Re: 2003 cadillac deville mpg
4,000 pound base Deville, 275 hp 3.11 final drive. 87 octane fuels.
11 - 15 city
17 - 21 near suburban
25 - 28 long highway run
On anything less than a 12 - 15 or so mile drive the engine and transmission never fully warms up. Oil drag is pretty high. Short, cold runs cause the most engine wear and oil additive depletion.
Put an egg on the gas pedal and don't break it. Run 5W-30 recommended oils, maybe synthetic. Run Top Tier fuels. Inflate the tires to 32 psi all around, cold - NEVER adjust tire pressures when warm or hot - the normal pressure rise with temperature increase is factored into the recommended pressures on the door sticker. The maximum inflation pressure molded into the tire sidewall is for your runs at Bonneville Salt Flats only. Use clean air filters and make sure the intake ducting has no leaks in rubber pieces or band clamps. Clean the throttlebody every year or so. Remove excess junk from the trunk.
Study how and why the average fuel mileage readout works like it does - and how long term resets or just "leave it alone" affects perceived mileage. You fill up and reset the average mileage to 0. You immediately get on the Interstate and drive a half hour at 60 mph. Your average mileage ratchets up to 26 mpg. You get off the I and are immediately stuck in heavy traffic jams for a half hour. Gas mileage drops to 0 and your average mpg for that hour is ....... 13 mpg.