Pump the tyres of your STRIDA up to about 4 to 4.5 bar (58 to 65 psi). This goes for both the 16 and the 18 inch wheels.
A harder tyre makes cycling easier
A well inflated tyre deforms less and rolls more easily, so you pedal less hard for the same speed. On the small wheels of a STRIDA you notice it straight away. A hard tyre also protects the inner tube: a soft tyre punctures sooner, for example when you hit a kerb.
Check the pressure regularly
Tyres slowly lose air by themselves, even without a puncture. So check the pressure regularly, for example every two weeks, with a pump with a pressure gauge. Never pump harder than the maximum pressure printed on the side of the tyre.