On my 2004 HCH which I bought last January the door sticker says recommended is 30psi cold pressure front and rear. I drove it off the lot new and they had 32. Sidewall max cold pressure rating is 51psi.

The best MPG I could get at the low pressure was mid 40's.

A few weeks later I brought them up to 51psi cold pressure and noticed 3 things:
*MPG went up by about 20.
*Ride was very hard, especially over those unavoidable larger bumps.
*Although what I still consider a safe margin, steering wander was noticed on the freeway.

On my 5Kmiles oil change I dropped them down to 30psi for the trip to/from the dealer and it felt like I was constantly driving either through a constant deep puddle or shifting sand etc. MPG dropped by about 20.

Just had my 10K maintenence cycle and drove it in with 51PSI tires.
Dealer dropped them down to 40psi.

I've left them there at 40psi now for 2 weeks and noticed:
*Those unavoidable sharp bumps aren't quite so jarring
*Freeway steering wander isn't as noticable
*MPG has dropped (I think by ~4) I wonder about this because at the same time my tires were rotated. Not sure if rotation will have effect on MPG but the next day I broke my all time record to work at 66.2MPG.

I'm not sure if it only feels like I'm struggling harder to gain/hold MPG and sometimes it seems like the car used to do better.
Toss an occasional cooling blast of A/C to lower my average and I'm not sure.

I've decided to keep the 40PSI at least for now.

Efficient drivers do it better.
1003 miles a tank personal record. 74MPG calculated. HCH1 CVT