Sudden drop in gas mileage.
 

O2 sensor has only close to 11,XXX miles on it. Spark plugs are about the same as well as air filter. No CEL code. Nothing was tampered with. Fuel/Air mixture is as it was when car came from the factory. I usualy would get anywhere from 280-300 miles on a full tank of gas (city and highway respectively) now I only get 240 miles city. Any suggestions

Any loss in power? hesitation? missing?

no loss in power, no hesitation, no stuttering or misfiring.

winter gasoline,more alcohol? If temps changed because of winter its normal

Rarely get winter get here.....temp is usually mid 60s to 70s during winter months

even in cleveland winters it does not drop that much.

Id check the duty cycle to see if it goes in to closed loop. Also, check you vacuum to make sure you have not sprung a leak.

Obviously duty should be around 55%, Vacuum should be roughly 15hg. 13hg is minimum.

Ismalley, pull the distributor cap and check the condition of the terminals and the rotor ... also, look for any tracking inside the cap. Verify that all ignition wires are securely fastened to the distributor cap and spark plugs.

He states no misfires,or hesitation, so I doubt that would help.

Ismalley, purchase gas from a different source and see if your gas mileage improves.

I don't think you can get an accurate mileage reading from just one tank. Realistically, one should fill up and note the mileage. Then record the gas used over the next few fill ups. Then fill it up to the top and note the mileage. This way, you even out all of the possible problems with a single filling.
Anziani,
'93 E420 128K
'97 S500 coupe, 135K

I agree - I keep a log book and write down every fill-up. It could just be that you didn't get the tank full last time you gassed up.

He said in the first post he usually gets 280-300 per tank, now he got 240. By reading that one would assume he keeps track of fill ups.

Are you guys posting just to increase your post count?

The point is that he shouldn't panic and go tearing into the car based on the results from one tankful, but thanks anyway for your helpfulness...

I never recommend tearing in to anything. I always recommend testing before throwing money around.

If the vacuum and duty cycle check out,(free to check) then I would say it could be a bad tank of gas.

He states a drop of 40-60 miles on a tank of gas. Have you ever under filled your car by 2-3 gallons? Really?

Give or take 1/2 gallon is more reasonable on a fill-up.