Tire Plug.

136,709 miles.

I finally got over to America's Tire to have the nail removed from the left rear. These guys are great and highly recommended. Not only did they remove the nail and plug the hole, it was absolutely free. 

These Michelin Latitude Tour tires (P235/70R16) were new in March 2013 and rated for 65k miles. Currently all four tires are at 5/32" tread depth with about 5 years / 50k miles on them. 

31.47 MPG

419.4mi, 13.325 gal @ $2.999 = 31.47 MPG

136,358 total miles. Shell on Bascom and Campbell.

34.11 MPG

454.8mi, 13.332 gal @ $3.199 = 34.11 MPG

135,939 total miles. Valero on Blossom Hill.

31.14 MPG

353.5mi, 11.352 gal @ $3.159 = 31.14 MPG

135,484 total miles. Mobil on Kooser at Blossom Hill.

Inflated Tires. Again.

135,456 total miles.

Got the low tire pressure indicator again, left rear tire pressure was back down to 29 PSI.

FRONT
38   36

29   36

I inflated all to 38 PSI. Last time it lasted about six weeks, 1000 miles. These tires were purchased four and a half years ago at 85k miles and were rated for 65k:
http://hybridescape.blogspot.com/2013/03/new-tires.html

I poked my head under the tire and believe I found the problem:

33.31 MPG

430.3mi, 12.919 gal @ $2.817 = 33.31 MPG

135,130 total miles. Mobil on Kooser at Blossom Hill.

31.41 MPG

405.5mi, 12.908 gal @ $3.099 = 31.41 MPG

134,700 total miles. Shell on Sequel Ave in Santa Cruz.