If you are going to measure pressure — then only on cold tires, and not in the process of driving. For the tires are heated from work, due to this, the pressure in them increases. So, the numbers on the sensor will be incorrect. And it turns out that in fact you drive with low pressure tires. Well, for clutch with the road it is good. But productivity is lost, efforts to move (and fuel consumption) are rising.
So the pressure is better to check before departure. Not every day — enough once in 3-4 weeks. If during this time the tires lost no more than 2% of the pressure, then this is normal. If more — then the tire may be damaged — inspect it. If they did not notice anything — go to the experimental vehicle.
And, by the way, after the pressure was solen, close all the safety caps. Of course, they are not saved from the loss of pressure. But they save from the dirt, which, when inserting into the hole, can make a tire of a leather — and the air will be waging faster.
If the air pressure in interest tires is 20 less necessary — it is impossible to ride them! They will be very quickly heated due to increased friction on asphalt. And the tire will rapidly lose its efficiency. But slightly pumped by 0.2-0.3 bar tire on the contrary good! The steering wheel is well, the fuel consumption is reduced and the normal pressure holds a little longer (you need to become less likely). But driving can be less comfortable.
If specifically, on numbers, then, regardless of the diameter of the rim 16, 17 or 18, with standard loading (three passengers) and the front, and the rear wheels are enough 2.1 bar. But if you ship «Niva» to the maximum, then the alignment changes: on the front 2.4 (2.5 for 18 disks), and the rear will have to download to 2.8.