I think for SUVs with fuel tanks of 20 gallons or bigger...running 1/4 to save weight helps for city driving. I fill up for longer trips and to check mileage...just starting this so no "hard numbers" yet...but 15 gallons missing will save me about 108# of weight.
2006 Jeep Liberty CRD...Founder of L.O.S.T.
OME 2.25" Lift w/ Toyo Open Country HTs 235/75/16s
ASFIR Alum Eng/Tranny/Transfercase/Fuel Skids
2002 Air Box Mod...Air Tabs (5) on Roof...(3)each behind rear windows
Partial Grill Block with Custom Air Scoop and 3" Open Catback Exhaust
Lambretta UNO150cc 4 Stroke Scooter
getting the engine tuned up and cleaned out would be a good idea. unless it says so in the manual, your car shouldn't need premium and that it does is a sign something's wrong. probably caboned up combustion chambers. that'll get you a few mpg and allow you to run regular. do the ol' seafoam treatment to help with that.
full tune up and filter change is always a good idea unless you know when they were last done and that they're good. (however, an ignition problem will NOT be fixed by running higher octane so that's not the primary problem)
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"