Puzzle: What caused my fuel economy to increase?

For the past 6 months or so, my fuel economy has been about 38mpg over the course of a tank, on every tank; it never went below 36mpg, but I could never seem to get it above 38mpg either.

I just today had my oil changed, oil filter replaced, and air filter replaced.

I have had my oil changed about every 5,000 miles or so. This was the first oil + air filter changes though, as I only recently became aware that they should be replaced about as frequently as the oil itself.

After doing some driving around town, about 10 miles worth, my fuel economy is about 50mpg right now. I didn't employ any special hypermiling tricks, I just drove the car normally through city traffic. The routes I took were the routes I drive every day.

The oil was changed on schedule, so I assume that was not it; the air filter did not really look dirty at all, but the new air filter was a shade lighter, so maybe you only need a little dust in order to have a "dirty" filter. However I know that clean air is essential to good combustion, so that may in fact be what it was, but again, it did not look that dirty.

The oil filter was replaced, and as I understand it, it filters the oil going into the engine, and having a dirty filter can restrict the flow of oil into the engine. I did not see the old filter, and even if I did, after looking at the new one, I'm not sure I could have just visually inspected it to see if it was clogged, like I could do with an air filter.

So my question is: what specifically could have caused my mpg to jump? Or, put it another way, what was it that caused my fuel economy to drop in the past?

Any help would be greatly appreciated.

 

Right, good points. Well, I always had the oil changed at the same place prior to today, but I wanted to save a bit on the oil change today because I knew I would be getting other stuff replaced too.

This new place charged half as much, but used the same oil ("Bulk penzzoil 5w-30"). I don't know how much they pumped in at the old place, but this time they put in 3.8 quarts I believe, and, after reading from some people here on Priuschat that overfilling can cause problems, I told the guy not to overfill it, and he said that he wouldn't; that he was aware of the risks, because they had overfilled some guy's car previously and his engine failed to start, or something. So he was nice about it, and was pumping it in slowly, and the pump had a digital readout to indicate how much oil was being put in. Again, I believe 3.8 quarts, of 5w-30 was pumped in.

I have never really checked the oil, I just never got around to it, because every time I meant to do it, I had used the car recently and so the various parts under the hood were hot, so I would say "I'll do it in the morning when its cooled off" but never did. Thusly, also never checked the oil levels after leaving an oil change...

I check the tire pressures maybe once every two weeks. I used to to it daily, but had the tires replaced a couple months ago, and they would read the same pressure whether I checked it once a day or once a month, so I just feel reassured that they are solid tires and therefore are not losing air.