audiomagnate

My bike is 11 years old and was built for 23s. I tried a wheel mounted with a 25 last year at my LBS and it rubbed up front (there's plenty of room in the rear). For the heck of it I tried 25mm Gatorskins today (don't attack me, I need them for where I ride) and lo and behold they fit (just barely) and the improvement in ride is massive!

audiomagnate

MORE DATA:
"Just barely," equaled about 1mm top clearance up front, which was beginning to cause intermittent rubbing, so I tried out a pair of 25mm 5000's. There is now a HUGE amount of clearance all around, about 2.5 mm. The fact that two 25mm tires from the same manufacturer vary so widely was a little surprising.

Branko D

Continental made the GP5000s more narrow than GP4000SIIs and other older Continental tires, or as they say "more true to size on modern wide rims".

Pretty clever way to make the tire more aero and lighter, make it nearly as narrow as the old 23 and write it's a 25

DangerousDanR

Just for giggles I went out to the garage and measured some tires:

Tire Nominal Actual
Conti GP4000 25 mm 27.5 mm
Conti GP5000 25 mm 28.3 mm
Conti GP5000 32 mm 32.2 mm
Schwalbe Pro One 25 mm 27.5 mm

The GP4000 and the Schwalbe are on DT Swiss RR 440 rims which are advertised as 19 mm.
The GP5000 X 25 are on Rolf rims which are advertised as 19 mm.
The GP5000 X 32 are on House of Tandems Tandem AC 3.2 wheels which are advertised as 18 mm. Take this for what it is worth.

TrackSmart

Yeah, tire manufacturer's have been making their tires smaller for the same labeled size as past years. GP4000 tires in size 28mm are typically 30-31mm on modern alloy road wheels and about 32mm on wide carbon rims. GP5000 tires in 28mm on those same wheels measure much closer to 28 or 29mm on those same wider rims.

Some brands haven't even waited for a new model year, which is really confusing! I have GravelKing Slick tires in "35 mm" and "38 mm" sizes. They are EXACTLY the same size tire. Panaracer just relabeled the new tires to 38mm because really dumb folks in the bike press would say things like "the GravelKing tires are a good choice, but we wish they came in a 38 mm width" while comparing them to tires that were exactly the same size, but labeled "38 mm".

There may be good reasons for this relabeling, even if it is confounding. A friend understood that his bike was rated for 28mm tires, so he purchased 28mm GP4000's at a good price (since the GP5000s had just appeared). Unfortunately, he had also upgraded to wide carbon wheels and rubbed away about 1mm of carbon fiber from inside his frame before he realized the tires are too big. After a week of expansion (tires get bit larger until they reach full size) I measured his "28 mm" tires as 32mm with my calipers! FYI: I warned him that the GP4000s would run much larger than stated, but he purchased them anyway. Now he believes me. Doh!

Now what would be better than all this stupid renaming? In addition to the stated size, manufacturers should also list the typical tire size on a range of rim widths (based on the internal width of the rim).

Example (which is probably close to reality):