I’m getting ready to set up my ENVE Foundation 45s for the upcoming road season and debating whether to run 28s or 30s. Either tire would be Continental GP5000 S TR, running tubeless. Wheels are going on a Cannondale CAAD 13, so sticking to the manufacturers maximum size of 30mm.
Thoughts?
What are your priorities? Compoert, rolling resistance?
I’m focusing on long Gran Fondo type events, so looking to balance both. Ideally minimize rolling resistance without sacrificing comfort.
I say 30mm without hesitation. I don’t ride anything smaller than 32 on my road bike anymore and I’ve seen no ill effects, only positives. You’ll add maybe an addition 30-40g to your bike. Other than that I can’t think of any drawbacks. And I think the 30-40g is negligible.
As a followup, the whells currently on the bike (stock DT Swiss RB470) have 28mm GP 5000 S TR with latex tubes)
Jeff from NC Cycling did a video on this recently. Worth checking out, FYI it supports daness40’s point.
That’s one of the reasons that im asking the question.
I have 30mm GP5000 on PCW Dual 5550. Used to run 28s. I am so happy I switched and still question what took me so long.
I’m an avid gran fondo rider myself, and I love hitting rural areas for long weekend rides. My bike originally came with 32c tires, but I’ve occasionally switched to 28c tires since I mostly ride on paved roads. However, many of the roads I encounter are less than perfect, often cracked and rough. In these areas, I find myself riding on very rough paved roads for at least 10-15 kilometers, and the 28c tires can be quite uncomfortable. I’d prefer to ride with 30c or higher size tires.
There have been instances where my bottles dropped from the cage as I hit a small pothole that I couldn’t avoid. I believe this wouldn’t have happened if I were using 32c tires.
Additionally, Jeff from NorCal cycling posted a video confirming (at least my assumptions) that 32c tires are actually faster than 28s, so it doesn’t seem like you’ll lose any speed by switching.
Idk I’d doubt Jeff’s tweet are accurate at all
Edit:
Well I guess the graf don’t lie. I’d wonder what the aero penalties are
Remember that Jeff used a wheel that is 35mm wide—among the widest wheels on the market. The Enve Foundation is 28mm wide, so while you’ll likely realize the rolling resistance improvements that come with a wider tire, you’ll likely have increased drag and less stability in cross winds due to the light bulb shape of the tire and wheel combination.
Might be helpful if you have’t already seen this.
That may be worth a trade-off for some.
You wouldnt run the same pressure on both widths though? It would be interesting to see how much the gap closes when the tires are both inflated to the optimum pressure for rider weight.
For arguments sake - you might run the 23@120psi; 25@100psi; 30@80psi 32@60 (actually might even be realistic 
) - then the 23mm are the faster tyre (on mythical perfectly smooth roads)…
We feel the ultimate test is adjusting all tires to the same comfort level. When all tires are adjusted to the same comfort level, rolling resistance is nearly the same (0.2 watts max) for all sizes of the GP 5000.
While you might have a slight aero penalty with the wider tires, it probably won’t matter too much to most riders. 
Oh I do love the feeling when someone who actually knows what they’re talking about agrees with you 
Everyone above is correct. 30 (or possibly wider) is the better option if you can match the wheel profile to your tire size. You’ll pay an aero penalty for going wider than your wheel though. It’s not enough to rule out other considerations but could make a difference in certain conditions.
My vote is 28 front 30 rear. Front wheel takes a bigger aero hit, and rear wheel carries more weight (and therefore transfers more vibration). Do check first and make sure a 28 actually inflates to 28 on the Foundation rims - a bit of searching should find someone that’s measured that combo (or at least on a wheel with the same internal rim width).
I change my vote to 30/30 if you anticipate riding in tight formation over bad roads for the bulk of your events - in that case you’re drafting more and also less likely to be able to pick an ideal line so extra cushion up front becomes more valuable.
I vote 23mm and some better cushioned bibshorts…
Then rolling resistance becomes the same within a fraction of a watt at 18 mph. Thus same rolling resistance with superior comfort from the wider tyre.
