In absolute terms, you'll likely see an aero penalty for going to the 30mm tire, especially if it exceeds the 105% of rim width. Rolling resistance is likely a wash if you adjust pressures appropriately. Whether it's worth it depends on your intended use for the bike. If you want absolute speed, stick with the 26mm tires. I know personally, I went with 30mm Schwalbe Pro One tires, at ~50-55 PSI (I weigh 167 lbs). I know I'm paying a bit of an aero penalty, but the rolling resistance is still pretty good and for the bulk of my training miles the extra comfort is worth it.

I rode a bike with 23C last night for the first time in ages after a lot of time on 38C gravel tyres and 28C road tyres. It felt like I was riding on the rims, it was brutal. Can safely say I wouldn’t go back, regardless of any aero penalty.

Today was the first time riding on a pair of 25mm tires on a set of HED Ardennes. They measured 28.5mm wide and they rolled great.

I am guessing you would not notice any difference in speed but the difference in comfort would be noticeable. I have the 28mm GP4000 which measure 30mm. With a latex tube, they're a great tire.

Continental makes the GP5000 now in a 32 width. It actually tests faster than the narrower widths. Also the mfg wind tunnel testing really is complete hogwash. The 1.05 rule is really bullshit. In my field test absolutely no differences between circa 2008 Zipp 808s and their latest 808NSW. Sure it matters in wheel only testing but add a rider and changing yaw conditions and differences since 10 years ago really don’t matter.

Just a way to sell more new shit to people.

Does a potential minuscule difference in speed on everyday rides outweigh comfort, handling and stability?

If the higher width tangibly improves your smoothness and confidence on the routes you're riding, it will probably improve your performance. Few performance losses are bigger than being hesitant and lacking confidence.

As far as performance on smooth roads goes, your bike will most likely be a smidge more aero on 26s than 30s. To what degree are you trying to optimize this? You'll probably be able to pick up a difference in the wind tunnel, or maybe if you do some careful Chung work, but the difference is very unlikely to be casually noticeable.

The really important thing is using a fast tire.
When I've got my 53mm performance-oriented slicks on my gravel bike*, my speeds on paved rides are typically within a mph or so of what I achieve on my skinny-tired road bikes with similar efforts in similar conditions. But I have some heavily-armored ~45mm tires laying around, and if I throw them on the gravel bike, it becomes more like 2+mph slower than the road bikes... the bombproof tire is far slower than the performance-oriented tire, despite being narrower.
If you switch from your stock 26mm tire to a super-tough wider tire, like a 28mm Marathon Plus, you'll probably end up going very noticeably slower. But depending on how fast your stock 26mm tire is, there's some chance that switching to a really fast 30mm could actually speed you up a little.

*With latex tubes, lol.

Does DT Swiss make tires?

I think the OP was looking at the rim decals when they said "DT Swiss." A lot of Tarmac models come with R470 Disc rims.

Depends on what you want to do. Just ride around and be comfy, then pick the wider ones. They won’t be noticeably slower at normal speeds. Or even in a hammer fest group ride.

But want to go KOM hunting or racing where you could be going 30+ mph, go with the more aerodynamic option.

As a first step, you could try reducing the pressures on your existing 26mm tires.

I've the same bike.

It's a DT Swiss R470 DB (= disc brake) rim. On Spesh's own hubs.
The rim is 20mm internal, 24mm external, amd 23mm deep. Alu.

They come with Spesh's Turbo Pro tyres at 26mm nominal. They measure up at 28mm real width on this rim. (Tubeless ready rim but Non-tubeless tyre, so come with a bog std butyl tube).

How about you install a set of continental 5000 700x32 tires on the rims and report back what you find. This form is getting pretty boring with no racing going on so we need some excitement!

What's your bike+rider weight?

Bike speed won't necessarily be compromised by a pressure reduction. If you're currently experiencing a rough ride, you may even go faster: smooth is fast, and if road irregularities are vibrating yourself and your bike, that's a waste of your forward momentum. From a performance standpoint, it's actually much better to be a few PSI too low than a few PSI too high.

Slight aero hit but something like a 28mm GP5000 or a 30mm Schwalbe One will roll quite fast... and comfy.

I think 95PSI would be reasonable for someone of your weight on 26mm tires on very smooth roads. In rougher conditions, you could get away with significantly lower.

It certainly doesn't make sense to be running 26s at a 35% higher pressure than 28s for similar riding circumstances. That's a huge difference in pressure for such a tiny difference in width.

Continental GP5000 clincher tires are available in 23mm, 25mm, 28mm, and 32mm.

Continental GP5000 tubeless tires are available in 25mm, 28mm, and 32mm.

Yep that was a typo on my part I meant 700 x 32, that’s what I rec’d earlier in the thread for him to get.

At 175lbs, and assuming a 45%F/55%R weight distribution, 95 psi sounds OK (perhaps you could go a little lower?) for the rear tire at 26mm, but definitely high for the front tire.

I don't have direct experience with them, but someone else in this thread has suggested that the tires in question measure closer to 28mm than the nominal 26mm, in which case you can definitely afford to go to lower pressures.

TL;DR - try 70-75 psi front, 90-95 psi rear.

This is not correct.

When narrow tires are properly inflated and when wide tires are properly inflated (so they won't be and shouldn't be at the same pressure), all other things being equal, the wider tires wiil always have more rolling resistance (and more aero drag too). And this is to be expected, it is simple physics.

(All of the "wider tires have less rolling resistance" stuff that is spewed is all based on the wider tires being MASSIVELY overinflated (overinflated for their size) which would make the wider tires essentially unridable as, among other things, they would feel rock hard to the rider.)

---

Advanced Aero TopTube Storage for Road, Gravel, & Tri...ZeroSlip & Direct-mount, made in the USA.
DarkSpeedWorks.com.....Reviews.....Insta.....Facebook
--

Roads where I live are a bit rough (tarmac not concrete, and no expansion joints - just not nice and smooth (old tarmac)).
I'm north of 200lbs. With the 26mm Spesh tyres on those rims, I'm running 75psi rear and 70 front.
If I went to 95psi i'd be battered to death and need all my fillings replacing.
(I often think people can mistake 'feeling fast' for 'being fast'. A go-cart feels fast because of sod all suspension and yer arse an inch off the ground !! Whereas a 186mph train doesn't because it's smooth)

I ride a Venge with 28mm GP5000 TL (measures 29mm).
I weight 78kg/172lbs and run them at 4.5bar / 65psi, but the roads around here are rarely smooth.