Over the years, I have frequently come across the ERTO tire width chart on Sheldon Brown's website with which most of you are undoubtedly familiar

Tire Sizing Systems

And then, just today I was looking at the specs of the Hed Jet 6 Plus

https://www.hedcycling.com/jet-6-plus/

...which lists the internal rim width as being 21 mm. So according to that chart, that would mean that the narrowest tire I should use on a Jet 6+ would be 35 mm. But, the Hed-recommended tire size is 23 mm

That has got to be wrong (the chart, I mean, not Hed's recommendation).

Now I realize this chart is merely for recommended widths. The note right under it says that it errs on the side of caution. but these two width numbers are so far apart that I feel certain I must be missing something obvious.

Could someone with a clearer understanding (or better eyes) tell me what I am missing?

HED recommends a 23mm tire for the Jet because tires of that size will combine with the external profile of the rim to form a nice aerodynamic shape.

The ETRTO chart is very conservative, and also fairly old. My guess would be that the minimum-width recommendations are based on assumptions which simply don't apply in current wide-rim road use cases, such as that the 21mm internal-width rim might not be designed to handle road inflation pressures, and that a user will pump their 23mm tire to 130PSI.

That chart is useless for the newer rims. These rims are wider because as HTupolev stated, it's for aerodynamic purpose, to reduce the transition from rim to tire. The bulbous shape of the tire is where most of the turbulance/aerodynamic issues occur on the wheel. That's why the rims are getting wider, 27mm rims with 28mm tires. Of course the wider tires have the added benefit of reduced pressure = comfort and wider surface area = reduced rolling resistance. For 28mm tires for example, I would definitely look at minimum 25mm wide rims.

Of course these are not set rules, if you want to ride 32mm tires on 21mm rims, and it works, go for it.

OK, great that makes a lot of sense. Thanks

The aero rims are a whole new ballgame that the Sheldon Brown chart predates. The HED recommendation for 23mm is to achieve the aerodynamics the rim was designed for.

Also to put a spin to the mix, HED Jet 6+ wheels with a 25mm tire are actually super aero and fast with low low rolling resistance. I think you can find their wind tunnel experiments

The chart on the Sheldon Brown site has been there forever. Tires and rims today are different than back then. The chart was also just a guideline for when you lacked specific recommendations. The HED site gives you a specific recommendation.

Sheldon's chart is still useful, but remember it has to be very conservative because it doesn't account for variables of rim type, variable of tire bead type, etc.