When it comes to front-wheel-drive vehicles, I've always run the rear tires a few psi softer since there's less weight back there. Might not apply with a "high efficiency" vehicle like the Prius though - as well as probably having more weight over the rear than say my Sentra due to the battery backs, etc. Start by maybe increasing the front tires 2 psi & see if there's a change. If none or negligible, then add a couple to the rear, etc. You may have to play around with them for a bit, fine tune, etc. You may ultimately settle on the same psi front & rear or different.

Since you're not using the OEM tires, the OEM pressures aren't right. Make sure that whatever you air them up to is not over the TIRE manufacturer's posted max PSI.
If the ride isn't objectionable, just run them at that max PSI. You'll get the best mileage without damaging the tire that way.

OK Ill give it a try. Thanks

Only problem I have noticed using max tire air pressure is they feel a little less stable in heavy rain.

But that might just be the tires I have tried it on.

This, combined with the tire weight issue already covered, is the most likely answer. The Prius OE tires would be designed to be low rolling resistance based on the tread pattern and rubber compound. You probably have a grippier tread pattern combined with a stickier rubber compound on the tires you just purchased.

The one (and only) time I took the first '87 to the MB dealership, shortly after buying it to have it checked out for major problems (I was totally inexperienced at working on MB diesels at that point), upon getting it back it seemingly wallowed around corners and the fuel mileage went to crap.

Getting it home, I noticed the tires looked awfully slack compared to before. Checking the pressures, they'd lowered them into the mid to high 20's per the door sticker. Pumping them back up to 38 to 40 psi restored it to it's old self.

When I switched to 205/70 Cooper tires, I kept the pressure in the same 38 to 40 psi range. Beast had no problem hitting upwards of 35 mpg on the interstate, and would slide absolutely flat around corners at speeds that would have sent the Jeep CRD into a rollover.

Low rolling resistance tyres are all the rage over here in Europe. See for example

http://www.michelin.co.uk/connected/tyres/michelin-energy-saver

The Max. PSI on these tires is 51. I have them at 35/35. I think i will go up to 37/37 and see what kind or results i get.

tire pressure is like oil weight.. big debate.. lol

But I run my tires at Max -10%

Its just depressing seeing the difference in marketing techniques used in Europe verses the US.

There are two tickers on the .co.uk site.
One for fuel saved.
One for reduced CO2 emissions.

http://www.michelin.co.uk/connected/

Makes us look like a bunch of knuckle dragging neanderthals.

Hows Finland this time of year ???

No, our emissions standards are a lot tougher than Europes.