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Q: When I purchased a new Audi A4, the salesperson wanted me to buy a maintenance plan that covers oil changes for four years. Can my local mechanic handle the changes, or do I have to go to the dealer?

A: Your warranty will not be voided if you chose to have oil changes, or any routine service, done by your local mechanic. But whether the plan you bought covers the cost depends on the plan. Some of these plans negotiate reduced rates with selected providers — like your dealer — and will cover only work done there. Check the fine print.

Gas mixology

Q: When I get down to three-fourths of a tank of gas, I fill up. Does this mean I always have mostly old gas in the tank?

A: No. You are regularly freshening it with new gas. If you sip half a glass of beer and then top it off with some cold brew, the older stuff does not sit on the bottom of the glass. It all mixes together.

Coolant a hot topic

Q: A few weeks ago, a reader asked about the coolant temperature in his 2005 Corvette. You replied that water boils at 212 degrees, or 226 with a 50/50 antifreeze mixture. Those would be the boiling points at sea level. Engines have pressurized cooling systems that allow water to reach higher temperatures than that without boiling. This is why if you remove the radiator cap from a hot engine, the water will immediately boil to steam and blast scalding coolant all over you.

A: You are absolutely right. Are you also a mechanic? Or a science teacher?

Ragtop regimen

Q: I got my car in 2018 during a dealer's "loss leader" sale. The car is in impeccable shape, and I want to keep it that way. I conditioned the convertible top with a waterproof product. I'd like to know how often that is necessary. Also, can you advise how I can best maintain the inner parts of the car's convertible roof such as the rubber seals? The car is garaged in the winter.

A: I would treat the ragtop each spring when I take it out of hibernation. Ultraviolet radiation from the sun causes deterioration. Be sure your cleaner is a UV blocker. Each fall, treat the rubber sealing parts with silicone protectant to keep them pliable.

Missing mileage

Q: I own a 2020 Subaru Forester. The trip computer tracks mileage. After I fill up the car, I reset the trip computer. When I manually calculate the mileage, it is always about 2 miles per gallon lower than that shown on the computer. I asked a friend about his Honda Civic, and he has had the same experience. Is this a case of the automakers jacking us around?

A: A couple miles per gallon either way is within an acceptable margin of error, so I wouldn't lose any sleep over it.

Bob Weber is a writer, mechanic and ASE-certified Master Automobile Technician. His writing has appeared in automotive trade publications, Consumer Guide and Consumers Digest. Send automotive questions along with name and town to