Don't run AC, Keep lights off unless legally required, keep windows rolled up, remove excess weight i.e don't carry junk around in the car. Take off any roof bars and check the tire pressures are as they should be, buy higher octane fuel. Other than that slow acceleration and keeping the top speed down.
Today I did a 140km (87miles) drive and I averaged 5.8L/100km (40mpg US) which is worse than the 5.3 (44mpg US) I normally manage, that was because today I was on smaller back roads with lots of bends so constant of braking and accelerating rather than just sitting at the 50mph speed limit like normal.
I agree with Joseph, try to reduce or eliminate the need to drive yourself. Many areas have public transportation.
However, to directly answer the vehicle question there are several maintenance tasks that can impact fuel consumption. The first thing I would do is consider the season. refineries modify the fuel blend for the time of year. I used to live in Los Angeles and there were two times a year where I noticed a very distinct drop in mileage for a few tanks, then it would pop back up to where it should be for my vehicle. You could be in one of those spots if it was a sudden and pronounced change and nothing else with your travel has changed.
Air filter - If you have not changed it, I would do that. It can help a little, or a lot, depending on the vehicle.
Oil Change - My vehicle is a little older with some wear on it. I watch the milage and when it starts to dip I check the oil. In my car when the oil is losing it's ability to seal and lubricate the rings properly my mileage suffers and it gets dirty quickly.
Tire pressure - This one can make a surprising impact. If you are not using nitrogen in the tires and you just went from hot to cold, your tire pressure could have dropped several psi. Make sure they are aired up to where they are supposed to be for your vehicle. It is usually on a sticker in the drivers side door frame.
Consider running higher octane fuel. I've found that in newer vehicles that I have used the higher octane fuel actually gets enough better mileage to justify the higher price. As in it's actually cheaper to run higher octane that the lower.
Fuel system cleaner - Try running a bottle or two of something like Techron or Lucas Oil fuel system cleaner through. You could have deposits on injectors that are causing poor mileage.
Spark plug - If your plugs are old and fouled you could be getting poor ignition on one or more cylinders. Check and replace if fouled. If one is much worse than the others, you may have a larger issue at hand like a leaky head gasket.
Hope that helps.
Not all those who wander are lost - J. R. R. Tolkien
Skandi Rogers wrote: Other than that slow acceleration and keeping the top speed down.
This is a good one I forgot. I had a Nissan Xterra that I discovered had a sweet spot of 53-55 mph on fuel economy. The best mileage I had with that vehicle was when I could drive longer distances and keep it at 53-55 mph. Your vehicle may have such a spot as well.
Have you made changes in your driving pattern or habits lately? Good questions to consider.
Not all those who wander are lost - J. R. R. Tolkien
I'm with Joseph.....the quickest and best way to improve fuel economy is not to use it in the first place. But having said that, others have given great advice on your own particular car and issue.
Somethings I've done to cut down on gasoline use...,.
.... When I needed to check on a pasture once a day that was 1 mile away, I walked. That's not much gas savings, but it also gave me the benefit of exercise and peace. The hectic buzz inside my head dissipated as I walked observing nature, listening to nature sounds, smelling things as I walked past. I returned home refreshed and calm.
..... I drive to town once a day (because I have a pick up route) and arrange in advance to get every other town task done along the way. So I limit gasoline use to one round trip where I pick up everything along the route plus as needed: stop at the post office, buy gasoline or propane, stop in the bank or hardware store, swing past the farm supply yard, or whatever else. One trip. I have a neighbor down my street who makes multiple trips a day, using 3-4 times as much gas as I do.
..... We often have things that can only be done in Kona or Hilo, both of which are a 2 hour drive away. So we try to limit those trips to 4-5 times a year. And when we need to go we arrange for as much as possible to be done on the same day. Plus we will stock up on supplies that are far cheaper to buy there, such as Costco items, Walmart, Home Depot.
..... When we take a pleasure drive/at home mini vacation, we will do any of our long distance shopping on the way home.
..... We use Amazon rather than driving 2 hours to buy something we can't get in our local small town. Living on an island with only a poorly supplied small town near by, Amazon.com is a godsend, not just for saving on gasoline. But we have reduced our long trips from 12-18 a year down to 4-5 by using Amazon.
It's never too late to start! I retired to homestead on the slopes of Mauna Loa, an active volcano. I relate snippets of my endeavor on my blog : www.kaufarmer.blogspot.com
I own a 125cc motorcycle, so whenever that can be used there is savings.
But for almost any car, especially if it's used in urban driving, you can gain a lot by surge and glide driving. Give it some gas and then let it glide. Anticipate lights and everything else and only apply power 10% of the time. Make sure your tires have enough pressure and the windows are up. At highway speeds you will waste more power with the windows down, than if you ran the air conditioner.
For me every vehicle has air conditioning, because if it's at all hot, I dampen my clothing. Works really well on a motorcycle.
If your fuel mileage suddenly dropped, then I would say you have a problem mechanically with the car someplace. I would have thought a warning light would have come on, but perhaps not. I would take it to a place like Autozone and have the car's computer scanned. It sounds like a sensor is not working to me, and not allowing your engine to properly mix the fuel to air ratio, but that is just a guess. A free computer scan at Autozone could tell you a lot, and for free.