Disclaimer: This is my opinion. Heed at your own risk.
Some people on the interwebs like to say you shouldn’t leave a car battery on a concrete floor. This was true — back when batteries had wooden cases—something to do with moisture and swelling. Since batteries haven’t had wooden cases since before I was born, and I’m old, it’s pretty silly now.
Some people on the web like to claim you should only use 80% of a tow vehicle’s capacity. I wonder about these people. Do they only drink 9.6 ounces of a beer or drive 52 in a 65 mph zone?
What can be asserted without evidence can also be dismissed without evidence.
Christopher Hitchens
Since I trust Toyota’s engineers more than self-proclaimed experts, I’ve mostly ignored the so-called rule. One reason is what I call proof-by-lawyer: if driving at rated capacity was as dangerous as fear-mongers portray, why aren’t there any class action suits against the manufacturers for bad ratings?
More recently, out of curiosity, I decided to look into it. Where do tow vehicles ratings come from?
It turns out, in the past, the manufacturers just kind of made them up, so naturally, they would spin a little bit to say they would tow more than the next guy, but not so much as to damage their brand reputation.
About a decade ago, the manufacturers realized it wasn’t a good situation and got together to standardize the rating. The result is SAE J2807, which, for a mere $87, you can buy a copy of. Fortunately, we don’t have to because it’s been reported on in the motoring press.
It’s not a cakewalk. The highlight is the Davis Dam test, where the vehicle has to ascend 3,500 ft in 11 miles, starting at 100o F, with the air-conditioning on full blast, and not slow down too much. In relatively cool and flat New England, someone like me is unlikely to stress my tow vehicle that much.
The standards include specifying specific tires, trailer configurations, etc. SRW means a single rear wheel on each side, and DRW is dually. The tests must complete without any warning lights.
A “dually” with four rear tires.
| Test | SRW | DRW |
|---|---|---|
| Davis Dam | >= 40 mph | >= 35 mph |
| Go 5 meters from stop on 12% grade, 5 times in a row | <= 5 minutes | <= 5 minutes |
| Hold full rig with just parking brake | 12% grade | 12% grade |
| 0-30 mph (level ground) | <= 12 sec | <= 14 sec |
| 0-60 mph (level ground) | <= 30 sec | <= 35 sec |
| 40-60 mph (level ground): passing test | <= 18 sec | <= 21 sec |
| 20-0 with trailer brake, staying in 11.5 feet lane, trailer lighter than tow vehicle: braking test | <= 35 ft | <= 35 ft |
| 20-0 with trailer brake, staying in 11.5 feet lane, trailer heavier than tow vehicle:: braking test | <= 45 ft | <= 45 ft |
| 20-0 without trailer brake, staying in 11.5 feet lane: trailer brake failure test | <= 80 ft | <= 80 ft |
| understeer test: increases from 0.1 to 0.3 g on 300 ft circle | ||
| sway test, 62 mph | >= 1.0 damping ratio | >= 1.0 damping ratio |
I don’t fully grok the understeer and sway test metrics, but they break down to the tow vehicle stays in front of the trailer and the trailer gets back in line quickly.
Here are some videos I found helpful:
- The RVProject has a quick 5 minute video that gives an overview, including a discussion of how much the weight rating of an example vehicle dropped when the standard was adopted.
- There are two videos based on the the same test demonstration
- Engineer Explained provides more detail and numbers in 13.5 minutes.
- TFLTruck Run has peppy music, a lot more video of the actual tests and some interviews with engineers. Runs 16:40.
Bottom line
The 80% rule is prudent if you have a pre-SAE J2807 tow vehicle, do not want to be meticulous about weighing your vehicle and trailer, or just like having an extra margin.
If you are good about understanding and staying below SAE J2807 rated limits — that means taking your rig to the scales from time to time — there’s no good reason not to use all the tow capacity you paid for. Ignore the fear-mongering of the “experts” who can’t produce data to back up their claims.
Credits:
Motor Trend description of tests.
Dually photo by Kevauto.