Bicycle Mechanics - Determining compatible tire sizes

Hi, I'm trying to find out the maximum tire size i can use. I have Alex PRD-15 rims and can't find specs on them. they came with 700cx25 tires. Looking on Sheldon Brown's page, it seems that most rims that will take a 25 can take a 28 as well, which is the size I want. can anyone confirm / deny? Also, what is the most accurate way to measure clearances? The bike is a Fuji Newest 3.0 54cm, if that helps any. Thanks.

Most accurate way is to spend about $10 and get a digital micrometer. On your other question, few of us would argue with Sheldon.

digital or analogue caliper.
http://www.finelinehair.com/home/Mitutoyo_absolute_caliper.jpg

if you measure on the outside of the rim, which is inaccurate, you subtract by 5.5mm, which gives you a rough size on inner rim width.

so say you get 20mm outside width rim, then you have an ETRO 622x15mm rim.
15mm rims can go from 23mm to 38mm

Most of the time the frame limits the size of the tire, not the wheel.

Sheldons chart is a good guidline, but it can easily be exceeded safely.

I haven't run into a rim on a lightweight that can't take a 28. S.Brown's page indicates that and he adds the bit about individuals going beyond...

A digital caliper shouldn't be nec., not that critical. Tire help desks will know off the bat and should agree. It's a popular bike, the wheels are commonly found

WHAT? calipers to check tires? OMG are we that critical? you are only talking 3 millimeters difference. I would say you have a 99.99999% chance that they will fit. my first real bike came with 700x20 tires and I have run 700x 38 cross tires on it.

Most accurate way is to spend about $10 and get a digital micrometer. On your other question, few of us would argue with Sheldon.

Well, yeah... because he's dead! But Sheldon left behind a treasure trove of knowledge that he selflessly shared with all of us via the Internet.

OP: I usually eyeball tire clearance. It's best to leave at least a couple millimeters since trapped debris can get jammed between the tire and fork. This is either annoying or dangerous, depending on the nature of the debris.

Remember that tire sizes are not always honest. You might be able to fit one manufacturer's 700x28C but not another's.

Jesus ****ing christ people. 25 to 28 doesn't require a caliper. Jesus ****ing christ. It'll work if 25 isn't tight on the frame/brakes already clearance wise. Rim is not a problem.

Jesus ****ing christ people. 25 to 28 doesn't require a caliper. Jesus ****ing christ. It'll work if 25 isn't tight on the frame/brakes already clearance wise. Rim is not a problem.

thank you operator, you said it more bluntly than I did, but more effectively I feel.

Thanks everyone. Looking closely at the rim, in tiny print, is the marking "622x16" and the frame looks like it'll fit fine. Hope everyone's having a good weekend.

Remember that the difference between 700x25c versus 28c is only 1.5mm per side.