Higher than 35 which is standard on a touring/mild performance tire will improve MPG but wear out much faster. For 38 psi, your inner/center tread will wear faster. Just stick with 32-35 psi all year long. Your DIC should indicate when it drops below 28psi with a TPMS fault light on your dash.
If a fuel filter/strainer is clogged, then yes, it will affect your mpg, but with 45k mileage, should have no issues unless you have to buy cheapest fuel around town. I stick with Shell fuel, no preference, but better known than a mom-pops place. I would check spark plugs not for just replacing, but look to see what color they show, that will tell you a lot.
By the way, is your cobalt a flex fuel LT? and have you used e85 more than once? just wondering cause the factory injectors cannot withstand a constant change of fuel types. It will deteriorate the metal on the fuel injector solenoid, causing the injector to stay open all the time, reducing fuel economy, but a CEL should trigger as well.
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille
Last edited by 5t3alth; 02-08-2014 at 04:00 PM.