (and repeat 5 times in succession).
Deceleration: Stop from 20 mph within 80' Parking brake must hold combination on 12% grade
Equivalent simulation to Davis Dam grade (3,500' in 11.4 miles) must maintain 40 mph, starting temp 100 F with A/C on max cold/fan on max.
There are some handling tests- think going around a clover leaf. i.e. no under/over steer while cornering at 0.4 gs
There is also J684 trailer couplings and hitches, which surprisingly, still doesn't recognize Class V (or VI) hitches.