Is there anywhere online you can find out how wide/tall a tyre is really? if my frame says it will take a 25mm, and i can see some 25’s are bigger than others. i can also see my frame has a fair amount of clearance. Just wondering if there is a way of finding out what tyres i can fit without buying first.

Which i assume once fitted can’t be returned??

There was once a site with details of this for MTB tyres, no idea re road.

There’s a lot of knowledge here if you have some tyres in mind.

LBS should fit them and refund if no go?

Too many variables.

You’re better searching for specific tyres, the better reviewers state real world widths on specific rims.

For what it’s worth, 25mm Specialized Espoir and 25mm Continental GP 4Seasons on 19mm internal DT R460 rims both measure 27mm at widest point.

This means Specialized are technically correct when they say the latest Allez can take 28mm tyres, but try fitting an ACTUAL 28mm Conti GP4season tyre and there’s virtually no clearance. I sent them back as you couldn’t tell they had been fitted, I even preserved the little bit of packing tape stuff they use to keep them rolled up in the box.

GP5000s are supposed to run a lot truer to size than GP4000S IIs, 700×25 variant is approx 25.5mm (max) vs 27mm.

currently i have a trek ALR with campag potenza calipers, campag c17 rims and gatorskin 25mm tyres. It looks like i can get a bigger tyre on, it is just trying to get an idea of how much bigger a bigger tyre actually is.

Just thinking if i can get something bigger that will cope with potholes etc.

Next size up Conti tyre would be a 28mm which in my experience measures up 31mm. That’s on a 19mm internal rim though, so perhaps 30mm on a 17mm internal?

How much extra space do you have?

Avoid 28mm Conti 4 Seasons – like trying to fit a fat bike tyre in!

How much extra space do you have?

I can just about getting a 6mm allen key around the tyre on the back and a 5mm on the front.

Old Schwalbe Pro one’s were about +3mm, new ones are true to size.

It depends on the rim apparently, the old ones must have been measured on some seriously narrow rims though! Which is weird as my old Ultremo’s and Blizzards were both true to size as well. just the Pro One’s that are massive (even on relatively conservative rims).

http://www.bicyclerollingresistance.com/road-bike-reviews has some real world tyre width info. I think it’s based on a 17mm internal rim and 100psi.

in case anyone does a search in future, the following works fine, Conti gp5000 28mm on c17 Campagnolo rims, a trek emonda ALR frame, and Campagnolo potenza calipers.

Schwalbe One 25mm on 17mm rim is ~27mm
Schwable One 28mm on 19mm rim is 30.5mm (but noticeably less tall so fits under a Shimano caliper)

My 28mm Conti GP5000TL measure exactly 28mm on DtSwiss PR1600 spline disc with an 18mm internal rim width.

My 28mm Conti GP5000TL measure exactly 28mm on DtSwiss PR1600 spline disc with an 18mm internal rim width.

I took from an earlier thread that new 28mm GP5000s were slightly narrower (i.e. actually 28mm) than older ones.

The wheel rim width means it will differ. A 25 on a wide rim might rub when it wouldn’t on a narrow one.