Gas mileage seemed to recently decrease from 280 miles (full tank)to about 210 miles. What can be the problem?

it is not unusual to lose some millage with age of cars ,also todays gas is junk,if running reg,run mid grade with a bottle of good fuel injector cleaner-lucus is great.

How has the weather been? I notice a decrease in gas mileage here in PA in the winter on the colder days. Basically, gas is not always the same density -- when it is hot out, gas is less dense, when it is cold, gas is denser. So when it is cold the same volume of gas that is fed to the fuel injectors is not the same volume as in the summer months.

Bad gas, dirty injectors, dirty air filter, spark plugs, timing, air to fuel ratio etc.

Timing? The ECU in these cars controls the timing. No matter what you do to try and adjust it, the ECU will just correct it to the proper adjustment. and A/F Ratio? Are you kidding me? This is something you cannot touch on these cars without any type of standalone tuner. All of this is preprogrammed into the ECU, and I highly doubt he is adjusting these settings from a tuner. And to the person above talking about going from regular unleaded to mid-grade unleaded, don't be so clueless. The 6G72 in his V6 Galant requires Premium Unleaded. True he might not be using the required Premium, but for the extra $1-$2 it costs per tank, it would be completely stupid not to use the correct fuel. Sorry. /rant, but I hate when people give B.S. info that in no way will help the OP figure out what he is posting for.

When did this start exactly? If it was shortly after the 'full tune up', it could be plugs were not gapped properly... It could be one of the many sensors that corrects the A/F ratio is dirty and throwing the mixture off... It could be a dirty fuel filter... It could be dirty injectors... It could be dirty valves... It could be that the gas station you get gas at has newer pumps that now are out-stopping sooner and you have slightly less gas in the tank... It could be the valve cover gasket is leaking into the plug wells and acting as a dielectric reducing spark propagation... It could be several other things... There is a decent list of what the problem could be... So where to start is your next question, right? 1) Pay attention to MPG in the future not miles per tank because how close to full you get depends on the gas station. 2) Injector/Intake cleaner... Not just the little bottle that goes in your tank but the two stage that includes running a cleanser through your vacuum lines to the intake manifold... Seafoam is the DIY'rs brand here. 3) Fuel Filter (If it's serviceable on your car... Some aren't... some are really hard to service(Inside the gas tank, but still supposed to be replaced)) 4) If your still having problems, pull your plug wires off the plugs and make sure there is no oil around the bottom... (Would mean a leaking valve cover gasket... a simple problem, but an annoying and often overlooked one)... While you're there pull at least one plug and make sure they were gapped correctly and that they aren't fouled up... If you're STILL having problems or have any questions post back...

The use an fuel injector cleaner on a regular basis is one answer to your problem. Another is if the car has been only in city driving is would be best to get a long highway at a steady speed at 70 -75 mph for about 1 hour one way. This will allow to blow out carbon buildup on your engine. Making it more efficient.

a little late to the party aren't ya there crawfish ? lol

Post to post. R u a mail man Ora male bug...lol