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96 3.8 Firebird Sudden MPG drop

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so I take a lot of long drives (300 miles+) in my firebird, and I've come to enjoy the stellar highway mileage I get with 89 octane. I was getting 28 mpg with the auto and 31 with the manual over the summer and the last 2 years I've had the car.

however, I just went up to Michigan and back (700 miles round trip) and I thought I was filling up more frequently than I should have been.... so I figured out my mileage and its about 24 miles to the gallon.

the weather was ~50s Fahrenheit going up with about a 30 mile headwind/crosswind the whole way. coming back it was ~30s F and little to no wind the whole way. going I tried to maintain a constant speed, but my speed varied a lot in the range of 55-75 with the speed limits and road conditions (2 hours of rainy driving). coming I did a pretty constant 70 the whole way and didn't have too much trouble with road conditions.

things about my driving: I did a lot of my accelerating in 3rd 4th and 5th gear. a lot of it was slowly climbing from 60-70 in 5th; I had about 50lbs of luggage and a 160 lb passenger; I drive like a grampa - slowly accelerating, coasting a lot;

I can't really think of anything off the bat that would cause this, as my driving habits were no different than they have been and there is nothing majorly wrong with the car. no DTCs in the computer that I've found, but i'll scan it again when I get up.

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Alright, it could be a few different things: need of a tune up (IE: spark plugs, wires, PCV change, air filter, fuel filter and oil change) catalytic converter could be showing early signs of getting clogged up, or just excessive carbon build up in the top end of the motor.

I'd Seafoam it, would help out immensely, and it's ten bucks.

What's the mileage on the motor?

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mileage on the motor is ~50K dropped it in summer 2008 from an 02 camaro and swapped out the DBW stuff for the 96 OE. plugs and wires are from 02 and the injectors are from 96.

what air pressure are you talking about? tire pressure or Intake pressure? cuz I just filled the tires to the spec on the door.

would bad differential fluid also help cause this? I noticed it was whining a LOT more than usual

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The L36 from the 00-02 models had Iridium AC Delco/NGK plugs from the factory, so you're fine there.

A bad differential could cause the drop. Added stress/friction on the motor from the lack of proper lubrication in the spider gears will totally affect MPGs. Some new fluid and diff cover gasket couldn't hurt.

IIRC, 75W-90 weight gear oil works best.

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what air pressure are you talking about? tire pressure or Intake pressure? cuz I just filled the tires to the spec on the door.

Tire Pressure. Not inflated to the correct PSI can cause your gas mileage to suffer.

Btw, when I said fuel system cleaner, I meant Seafoam but for some reason I couldn't remember the name of it lol

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The cooler temperatures will result in decreased fuel mileage. Also winter fuel since it has an additive in it will also decrease the fuel mileage.

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