Tyre Pressure for Challenger

Yeah I guess thats why they recommend 26 so you get a squishy ride.

The goodyear bloak also said not to bother airing down on sand. I have run 36 no worries on Double Island a few times although I guess if it was Dessert Sand or servere sand dunes it may be a different story.

Use the OEM recomendations, Long term if you get a set of verniers and measure the tread depth now, in the centre of the tyre then at the inside then on the outside just off centre of all 4 tyres (probably 12mm new) write the measurement down plus the presures and the KM, Excels good for this.
If you could expect to get 60,000km from the tyres that would be about 2mm for each 10K.
In 5,000km time measure the tyres again, you would expect to see 11mm in the centre and either side of the centre, adjust the tyre pressure up or down to compensate in incriments of only 2-3 psi, (write down the results) in the scheme of things this is a change of approximatly 10% ie if the tyre is worn more in the centre lower the presure etc.
You may not get it right for this set of tyres but you will have a good starting point for the next set.

I run the same pressure 36psi on my Challenger for tarmac duties
(255/70-16) standard rubber

I have checked them after a couple of hours at highway speeds and found they only went up to 38psi so the 4 psi rule is there