There are other things too. When any service is done that requires disconnecting the battery, the Engine Computer loses its memory and will have to relearn the characteristics of all the engine sensors by comparing them and various operating conditions to each other. That doesn't take real long, but fuel mileage can improve over a few days. Those fuel trim "lockup tables" have to be rebuilt too. You'll never notice that is happening, and that too shouldn't take too long.
Be aware too that thanks to our wonderful politicians, we have some real miserable winter fuel blends now. The exhaust on today's cars is so clean you can suck on the tail pipe and live to tell about it, but it comes at a cost, and that is greatly-reduced fuel mileage. My 1980 Volare consistently gets 28.3 mpg in the summer but only 19 mpg in the winter. A 1968 Buick Wildcat was so big, you needed binoculars to look in the mirror to see the tail lights, but they easily got 22 mpg. The problem is those cars are dirty as far as emission is concerned. With today's technology and seriously lighter cars, we should be getting 50 - 60 mpg, but the trade-off would be increased emissions.
I wouldn't be too concerned with your fuel mileage yet until it gets warmer I have a friend who specializes in rebuilding smashed one and two-year-old Dodge trucks, mainly dually diesels. His latest project started out getting less than 16 mpg, but the more he drove it, the better it got. Today it's up to over 22 mpg. That's in winter, but not when pulling a big trailer. It took almost a month for the Engine Computer to relearn everything as he drove it.
Monday, March 2nd, 2015 AT 3:28 PM