Quick tire and wheel math check
 

2012 Giant Defy Composite needs a new rear wheel. The factory wheel is 22mm outside width, and I am running 25mm tires. I have a few mm clearance on the sides, but only about 2mm clearance to the seatpost. One of the rims I am looking at is 25mm wide, which in my mind will result in a slightly wider and slightly less tall wheel.

Not necessarily. HED has a diagram (which I can't find at the moment) that shows actual profiles for a GP4000 on 19, 23, and 25mm (external) width rims. Going from 19 to 23 on both 23 and 25mm tires resulted in a significant height increase. Going from 23 to 25mm width rims, the height of the same tires remained the same.

In a simplified "theoretical" model, the tire assumes a circular shape. Adding width to the rim is effectively increasing the tire cross-section circumference. In this model, the tire diameter will increase by (rim width increase)/pi. So a 2mm width change would be a 0.64mm increase in both width and height. However, real tires aren't uniform in cross-section and don't assume a perfectly circular shape. Under pressure the casings tend to flatten at the sidewall so more of the rim width increase goes into increased tire height than width (up to a point).

tl;dr; your tires probably won't get shorter on wider rims and may even get taller. If you're tight on clearance, make sure you can test fit before committing to purchase.

As the tyre size gets bigger so naturally its circumference will increase, I’ve just had this problem on rebuild where I had to drop the trye size to fit between the chain stays.

Without a photo I’m trying to understand what exactly you mean maybe I’m missing something :-

You say the distance to the seat post do you mean the seat post half way down to the BB when looking at the wheel at 9.0’ clock ? or are you referring to the seat post bridge where the brake caliper is ?.

Either way it seems a very small distance,

I’m asking a stupid question – is the rear wheel fully back in the drop out slots ?

Visualize an incandescent light bulb.
Without changing the diameter of the "globe", imagine installing it on a larger diameter base.
It doesn't have to "curve in" as far.

Also keep in mind that tire sizing isn't always consistent.

If my heart was set on the 25 mm rim, I'd buy it, test fit my existing tire on it and see if it works. That's knowing for sure. Everything else is just guessing.

If it was my bike, however, and my tire clearance was already tight, I'd be looking for a 22 mm wide rim to match the one that I already know just barely works.

Hi Podagrower,

I agree with the previous responses that you probably won't see a significant decrease in tire height with the larger 25 mm wide rims.

Here's something else to consider: For best aerodynamics it is best to use a tire that is slightly wider than the rim. A 25 mm tire would be ideal for a rim whose outside width is 22 mm. Since your frame won't accommodate a larger tire, I would hesitate to go to a 25 mm rim---I would stay close to the 22 mm rim width. Unfortunately for those of us with older frames, the current trend is toward wider rims and larger tire widths. There are some good advantages to this. Unfortunately, there are many, many road frames in use (like yours and ours) that don't have the clearance for the bigger tires. This may limit your rim selection if you are looking for the "latest and greatest" on the market.

My Team Fuji and my wife's Specialized Ruby frames (both carbon frames) max out at a 25 mm tire size. So we're in a similar boat as you. Where we ride, wind is frequently a major factor and crosswinds are very common. For training, we've been running Shimano Ultegra wheels (WH-6800 and WH-6700). Their outside width is 21.4 mm. I'm running 25 mm wide tires and she's running 23 mm wide tires.

Kind regards, RoadLight