Get it checked out from a service shop (dealers can be way too expensive) and a second opinion on the car is as valid an option as getting one from another doctor for a medical problem (but usually a lot cheaper).
OK, some thoughts on some really, really basic stuff (you probably know it already but there are some who might not):
- When was your last oil and filter change? Some vehicles will see a drop in mpg and it's a simple fix you should be doing anyway.
- When was the last time you added fuel injector cleaner? Ditto for above. Castle Fireball Plus is good stuff but there are others. Follow the instructions on the bottle. Cheap preventive maintenance that actually works.
- Yes there are different gasoline blends for different seasons. That will change your mpg. If this is the cause, there is probably little to be done for the rest of the season but the change in the mpg should not be all that drastic.
- Are your tires inflated correctly? Check them when the tires are cold. Inflate according to the information on the inside of the door or door panel on the drivers door (there should be a sticker) or per the owners manual.
- If it really is an alignment problem your tires will start to show different patterns of wear. Most car owners manuals will show you what to look for but if not, it should be easy to find some pictures on the web.
- Also regarding alignment, the car will likely start to pull to one side or pick up a vibration even if only at certain speeds (examples would be between 40 and 45 or 60 to 65). Carefully drive down a flat, smooth, and straight road. When it is safe to do so, you might want to hold the steering wheel as lightly as possible for a few seconds to see if the car pulls to one side or the other. Some roads are "crowned" (have a slight incline at the middle of the road so that water drains off) and that will naturally cause the car to pull toward the shoulder to some degree so be aware of that. If the car has a vibration the easiest way to tell is to hold the steering wheel at the twelve o'clock position with one hand while holding onto the wheel normally with the other hand. With the hand that is at twelve o'clock, stick the index finger straight up and watch for it to wiggle. Any pulling or vibration should get attention and not just because of the loss of mileage. This is a big safety issue as well as causing increased tire wear as well as poor gas mileage.
- Check the oil and radiator for cross contamination. Water in the oil or oil in the coolant will make the engine run worse. Unfortunately these are signs of a blown head gasket. $$$
- Pull the spark plugs and inspect them. Do they all look OK? Are any of them too dark, too light, or oily? This is an easy diagnostic check for several different problems. A mechanic can easily tell what's going on or you can get a good repair manual and DIY.
- Bad plug wires can also lower mpg. When it is darker outside, pop the hood with the car running (outside -- not in the garage) and with a mist-like spray on a garden hose spray the wires. If you see any arcing or sparks, replace the spark plug wires and/or the spark plug boots (rubber covers over the plugs).
Again, this is old school, really basic stuff that a lot of people just don't get taught any longer. These are standard to-do items for any car owner and should help you in the direction for getting the problem fixed. If you were to do all of these checks, it should take about an hour and it is a lot cheaper to go to a mechanic if you already have an idea of what is going on. Cheap, fast, and easy!
Good luck.
