There is certainly a break in period for any new car. The factory break in period for your Civic is 500 miles, but you will see gains over the first 5000 or so. For starters, the oil shipped with the new car is likely heavier than what it will be changed for (do not read this as advice to change the oil prematurely), the bearings and journals still likeley aren't smooth.
Also working against you is the cold weather. Hondas use a MAP and temperature sensor (instead of a maf sensor) and get significantly worse milage below about 35/45 degrees. Add in the winter gas, lower tire inflation and "suppleness" due to cold, and the greater resistance of heavier cold air and snow... I guess I could go on and on.
Expect another couple mpg due to the cold, and perhaps another 1 or 2 over the break in period of the car. For kicks, check out a car and driver long term test. Most cars get better milage and accelerate faster at the end (40,000) than when new due to the break in.
It will be hard to beat the tercel since you have so much more weight, but you'll get closer than you're thinking now.
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