+2 psi in front

The physics is the same for your bike and your Prius. On your bike, however, the weight is pretty evenly divided between front and rear; not so with the Prius as you have observed.

Lower pressure means more tire surface in contact with the road, resulting in better traction especially on wet or icy roads, but at the cost of higher gas consumption. Many people on this forum have talked about tire pressure and lots of them, including me, have raised the pressure several pounds, but still keeping the same differential between front and rear. Factory specs are 35 front and 33 rear; I run my Prius at 38 front and 36 rear which is a good compromise between mileage and traction. If I were expecting snow, I'd probably drop the pressure down to factory specs.

 

I've been running 42/40 since we got the car, still do (Maine). No problems in rotten weather and we've had a bunch already this winter.

 

Another major difference between your bike and your Prius, fortheanimals, is the shape and engineering of the tires. A section through the bicycle tire is basically round, an O, with tread running up the sides. This makes sense since you lean your bike going around a corner, so you need tread on the sides and the transition from the bottom of the tire to the sides must be continuous.

The Prius' tire section is more of a box, with a flat bottom on the road and vertical sides. A car tire with low pressure tends to have the sides bulge out, and the bottom, which should be flat to make good contact with the flat road, cups up, so you ride on the transition between the tread and the side. If you see a car tire that has great tread in the middle and is all worn on the edges, it was probably under-inflated.

The opposite is true for a car tire that is over-inflated. It tends to be more like the bicycle tire, round, so the patch of tread touching the road is reduced and gets more wear. A car tire with great tread on the edges and little tread in the center was run at too high a pressure.

So, "over-inflating" the Prius' front tires by a couple of pounds tends to make them more round, but the extra weight of the engine tends to flatten them out, with the overall effect of having good flat contact between the tire and the road.