Miles per gallon?

I've been through the winter gas mileage thing with my slant a body, especially coming out of a 95 Saturn that would get 35 mpg year round. I googled slant six mileage figures and came up with the same figures you were expecting. And those figures ARE possible.

The fact that you mention that a new thermostat helped keep you warmer while driving indicates it's cold where you live. Winter is the worse time for fuel economy, especially in these older cars. The colder it is, the worse it is. The technology in your car is 60 years old and very crude. That's why the car companies went with computer controls 35 years ago. The first few miles of driving in really cold temps use the most gas. The further you drive once it's warmed up, the better your fuel economy average will be. But if you're driving only a few miles to work each way, the car is subject to 2 dead cold starts a day to cover very little distance. Do this 5 days a week and add in a trip or two to the corner store to get beer or smokes and 12 mpg could be the painful truth. Once the temps warm up it'll be much better.

I've gotten some really great figures with my car by getting it running great (with help from this forum) and driving very carefully. 24+ on the high end and high teens on the low end in warmer weather. Average summer driving was about 21 combined, but the average is much lower in the winter. It started driving me nuts so I stopped topping it off and tracking the mileage and just try to enjoy my car now that I know it's running it's best and what it's capable of under the right conditions.

I have gotten better than 24+ on long trips but those figures seem to be an anomaly. Not all filling station pumps shut off that the same time. But high 23's and low 24's were a very repeatable figure.

Driving style can't be emphasized enough, btw, as others have mentioned.

Once you think you've got it all dialed in, drive it to a gas station (it'll be warmed up by then), top it off and then get on a highway or nice winding back road where a steady speed is possible. Drive carefully and easily for 100 miles and refill. That'll show you what the car is capable of under the best conditions.