[CANCarbGuy]

Hey guys and girls, long time visitor to the site, first time posting. Not long ago I was in a situation that raised some questions that I'd like some opinions on.

Recently, I helped my cousin buy her first vehicle due to her heading off for university. Due to a low budget, we only had 2 options. The first was her friend's 2001 Honda Integra sedan (1.8 I4, 4 speed auto transmission). He let us borrow it for a few days and I expected it to do well, but MPG came back at a disappointing 19.5/ 28 (hand calculated). I thought for sure we'd at least break into the 30's. The other option was her next door neighbour's 1989 Buick Electra Park Avenue (3.8 V6, 4 speed auto transmission) that the family was selling (the owner had been moved into an old folks home). We convinced them to let us borrow it for a weekend to test it out, and we got a solid 19/ 29 mpg. Between the two, the big difference between them was the condition. The Integra, maintenance wise, looked to been beaten and neglected as far as I could tell. it sounded fine, but the oil, spark plugs, air filter, all were old and black with use. One of the spark plug wires had electrical tape wrapped around it in one area, and I didn't even want to think about things such as alignment, tire pressure, fuel filters, etc. The Buick on the other hand, was in mint condition. Everything from rubber suspension parts to the air cleaner looked new. As far as I could tell, everything was %100 up to date maintenance-wise.
Also, the Integra had roughly 201,000 KM's, the Buick had 186,000.

Now here's the weird part: After my cousin bought the Buick, I checked fueleconomy.gov to see if what we got MPG-wise, was normal. For the new ratings, the Integra is rated at 20/28, so we actually got pretty close. The old ratings rate it at 24/30. The new ratings for the Buick is 17/26, but the old ratings are 19/28, which again, we almost hit spot on.

So, to close, my questions are: How much does long term maintenance really effect fuel economy? And, how the heck did fueleconomy.gov reach their conclusions about what MPG ratings older vehicles should get? To be honest, I'm still a little skeptical that a 27 year old, V6 powered Buick was able to meet/ exceed it's original ratings while the lighter, 15 year old I4 Honda wasn't even capable of meeting its new ratings. Don't get me wrong, I know that a full, in-depth tuneup would do it wonders, but would it be enough that it gains back the 4.5 city and 2 highway MPG's that it would need to reach it's original rating as well? All thoughts, options, and general speculation welcome.