If a rim has 27.5mm outside width, should I be using 25c or 28c?

[downhiller]

My initial thought was 28c, and indeed I ordered those when I ordered the wheels, but I now read that 28c tyres tend to measure 29/30mm (these are GP 5000 TL), and that from an aerodynamics point of view my 50mm deep rims might be rendered a bit moot if the tyre is wider than the rim.

So is 25c the right choice? It’s what I’ve been running for a few years, and I have no problems with comfort or pinchflats. I tend to run 80ish in the back and 65ish in the front.

I’m a loser who’s more interested in going fast than being comfortable, what size should I have?

I know the theory behind wider tyres rolling faster on anything other than glass-smooth surfaces, but then there’s the aero side to consider.

What wins, on 50mm deep 27.5mm wide rims - 28c, or 25c?

Thanks!

[downhiller]

Additionally, I’m open to 25c front and 28c rear, if that’s considered a good thing to do!

[kevstraining]

Rule of thumb for aerodynamics is that the tyre should be around 95% of the external rim width and never wider so the 25mm is the correct choice.

[kevstraining]

@downhiller

Additionally, I’m open to 25c front and 28c rear, if that’s considered a good thing to do!

That’s a good compromise

[DwayneB]

How wide the tires end up being is dependent on the inner rim width. What’s the inner width on yours?

My rims are 23mm internal, and I run a 25c GP5000TL up front, which measures out to 29.5mm 28.5mm, and a 28c on the rear, which measures out to 30.6mm.

Edit: Had a typo in the front tire width, thanks to @downhiller for catching it!

[downhiller]

19.3mm

Seems crazy that there’s only 1.1mm difference between your tyres. Are they the same age?

[DwayneB]

@downhiller

Seems crazy that there’s only 1.1mm difference between your tyres. Are they the same age?

Thanks for catching that, should have been 28.5 for the front, not 29.5. The measurements are on tires that have been mounted for a while, so they’re stretched out. They were a bit narrower when they were new.

According to bicyclerollingresistance.com’s data on the GP5000TL, on a 17.8mm inner width rim, the 25c measures out to 26.8mm. Using my data point and theirs, it should measure out to 27.3mm on a 19.3mm inner width rim.

[Cleanneon98]

My rim is 27mm external, 17.5 internal, running a 25c GP5000 measuring 26mm

[Weissjoel82]

I have 27mm ext 19mm internal Fast Forward wheels with 23mm GP4000 SII. I upgraded the wheelset on a 2013 Cervelo S2, which has very low vertical clearance, so its the biggest tire I could fit. The 23 balloons out to 25mm, seems like a pretty good interface with the rim from an aero perspective.

I discovered through trial and error that unless about 3mm clearance is needed between the tire and frame otherwise a wet ride will turn your tire into a sanding wheel and take the paint off the bottom of your fork. No more 25’s on this bike.