I bought a used BMW from a Jaguar dealership. They said they did a 50 point (or whatever number) inspection on all used cars they sell. I buy it. It runs fine. But, it's really bad on gas mileage. My previous car had a 4-cylinder and I knew going to a 6-cylinder would use more gas but not in the low-20 MPG. Anyway, I start teaching myself some basic stuff. I buy a Haynes manual. On my first foray into doing a tune-up, I check the air filter. Now, I've only had the car for two months in pretty much a non-dusty air environment in Pennsylvania, not the Baja Desert. I take out the air filter and it's got huge amounts of dirt in the filter. Literally, I tilt it over and sand is falling out. It's like if they did a 50 point inspection, they would have replaced the air filter. Anyway, I replace that, change the spark plugs, distributor cap and rotor and my MPG goes up by 6 MPG.