I bought a brand new 2008 Jetta SE (2.5L petrol engine, automatic transmission, 21mpg city, 29mpg highway) in May 2008, which had under 200 miles on it. I immediately within a week drove it from eastern North Carolina to southern Ontario in a day, going above 100 mph for some parts of the trip, possibly averaging above 74mph.
I'm not sure when exactly, but I've noticed from the very start that my car had a really-really bad gas mileage. I would usually get about 200 miles until the gas light would come on, sometimes even just under 200 miles; the gas tank is 14.5 gallons, IIRC, but it usually fills at about 12,5 gallons when the light goes on. 200 ÷ 12,5 is 16mpg, ridiculously low. I think even going on the highways at 79mph to 89mph never brought me above 250 miles on a 13 to 13,5 gallon tank, and I don't drive in the Sport mode all the time, either.
I've had a number of oil changes at the dealerships, where they obviously supposed to use "fully" synthetic oil, and charge and itemise for the oil changes as such. Mileage has never improved.
Except that recently, I've noticed that my oil level was non-existent after about ~10k to ~12k miles from the last oil change (I got lazy at visiting the dealerships, being dissatisfied with their service), and added an actual 5W40 "fully-synthetic": ~1L Castrol, 1L Pentosin, then a couple of k miles afterwards, after learning about ester-based oils and getting low on oil again, 1L of Redline, then a couple of more k miles, one more Redline (yes, 4L in total!); I also got new tires of the same size as OEM, but a different brand (Yokohama in place of Michelin), and also started driving longer distances on highway and also higher altitudes; I've noticed that now it's unusual for me to have less than ~300 miles prior to the low-gas light, and once through Wyoming, I got to almost 400 miles on a single tank, getting above the promised 29mpg (I think I got even 30 or 31mpg!) with now nearly 20k miles after the last full oil change, with 4L of top-up oil, 2L of which is ester-based Redline.
Why did my car always had 16mpg when I just bought it, when it's supposed to have been 21mpg city and 29mpg highway? I do use AC, but I don't idle. I drove highway, from NC to ON, and never got anywhere close to 29mpg.
Did my mileage really improve recently?
Is there anything that I could do to be compensated for bad mileage from the beginning of owning the car? I might have all the gas receipts; will have to check.
Do I need to change the oil at all? Is it at all weird to be getting such great mileage when the original 6L of oil was reduced into 2L, and has now been running for close to 20k miles?