1986 honda civic, HORRIBLE gas mileage and why?

Hello all, a newbie to this foruma and I'm one of those people who have no clue about anything on a car, parts, how to fix, its all a mystery. Anyway i bought a cheap 1986 honda civic and it's a quiet little car, runs good. One thing i've noticed is the horrible horrible gas mileage so far, i can't quite understand it. The car is really light weight and small, but its sucking gas like a battle tank. I haven't calculated the exact mileage yet, not fully sure how to see how many miles to the galon it gets...but i did fill it up once and i set the mile gage thing to zero. By the time my damn gas tank was empty...it only chewed up about 120 miles...so a whole tank a gas after driving a little over 100 miles? something seem terribly wrong with that?

they've got small tanks, but that still sound pretty low. sound like time for some basic stuff like an oil change, new filters, and sparkplugs.

Which model? DX or HF? Civic or Civic/CRX? The HF CRX has the smallest gas tank of all models so if there's a problem with fuel efficiency, it will drain pretty quickly.

I'd start by giving it a tune up, and checking for vacuum and fuel leaks.

fill it up 3 times, write down mileage and the amount of gas each time, then add the mileage, then add the amount of gas, then divide mileage

if you have an 8 gallon tank and your getting 120 miles on that 8 gallons then your mileage would be 15 miles to a gallon, my old honda got 300 miles on 8 gallons of gas during the summer, and around 340 during the winter