28 with latex is noticeably less grippy and smooth than 30mm tubeless with 23mm internal wheels. I bet 32mm on 25mm internal wheels is next level!
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This year I traded in an SL7 for an SL8 (I know…). While I certainly like the new bike I am getting slower strava times for the same or greater power, at least on climbs. The bikes are very similar weights and I am roughly the same weight. Key differences are narrower bars, slightly shorter cranks, SES 4.5 instead of Rapide CLX, and most importantly tires: SL7 was 26c turbo cottons and 4.5 is 30 mm GP 5K tubeless. Was running turbo cottons at ~80 psi and I’m running GP5k at ~55-60 psi. lower than that and they feel bouncy and I can see them bulging out beneath me at 165 lbs dressed for riding/17 lb bike.
What else could be causing me to go slower at more power? I think it might be the tires.
Which bike has narrower bars? And are they the same handlebar or different ones? Same power meter?
maybe the wheels? I feel like Rapides always test super fast. Not sure on the Enve’s. Bars might be too narrow causing you to chicken wing. Do your cranks feel ok? Maybe not getting same power from shorter crank. Otherwise I would say mount the 26c turbo cottons and see if thats the culprit
sl8 has 40 cm enve AR bar, levers turned in. I have 39 cm shoulders. SL7 had 44 cm rapide bar. SL7 was Sram/quark and SL8 is ultegra PM. I know those are thought to run high but the difference seems to be more than that to me.
the rapides did feel a little faster but I dont think thats it. Have not nailed the fit on these shorter cranks but meter shows same or higher power at slower times. new PM is shimano and the old was SRAM but I really don’t think its that…
Oh wow, I was about to say damn that is expensive, then I checked my bookmark and they increases the price by 20…I knew that price was too good to be true, but my current ones have some way to go and then I have one more new pair so thought not worth it buying.
Still only about £90 though, they were about £72 just some days ago.
That is some wide wheelset!
Unfortunately mine is pretty narrow and I will try 30mm but I think that is already too wide (22 inner, 29 outer and they say 28mm is best).
£72 for a pair of GP5000 S TR? I really struggled to find any 30mm in stock when buying them before, but I can’t see anything less than £57 per tyre at the moment. If I had seen them at £72 for a pair I would have probably bought them for next year 
Although, I’m tempted to try the Aero 111 at some point. Currently there is only one European distributor, until that changes the prices won’t come down - 120 euros at the moment. Ouch!
That’s only best for aero potential though right? Nothing stopping you putting 28’s on the front and 30’s on the rear. That could make a huge difference in comfort without really compromising the aero benefit of the best fit tyre on the front.
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Yup, but I guess that’s why they raised prices now.
That’s a good point!
Maybe the shimano pm is reding high . Or the sram pm is reading low .
If it’s a dual sided Shimano I wouldn’t trust it to compare to another brand. They are famous for being random number generators on the right hand side.
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wow that feels super cheap. I’ve seen clinchers at that price but not TR. Hopefully with some newer tyres coming out they might drop in price again
What about increasing wheel diameter from larger tires? Is it easier to turn over a slimmer wheel? When climbing under 10-12 mph and pushing under ~80 RPM, does it take more power to turn over a big 30-32 mm tire than it does to turn over a tire that measures a true 28? Have been noticing the bike with 30 mm (32.7mm) tires just look so much taller than the bike with 28 mm (28.8 mm) tires in my garage.
This is almost certainly it… Not sure why you’d think otherwise, but it’s not uncommon for two power meters to measure 5% different or more. Which at a 300w type power output is massive. The recent batch of Shimano PM’s have been shown to be all over the place, and 30w of variability would not surprise me at all. My recommendation would be to go tackle a couple of your past PR’s or KOM’s with full gas efforts, ignoring the PM. Sorry to say, a few watts here or there whether from tires or bars aren’t really going to be tangible, but at least it would give you the confidence that the bike isn’t an “issue”.
What you describe is something @gpl and @dcrainmaker have uncovered in their reviews as well. Basically, the power numbers from the right side are unreliable. Shimano is trying to “fix” the issue by e. g. changing algorithms depending on what chain ring you are in (small vs. big), but it isn’t really a fix. That is especially inacceptable seeing how expensive Shimano’s power meters are.
Quarq power meters on the other hand have, by and large, been very, very reliable. So I’d trust those numbers.