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I have a urban/hybrid bike ( Riverside 500 that came with 38mm tires that I don't really have complaints with except some visible cracking since a year or so (which hasn't impacted feel, air retension or anything at all) But I have been thinking of replacing these 700x38 tires.

I can go with 38, 35, 32, 28 or 25 mm tires to replace these. I am speculating about going with 28mm as of yet.

I dont do any off-road stuff , I stick to road riding and commuting mostly on great highways but the occasional pot-hole riddled by-lane does come up , so fragility or sharp feedback is best avoided .

Which size is ideal in this scenario ?

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    I'd stick with some slick, lightweight 38 mm tires on there. As of 2020-10-12, the Panaracer Gravel King is (especially when set up tubeless) is a delightfully versatile tire and doesn't come with much of a weight penalty compared to other 28 mm tires.
    – Paul H
    Oct 12, 2020 at 16:47
  • What is the motivation for the change? Weight, rolling resistance, looks?
    – mattnz
    Oct 12, 2020 at 19:41
  • @mattnz The motivation is getting the bike to be faster , so rolling resistance, aerodynamics (if they at all matter), and everything else that makes bikes go faster matters , not really looks , maybe weight but only if it is a big enough factor.
    – An Ant
    Oct 13, 2020 at 3:24
  • Do some research on tire width vs rolling resistance. e.g. road.cc/content/feature/… Essentially there is little difference, and tire brand/model choice rather than width makes a much more significant difference. Comfort and shock absorption of wider/lower pressure make them fast for all but the elite class racers. (No saying don't do it, just do home work so you are doing it for the right reasons)
    – mattnz
    Oct 13, 2020 at 7:35

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Two things to take into account.

Firstly, 28mm tires lower your bottom bracket by one centimeter when compared to 38mm tires. If your bicycle was designed with 38mm tires in mind, it probably has a quite large bottom bracket drop. With 28mm tires, you can touch the ground with pedals when cornering pedaling.

Secondly, from 38mm to 28mm is quite a large change. Are you certain that 28mm tires are what you want? They require far higher pressure and the ride is less smooth.

Considering that one of the best tires, Continental Grand Prix 5000, is available as 32mm width, I would pick this width in your case. (Don't do what I do; instead, do what I say -- I use the 28mm width of GP5000 but in my opinion 32mm may be even better for people not used to narrow high pressure tires.)

The rim width may also limit your tire options. A 28mm tire requires a rim at most 622 x 19C. If the rim is 21C, you should not use 28mm tires.

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    Your rim width info is outdated - Zipp explicitly designed their 25mm internal width wheels for 28mm tires.
    – MaplePanda
    Oct 12, 2020 at 18:36
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    I sometimes run 28s and sometimes 32s on my tourer. The 32s aren't noticeably slower on the flat, but are a bit more comfortable (a lot more comfortable for my wrists after a long day on poor surfaces). I agree, slightly bigger tyres, slightly less of a change
    – Chris H
    Oct 12, 2020 at 19:10
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    @MaplePanda is correct that the rim width restriction mentioned is likely based on old information. I documented that here. bicycles.stackexchange.com/a/69522/38270 However, 19mm internal width is actually fairly large, and the OP's bike may have rims that actually conform to the ISO standards that juhist alludes to. I can't find a spec sheet for the bike with a rim model, so I can't confirm.
    – Weiwen Ng
    Oct 12, 2020 at 22:47
  • juhist , thanks a lot for recommending the Continental GP 5000s , I had no idea which tire to get and now I do, thanks a lot. I will be ordering these and getting them installed at my local bike shop, wish me luck :) .
    – An Ant
    Oct 15, 2020 at 6:38

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