What's the real-world size difference between 2.2 and 2.4" tyres? Am I right in thinking they'll be about 2.5mm wider all round?
Partly depends on brand and partly depends on rim width I guess...
In linear terms yes
But the difference in volume is greater as the 2.4 will be taller too
5mm difference between 2.2 and 2.4, 0.2 of 25.4.
Obviously that's comparing 2 tyres of the same brand/model on the same rim.
Even within a brand or specific model of tyre it’s incredibly inconsistent. On their last generation of tyres Specialized did a lot in both 2.3 and 2.6 widths. On 30mm rims the 2.3 measured 2.4 and the 2.6 was 2.5 - both preferable actual widths anyway!
The 2.4 will be bigger.
Usually, but not always.
After doing similar overthinking last year I ended up shoving a 2.25 on the back of a bike last year because I had it sitting in the garage. Did it make any noticeable difference? Not a bit.
Any brands that size up small or is it a complete lottery?
Have also spotted that the rim on by back wheel seems to be slightly to one side…
It’s pretty random! Generally stuff is truer to size than it used to be, not a lot sizes up bigger.