I've got a set of older Alluminum Shamals with 40mm deep clincher rims.

I've been toying with the idea of running 19-20 mm tires on these since I read somewhere you need a narrow tire to get full aero benefit.

I am currently running 23 mm Vred Tri Comps and these have a smaller diameter than a 23 mm Conti GP 3000 but they do stick out a bit on the Shamals so my assumption is they could have a negative effect on maximum aerodynamics.

I've noticed that good tires in 19-20 size are not very common. I've seen Conti's and Paves in 19 or 20 size. I've been thinking about a set of Conti Supersonics.

They are race only wheels for flat and rolling hill courses (weight not a big issue). It will be a mix of TT, road, crits and ovals. I like high pressure for racing so I'm assuming a 19-20 will support my weight at higher pressures. I weigh 170# (hoping to be down to 165 for next season).

What are the thoughts, stick with the Vreds or spring for the supersonics? Other options?

PS: If I do well this year with them I'm going to save up for a good set of tubular carbon wheels.

I read on a triathlon site a few years ago that someone did a study and found the most aerodynamic tire for his deep rims was a 25mm. Something about the profile of the tire and the profile of the rim in unison. A narrower tire wasn't as smooth somehow. They might have linked to a Cobb site study, or a Michigan tri bike shop near Dearborn, or else my memory is whacky and just ignore me.

With the tire wider than the rim, the air will flow more smoothly past the tire and then from the rim. If the tire is the same width as the rim, there is a discontinuity at the tire/rim interface and so more drag. John Cobb did a study on this several years ago.