1993 Nissan Sentra fuel economy

1993 Nissan Sentra mileage: 134,000. Since March of this year, I am keeping records of my car's mpg averages. I began to notice a significant drop in fuel economy. In the city, the averages were between 22 and 24 mpg, and in the highway, the average was 26 without reaching 27. All this with new aftermarket spark plugs, wheel alignment in all four wheels, correct cold air pressure (32 psi front, 29 psi rear), new set of spark plug cables, a nearly new distributor cap and rotor, and an rpm adjust made at a local dealer. I was using an oil that wasn't energy conserving, 15w 50 synthetic, the one I was using since the car was new, and I decided to use 10w 30 synthetic, so fuel economy improves, and it remained the same. Here in Puerto Rico, heat is a problem and that is why many mechanics including dealers, use oils that aren't energy conserving. But, well, even with the 10w 30 oil, the problem was there. I read on the Internet, that the oxygen sensor is related to fuel efficiency and that the best sensors are usually replaced at 100,000 miles. So I decided to replace the oxygen sensor with an equal aftermarket oxygen sensor, the same one my car had since new. And, the result was good. The highway mileage improved, but not exactly to the 34-35 mpg average, I obtained in the past. I obtained 31.4 mpg controlling the speed, the acceleration, and the braking in a long trip. The problem is that I still obtain between 22.5 and less than 24 mpg in the city, using the car under the same roads I used it when it was new. This car is capable of delivering 26-27 mpg under these conditions because I calculated its fuel efficiency when it was new. Some people around me say that this is not a big problem, due to the fact that the engine has nearly 134,000 miles and there is wear, even after using synthetic oil since the car was new. Also, this car is automatic not standard. In spite of this, the fact is that I am running at least 20 miles less with each and every fuel tank, having all of the above checked, and this happens consistently. This means more fuel expense, and I think that probably something can be done to put the fuel efficiency of this 1993 Nissan Sentra (automatic) closer to where it is suppose to be, which is 26 mpg in the city and 35 mpg in the highway. I did something that two different local Nissan dealers did not, which was replacing the oxygen sensor for an exactly new oxygen sensor and it worked, so that was part of the problem. But, the car is still below its average. One more thing is that this car had its accelerator cable adjusted by a local mechanic, not a dealer. Can this affect its rpm? Sometimes, after driving and for example, stopping at a parking lot, the car seems to remain at a higher rpm than usual, instead of reaching a lower rpm level. This happens, but not all the time, maybe 50 percent of the time. Other thing is that in the mornings when the car is cold, I heard a sound similar to the one you can hear on a blender immediately after start up, and it disappears quickly, happening only when the car is cold. In the past, the car made a similar sound and the starter was replaced, but somehow, this sound is different. The catalyst hasn't been replaced, is running with the same one it has since new. The other thing, are well. The car runs and accelerates well, but the accelerator feels hard when I compared with the past. This is since the accelerator was adjusted, what mechanics here call the accelerator leg, and other call it the accelerator cable. I don't know if that adjustment is making a difference, but I am going to try with other spark plugs I have, to see if there is a difference with the ones my car is using right now. Since I am using these spark plugs, I haven't heard the usual engine knock that this car makes in heavy traffic, using premium fuel all the time. And, because the owner's manual relates the small amount of engine knock with fuel efficiency, maybe that is another fact. Also, I don't know about the rest of the sensors this car uses, (if they are working well), but I do know that there are many other sensors or parts related. (I have the big manual also). I don't know about the new gasoline's that are being sold, but here in Puerto Rico everybody uses premium 93 fuel all the time. Well, that is all the information I have. Maybe others have bigger problems, but one of the virtues of this car is its fuel efficiency, and if it looses it, the car looses a great part of its value. This is not a Porsche, or a BMW M3, or one of the other monsters around, but is a reliable car, that requires little maintenance, and, it usually delivers better fuel than the average by far, which I read that for 1999 the mpg average was 23.8. Pablo in Puerto Rico