Why Tow Ratings are Different in the EU vs. the US

A question that has come up in regards to not just Prius, but other car models as well, is answered in this article. And also concisely by a poster in one of the gen4 or Prime threads, whose name I cannot recall, sorry.

Tow me down!
"The most important number to look at is the % of weight over the "nose" or tongue for the UK{4% to 7%} vs the US{10% to 15}. In the UK the thinking is that if you reduce the tongue weight you can pull more without risking maxing your GAWR out or nosing up the vehicle dangerously and as a result the trailers have their mass much more centralized over the axle which means that the only thing the tow vehicle needs to do is pull in a line, which is less stress on the components. Sounds good right?

However, if you're still awake you will recall that at low tongue weights with the trailer mass more closely centered to the axle you have a much increased risk of destabilizing oscillations. Given that the trouble really starts, according to the math and the real world example, around 65 mph its no wonder that the UK has a strict limit on trailing and why Jeremy Clarkson has a rational hatred of caravans.

To boil it all down, the reason that American vehicles are rated to tow less is based on a difference methodology for tow safety.

US tow safety prioritizes a naturally safe dynamic situation so that more people can pull at freeway speeds with less likelihood of incident. There is no doubt that using this method results in an inherently safer tow situation as it relates to vehicle dynamics.

UK tow safety allows for the possibility to carry more weight, but erring on the side of caution with far less safety margin in respect to the physics."

The author actually compare the licensing and trailer requirements in the UK to that of Utah before the quoted bit, with a decent explanation of the science that goes into trailer dynamic in the beginning.

The UK, and likely most of Europe, allows the tow rating by allowing trailers to have a more unstable distribution of weight. That lower physical safety is countered by requiring increased driver licensing and trailer brakes at lower weight limits than in the US, and a max towing speed of 60mph on the highways.

In regards of the gen4 Prius, yes you could try to tow that 750kg tow limit in the US. However, load the trailer as recommended here, you'll be putting more weight onto the car's rear axle. This reduces the payload and even passengers you can safely have inside the car. Increases the weight on the rear axle, possibly over its safe rating, and decreases the weight on the front axle with the risk of reducing the traction of the front wheels.

Load the trailer up as recommended for Europe, and you have to stay off the highways and interstates. If you don't want to be an nuisance to others on them by driving slower than even you would for fuel economy there, or risk their lives and your own when your trailer starts wildly swinging side to side as you drive to keep up with traffic.

For added info, this is the SAE standard for determining a car to big truck tow rating that GM, Ford, and Ram (don't know if any others have agreed to it or not) have all agreed to use, J2807: Performance Requirements for Determining Tow-Vehicle Gross Combination Weight Rating and Trailer Weight Rating - SAE International , and an article that goes into it for those that can't get pass the SAE paywall, SAE J2807 Tow Tests - The Standard.

 

Not really.
NASA wasn't confused about unit conversions. The real issue was communication break down between the project groups developing the Mars orbiter.
The software code wasn't written from scratch, but reused from an earlier, successful mission. The new craft had one solar panel versus the two on that earlier one. So course corrections where needed enroute to Mars to compensate for the solar winds pushing the craft evenly. The guy that wrote the program for that did it right, and the software from the first craft contained the subroutine for the unit conversions, but no one told him that another group of the project had removed that subroutine.

Back to the Prius and towing. The difference between US and UK approach to trailer weights and towing doesn' explain why Toyota doesn't recommend hitch mounted bicycle rakes for the gen4. Going by the UK tongue weight limits, the car's frame should be able to handle 52.5kg, or about 115 pounds without issue. The CURT hitch and basic 4 bike rack is 71 pounds, leaving 44 pounds for bikes.

 

I didn't see the different unit aspect to the tow issue.

While the different approach to tow ratings can lead to different outcomes dictated by hard science, the reason for the different approaches starts with the different driving cultures.