1992 Tercel, 30mpg?

Questions, questions, lots of questions.

Do you have a manual or automatic tranny? According to autos.msn.com, the automatic (3-speed) was rated by EPA at 26 city / 29 highway. The 4-speed manual was rated at 32/37 and the 5-speed manual was rated at 29/36. So if you have an automatic tranny, your mileage is actually on par.

Is this city, highway, or mixed?

Are the new O2 sensor and plugs OEM or aftermarket? How do you know the wires, rotor, etc. are good? Were they ever replaced?

Are the tires the correct size?

Are you a lead foot driver?

Is the weather becoming cold where you are?

All of the above will affect your mileage in some way.

How is it that the 4 speed manual gets better highway mileage than a 5 speed????What is the point of the extra gear if it gets worse mileage?

The 4 speed could get better highway mileage if the 4th gear's ratio is such that the engine turns at a lower RPM than the 5th gear in a 5 speed.

The point of the extra gear is usually performance, mileage, and/or selling feature

Either that or flog the poor thing at 70MPH.

On *all* my Toyotas, I have found that 70MPH in 5th gear gives the best fuel economy. And believe me, I've had enough to know!

Anything under 60MPH, put it in 4th.