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Next month I'm embarking on a 250 mile ride over one day - I've done plenty of training and I'm ready. Currently my bike has 25mm tyres and some of the 100 - 175mile training rides I've done have proved by the end fairly uncomfortable.

I'm wondering what real world difference in terms of watts having 28's on instead of 25's would make? Obviously over 250 miles we won't be going very fast, perhaps averaging 16mph or so.

The route is fairly flat (St. Ives Cambridgeshire -> Newcastle (in the UK)). So I guess weight isn't a huge issue.

Is there a calculator or something out there that can help me decide?

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  • If you can maintain 16mph you'll be looking at a journey time of 15.5 hours, I'd personally have a rest break every 3 hours and forget about the slightly wider tires.
    – Dan K
    Jul 11, 2019 at 8:43
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    Wider tyres will let you run a lower pressure which, in theory, will be more comfortable. Personally I have moved from 25mm to 28mm in the last few months and the difference is small, but noticeable. You're going to be on the bike a long time and towards the end it's going to be a chore, comfort probably a good idea.
    – Lou O.
    Jul 11, 2019 at 8:49
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    not sure if a calculator is possible, but there's always bicyclerollingresistance.com if you want to become obsessed over individual watts :) (danger, madness etc etc...)
    – Swifty
    Jul 11, 2019 at 8:51
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    Comfort is the main factor in my pondering of this purchase, and there will be frequent short (10-15 min) stops (I'd be surprised if anyone can do 250 miles without stopping?!) If there's not really any noticeable diff in comfort then it's not worth the bother I guess
    – John Hunt
    Jul 11, 2019 at 9:26
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    You may want to research some previous questions on tire sizing. Unless you change just the size, but keep to the same brand and model of tire you can't tell for sure if there is an appreciable difference in size. One brand of 28mm may be the same size as a 25mm from your brand.
    – mikes
    Jul 11, 2019 at 15:17

1 Answer 1

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If you're going to change anything before the big event, change it now and allow time for any problems to arise, then be sorted out before the event.

Generally, a 28mm tyre is said to give lower rolling resistance over a 25mm and has been tested, though using a 28mm at the same pressure as the 25mm could spoil some of the comfort advantages. The numbers tell us that you could run a 28mm tyre at 20 psi lower than a 25 with very little performance difference.

More significant might be saving a few more watts with tyre choice to a different model in 28mm, but beware increased cost and (possible) lower puncture resistance.

So, if there's little difference in performance, or even an improvement and a 28mm proves to be more comfortable for you (higher volume, lower pressure) then you're looking at a win-win. Just watch out for frame and fork, brake, clearance for the larger tyres. Another reason to test now, not in a month's time.

Up to you if you want to spend the money, but I would try it for yourself rather than focus too much on what you can find written on the internet, some of it may be written by advertising experts for a start.

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    The linked test doesn't mention that, but at same pressure a wider tire will be harsher. At usable pressure there is almost no difference in rolling resistance, but wider tire will have more margin before the rim hits the road.
    – ojs
    Jul 11, 2019 at 9:29
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    @ojs good point, I'll edit something in
    – Swifty
    Jul 11, 2019 at 9:32
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    Despite potential rolling resistance savings (not getting into this debate), wider tyres will almost always come with an aero penalty as well. Some modern aero wheels are wide enough to negate this, but most will be optimised for 25mm or narrower.
    – Carbon side up
    Jul 16, 2019 at 12:46
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    If you're going to change anything before the big event, change it now and allow time for any problems to arise, then be sorted out before the event. And to find out if the larger tires actually are better for long-distance rides. They may not be for some reason.
    – Andrew Henle
    Jul 16, 2019 at 13:04
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    @JohnHunt For a 400km ride, the key point is puncture resistance anyways. Depending on road conditions, a non-puncture proof tire may force you to patch tubes several times, while puncture resistant tires only have a very slight chance to fail you. (I'd estimate that chance somewhere in the ballpark of about 4%, depending on brand and road conditions.) I mean, what good is the fastest tire when there's no air in it?
    – cmaster - reinstate monica
    Jul 17, 2019 at 9:53

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