You bought a used car, signed the paperwork, and headed home. Before you even reached your driveway, the transmission gave up. Now the seller keeps repeating two little words: “As is.” Surely those words can’t cover a car that barely survived the trip home. Can they?

Everyone wants to squeeze a few extra miles out of a tank of gas, especially when prices keep climbing. Unfortunately, some supposedly clever fuel-saving tricks can actually make your car less efficient. From questionable driving habits to expensive products promising miraculous results, plenty of popular advice doesn't hold up under scrutiny. Here are 29 things that might be doing your gas mileage more harm than good.

I thought replacing my car battery would solve my starting problems for several years. Instead, almost exactly two years later, I'm standing in a parking lot with another dead battery and another replacement bill. The mechanic says batteries wear out, but this is becoming an expensive pattern. Am I just unlucky, or is something about my car or driving habits causing the problem?

The whole point of paint protection film is to prevent expensive cosmetic damage, so it can sound backwards when a dealership warns that installing it could put the factory paint warranty at risk. Before abandoning the idea or accepting a future warranty denial, you've got to figure out exactly what the factory warranty covers and what an aftermarket modification actually changes.

If your mechanic says, “Since we already have this apart, you should replace the water pump too,” it can sound like classic upselling. Sometimes it is not. In many cars, preventive water pump replacement really is cheaper when the timing belt, timing cover, or related front-engine parts are already off.

When I found an inexpensive used car with surprisingly low mileage, it seemed like the perfect deal. The odometer suggested plenty of life remained, and the purchase price was comfortably within my budget. But after several trips to the repair shop, I'm beginning to wonder whether I overlooked something important.