[Closed] Should I run different pressures in different size tyres?

1. yes. bigger tyres = higher volume/lower pressure for the equivalent "firmness"

2. Trial and error

Should you? Do either of them do something you don't want them to? It's only air. Add some, remove some, find a sweet-spot you like.

Tyre width can be a reason for different tyre pressures...so can..tread pattern, tyre shape, rubber compound, rim width, wheel size, bike geometry, rider weight, size of morning poo, terrain, weather, fork setup, shock setup, riding position, temperature, tubeless, not tubeless..

Unless there's a specific issue you are having it's not even worth thinking about.

<span style="color: #444444; font-size: 16px; -webkit-text-size-adjust: auto; background-color: #eeeeee;">It’s only air. Add some, remove some, find a sweet-spot you like.</span>

This

"<span style="color: #444444; font-size: 12px;">Tyre width can be a reason for different tyre pressures…so can..tread pattern, tyre shape, rubber compound, rim width, wheel size, bike geometry, rider weight, size of morning poo, terrain, weather, fork setup, shock setup, riding position, temperature, tubeless, not tubeless.."</span>

You missed carcass construction/type.

Your measuring the pressure, not the amount of air in the tyres.

Experiment as sr0093193 suggest.  A few psi can make a big difference.  Don't use your thumb as a gauge, get a proper pressure gauge and enjoy fiddling about.

As above find the sweet spot for each tyre by trial and experimentation. For example a run 2-4psi less in a 2.5 shorty than a 2.3 DHF both tubeless on the same 27.5 bike, but also the shorty is used in softer conditions. I also run similar pressure to the 27.5 2.5 shorty in a 2.3 vigilante with tubes on the 26er HT as the vigilante is a tough casing and as such gives much more sidewall support than the exo shorty carcass. So yeah find what works for you for each tyre situation, but as a general rule all other things being equal wider tyres equal lower pressures.

Or just ignore all the tech advice above, don't worry about it and enjoy riding your bikes.

How to tyre pressure, IME:

Getting pinch punctures: put some air in.

Bouncing when you pedal: put some air in.

Too vague in corners: put some air in.

Skittish in corners: let some air out.

Feeling every bump in the terrain: let some air out.

Poor traction on technical climbs: let some air out.

Can't find a sweet spot where you get none of the above: try a different tyre.

Bouncing when you pedal: put some air in.

On a plus tyre take some air out.

There's a few tyre pressure calculators around. I've used this one in the past

<span style="font-size: 12.8px;"> http://mtb.ubiqyou.com/</span>

Just put in rider weight, tubeless or not, tyre diameter and width and it'll give you an estimate for your front and rear tyre pressures.  Useful as a guide pressure to start with and then adjust if you feel the need.

IMO people get too hung up on figures. Whilst they're a good starting point, every tyre is different and simply putting the same pressure in every tyre is highly unlikely to get you optimum performance. Even the ground conditions can have a significant effect on how a tyre behaves.
As others have said, experiment.

"Or just ignore all the tech advice above, don’t worry about it and enjoy riding your bikes."

I agree I'm a little to fussy/concerned over optimum tyre pressures (and suspension settings) and there is a lot to be said for just riding, but that assumes pressures (or settings) are in the right ball park. I've had people come on trail rides with 40+psi in their tyres sliding all over the place with no grip or dinged a rim as pressure was too low. IMHO there does need to be at least a little bit of attention to the right pressures etc to help get the best enjoyment from a ride.

"IMO people get too hung up on figures."

Yep. That's why I deliberately avoided listing specific pressures above. Besides using different gauges makes it all a null anyway.