I've been driven mad lately trying to figure out why my '99 VW Beetle with 170k miles seems to be chugging gasoline. I'm obsessed with trying to get this thing to be more efficient considering gas is above $3 again. It gets 25mpg on the highway these days. It used to get over 30. Unacceptable for a small car IMO. Parts that have been changed within the last 1.5 years: catalytic converter, both O2 sensors, mass airflow sensor, coolant temp sensor, air filter (with a K&N), timing belt and related accessories. It also has new spark plugs. My mechanic told me it has low vacuum under heavy acceleration and that it could be due to the foreign matter inside the muffler (probably from the old cat converter). But a muffler shop told me that there's no obstruction in the system. It doesn't burn oil so I assume it still has good compression. So, before I bring it in again, I would like to try to figure out why the vacuum is low. I suppose this could be related to the decreased fuel efficiency?? What else should I be looking for?