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

Is everything else on your bike, body, position and clothing so perfect that you are worried about 3mm differences in tyre width for day to day riding on normal road surfaces? I think you might be worrying too much about changes you probably won't notice in the real world. 

http://engineerstalk.mavic.com/en/the-right-tyre-width-on-the-right-rim-...

whats the internal width? according to Mavic a 25mm tyre should not be fitted to a rim with internal width 22mm or greater. Going through the comments Mavic state that the table displayed is as per the ETRTO recomendations in the process of being agreed by wheel and tyre suppliers. (as of 2019)

I know that my Canyon aeroad was supplied with wheels with internal width 17mm and external width 25mm. The tyres as supplied were 25mm rear and 23mm front. So the answer for your wheels might be 25mm front and 28mm rear, provided the inner rim width is at least 22mm

Given that speed and aerodynamics are important considerations for you, you should definitely go for 25mm tyres over 28mm ones. It makes me laugh the amount of expensive aero bikes I see with un-aero positions, riders with baggy cycling gear and wide tyres. Must be a Surrey thing...

If you're still not sure, some brands like Specialized and Pirelli make 26mm tyres.

Clear answer - 25mm.. because of the rule of 105%

The Rule of 105 states that the rim must be at least 105% the width of the tire if you have any chance of re-capturing airflow from the tire and controlling it or smoothing it. 

When discussing comfort, it's subjective. Yes a bigger tyre at a lower pressure will be like riding a couch.. BUT I'm really quite happy running 25's at 80psi on my roads at my weight they are more suited to how I ride. I also run a 23mm front and 25mm rear on my 'best wheels' for speed and I run 28mm's on my winter bike which is lovely for plodding about on but I don't usually beat any PB's on it. 

FWIW, I'm curious as to why you run such a differential of F/R tyre pressures? Is there a 'bike fit' reason for that? Having experimented a bit with pressures I found 65psi to be a bit too low for riding at speed.. general rule of thumb is about 5psi lower in the front than the rear. 

Actually the trend with tubeless tyres is that they are made to be more "true" to size, if mounted on the specified internal rim width. 28mm tubless tyre should measure 28mm....only if you've mounted it on the size of internal rim width it is optimised for.

(This BTW is why you'll see increasing levels tegration between tires and rims, with more rim makers selling their products in packages with tires optimised for the specific rim.)

So the question you need to figure out:

• what's the internal width of your rim?
• what's the stated spec for internal rim width that Continental provides on the product instructions for the spesific size of tyre that you want to mount?

I've read that for optimal aerodynamic benefit you don't want your tyre section to be more than 105% of the external rim width. However, you won't know what the optimal width is until you've fitted the tyre; and various brands measure width differently.

One answer to the last bit is to check out https://www.bicyclerollingresistance.com/ where Jarno gives the flat bead-to-bead dimension as well as width when fitted to his test rim and inflated. The GP 5000 TL is on there but also a comparison of 3 widths of the 5000 clincher.

Having said all that, in the real world when you're not doing 25-30 mph in a TT, any difference would be immeasurable while comfort definitely trumps aero which in turns trumps weight. If you ride on UK roads then go for the widest tyre you can - for 28mm drop the pressures a further 5-10 psi from those for 25mm. I'm only 60kg and find 28mm nicer than 25mm in the same tyre (Durano).

I reckon it's going to be marginal either way but 25mm is likely to be the better aero choice and I reckon that will be good on rough surface anyway. Worth checking with the manufacturer recommendations. 5000TL will come up wider than 25mm on a wider rim and the 28s will be wider too.

Those pressures sound like you are quite light, if you were a heavy rider I'd suggest 28 would be a good option.

I'd say 26 would be ideal. I'd still go 25 on the 5000. TBH I wouldn't consider the 28 other than maybe to experiment for my own curiosity.
I'm around 73kg on Conti 4000SII 23mm, FLO wheels 25ish ext and 30mm/60mm deep. 

I run 303s tubeless with a 28mm @ 60psi on my aero bike and they are faster than my old clincher 50's. The biggest difference in real world speed is comfort not rolling resistance. My average power tends to be higher because I can carry more speed because road conditions are less of an issue. 25mm with latex inner tubes on a test rig are marginally faster. A side benefit of latex tubes is the are also more supple so more comfortable as well it's just a pain pumping them up all the time

Zipp303s have external 27mm, and internal 23mm. They recomend 28mm as the optimum tyre size, but advise the absolute minimum should be 110% of the internal rim width  (23mmx110% = 25.2mm). Hav you checked for recomendations from the wheel manufacturer?

I'd be hesitant to go down to 25mm, as it seems the tyre could well be narrower than the rim.