That really depends on the engine, transmission, rear axle ratio and how you drive it. I had one with a 307 4bbl, 3 speed auto, 2:29 gears. Running 70-75 from Flagstaff, AZ to St Louis, MO it averaged 26 MPG.
15 20 miles per gal with the 3.3
17 miles to the gallon city. 23 miles to the gallon highway.
what about it?
In the gas tank
According to my 99 Cutlass Owners Manual, the capacity is 15 gallons.
electronic fuel pump inside the gas tank.
See link.
I get about 30 miles to the gallon with my olds achieva.
87 octane should be sufficient
The stated gas mileage is; 23 mpg on the highway, 19 mpg in the city. The actual gas mileage is dependent upon the driver and the driving conditions
you may have changed the tire size which will give you incorrect mileage hence causing an erroneous gas mileage reading.
20 miles to the gallon