Bike Forums - 700x28mm tyres

Tires vary in how true they are to advertised dimensions. As an example, the Continental Grand Prix 4000s II in a 28mm size tends to run larger than specified. On the other hand, the Continental Grand Prix 4 Season and Continental Grand Prix 5000 tend to run closer to specification in size.
Dimensions once mounted on a rim will also be dictated by the rim's inner width. A wider rim will result in a wider mounted tire.
Tire age and pressure will have some effect on the mounted width. As a tire ages it is possible (though not universally the case) that the casing will relax a little, allowing it to fill out a little wider.
Wheel stiffness, and frame stiffness in the rear triangle will have a bearing on whether a tire mounted in back that seems ok will actually work out ok under riding conditions.

All these factors come together to make it nearly impossible to know whether a given tire will be ok on your bike, if you're cutting things close. If the frame is made for 25mm tires, but you find that a true-to-spec 28mm tire mounts ok, you're lucky. So start with what the manufacturer says will work.

The best I've been able to come up with for my own purposes is to do the following:

Measure my existing tire. Let's say it's a 25mm tire. Measure my remaining clearance in various points along the fork, frame, and both open brake calipers and closed brake calipers. Lets say the tightest point I can find has about 5mm clearance.
Now look at what the next size up is: 28mm. The radius of a 25mm tire is 12.5mm. The radius of a 28mm tire is 14mm. So if the 28mm tire is true to size, the new clearance will be 5mm - (14mm - 12.5mm), which comes out to 3.5mm. I've seen it suggested that you should not try to mount a tire with less than about 3mm clearance. So I should be ok to go with a tire that is known for running true to size in a 28mm size.

The next step is to read some reviews, paying attention to the part where the reviewer discusses spec size (or nominal size) compared to actual measured size. Also look at your own 25mm tire to see if it runs larger or smaller than 25 when mounted on your rims. If your existing 25mm tire is known to be fairly true to size, and is fairly true to size on your rims, and if you can find a 28mm tire that is reviewed as being close to true to size, you should be ok.

I can say that the GP4000sII in the 28mm size run large, and on my rims measure 31mm. On my particular bike there is one place in the rear triangle where I have only 2mm clearance. I'm not getting any tire rub, but when I finally wear out this tire it will be replaced with a tire that is reviewed as running closer to true to size such as a GP4Season or GP5000. But the tolerance is so tight that it could have turned out poorly, and I might have ended up with an expensive tire that is not suitable for my bike. Do be cautious. In my case no amount of measuring could have predicted that the GP4000sII 28mm tire runs 31mm. But there are reviews online indicating such; that the tire runs large.