When I did research on 25mm tires last year, for club rides, I wound up choosing the Michelin Pro2 Race 25mm tires, based on recommendations made on bikeforum. I already know I'll like them a lot.

Down the road, I may want to try the roadie as a touring bike. Just eyeballing the 25mm Pro2 tires inside the rims, I can see it would be an adventure to get upwards of a 27mm tire in the forks.

A couple of days ago, I ordered a Sears Special: an 8-buck caliper for taking measurements:
< https://www.sears.com/shc/s/p_10153_1...eyword=caliper >.

At a minimum, I can do more than just eyeball the actual width of the 25mm Pro2's, while they're on my bike. I was thinking I could also take measurements of tires ... which are sold as 27mm or 28mm width ... but are actually around 26mm or 27mm width. The drawback is finding those tires while they are under pressure and mounted on bikes.

A good thing I like about Rivendell is that a lot of tires they sell, they show a picture of their caliper-measured width ... and their caliper probably costs more than 8-bucks.

Anyway, here are two questions to begin the OP:
* Is there a website that lists tires not only by their advertised widths, but their measured widths?

* What risks are there to riding a tire that is very snug within the forks, say only 0.25 or 0.5 inch of clearance on either side of the tire ... and do you ride with tires that are this snug within the forks?

Good luck on finding the actual widths. I've searched for months on that and only find what people have posted. No company data.

In the end you may have to take your best guesses, from what data you gather, and then buy locally, where you have the option of inspecting the result and returning the tires.

One of the reasons why I went with the bike that I recently purchased, was that it can fit tires of up to 38mm. Lots of future options remain viable. I have a peeve with bikes that can't accept a true 28mm.

[QUOTE=Motorad;6321679Anyway, here are two questions to begin the OP:
* Is there a website that lists tires not only by their advertised widths, but their measured widths?

* What risks are there to riding a tire that is very snug within the forks, say only 0.25 or 0.5 inch of clearance on either side of the tire ... and do you ride with tires that are this snug within the forks?[/QUOTE]

1. I've got identical 28mm tires (Continental Gatorskins) mounted on two different bikes. On my tandem, with Velocity Dyad rims, they measure 28mm when inflated. On another road bike, with Velocity AeroHead rims, they measure 26.5mm. 1 1/2 millimeters isn't much but it does show that rim width matters.

2. I would guess that 1/4 inch would be plenty of tire clearance. The problem is making sure you have 1/4 inch clearance everywhere. Most of the time I encounter problems squeezing a tire past the brakes before I have frame clearance problems. Another point of interference is the front derailleur when it's in an extreme gear position.

Thanks Tom, and good point RG about rim being a big factor.

The roadie rims are DT Swiss, 700C, 28 spokes, and I'm pretty sure they are the RR 1.1 model.
< https://www.youngwheels.com/prices_rims-spokes.html >

How would this rim impact the fork-clearance of 27mm & 28mm tires? For example, what would be the expected width of the 27mm Rolly Poly or 27mm Ruffy Tuffy ... on the RR 1.1 rims?

Did you ask anyone at Young Wheels? As they sell both the rims and the tires, they might know what the combination measures out to.

Also keep in mind that many tires get slightly larger as they wear. The one place I recommend not having super tight clearance is between the tire and the fender.

MKahrl, what causes tires to get slightly larger as they wear? Is it the width that gets wider ... or is it the diameter that gets bigger?

I have new fenders on my Trek, and there's not much clearance between the (32mm) tire and the fender.

Max tire size may also be a function of how your ride. If you generate significant lateral deflections of the wheel in corners you may find that you get frame rubs that others don't. Examine the inside of the front fork and the inside of the seat/chain stays, if you see rubs (even though the static position has gaps), then the tire is too wide.

Terrain also makes a difference. If you ride where mud can accumulate on your tires you may need to be careful about tire widths. Cyclocross bikes are designed around extra wide spacing to accomodate mud. This is one of the reasons that they make good frame choices for people who want to ride extra wide tires.