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

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?


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

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.

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That’s a good compromise :+1:

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!


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

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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.

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

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.