By: Mike Allen, The Saturday Mechanic

In my more than two decades of answering car care and repair questions, I’ve had readers who push me to my limits to expand my automotive knowledgebase. I’ve also received questions that seem so simple it makes you wonder if the writer knows which end of the car is the trunk (tricky with older VWs and Porsches). But the following are probably the eight most common questions I’ve received over the years.

Q1.    Are there any emissions control devices on my car that I can remove or disable to improve the economy and performance of my car?

A: No. Okay, back in the early days (‘60s and ‘70s) of emissions devices, you might have been able to improve drivability by unhooking an emissions-control vacuum line or taking the belt off of the air injection pump. Not anymore. The emissions system is completely integrated with the engine management system.  If you try to tinker with it you’ll just make the CHECK ENGINE light come on, likely reduce your fuel economy and increase emissions, and make it impossible to pass an annual DMV inspection.

Q2.    My mechanic says I shouldn’t use that new 0W-20 oil in my new [insert your new vehicle here]; it’s too thin. Should I let him put in the thicker 20W-50 oil I used to use in my older car?

A: Vehicle manufacturers have re-engineered newer vehicles to use thinner and thinner oil, in an attempt to reduce viscous and hydrodynamic drag inside the engine. Thinner oil takes less power to pump around, and that consumes less gasoline—which is good! The bearings and wear surfaces inside the engine have been specified to run in that thinner oil. Using thicker oil may actually damage your engine, as it may not make it to needed places when the oil is cold and thick and hard to pump. Think molasses on a cold morning.

As always, the definitive source for the appropriate type and viscosity of lubricants is your owner’s manual.

Q3.    What additives should I add to my gas and oil to improve fuel economy?

A: If your car is operating normally, there are no additives that will improve the economy, and I’ve tried them all. If your older car has seen deteriorating mileage or performance lately, then a couple of cans of fuel injector cleaner or a couple of tankfulls of a top-tier-rated gasoline may restore the original performance.

Q4.    Is it okay to use a brillo pad to take bugs off of my hood and grille?

A: Never use any household product on the paint of your car; it’ll leave scratches. My remedy for bugs is to soak them with water for a half-hour, and rub gently with a clean terrycloth or microfiber cloth dampened with plain water and a drop of carwash detergent. Stubborn bug? Soak longer, try again. A really stubborn spot might need a spot of rubbing compound. Keep your car waxed, the bugs won’t stick as persistently, and if you remove them promptly, they’ll be less tenacious.

Q5.    It’s [insert season].  Is it okay to use plain water in my cooling system, instead of antifreeze until it gets cold?

A: No. Coolant (which is what it’s appropriately called, because it does far more than keep the radiator  from freezing) has a higher boiling point than plain water, which will provide extra protection from overheating. Also, plain water has no anticorrosion additives, which will sludge up your cooling system within months. Always maintain a 50-50 mix of the appropriate coolant and water in your system, winter or summer.

Q6.    Will I save gas if I throw the transmission into neutral and coast down hills and up to traffic lights?

A: This doesn’t save any gas. Modern fuel injected cars don’t spray any fuel into the engine when coasting with your foot off the accelerator and the car moving faster than about 10 mph. None. But when you’re coasting in neutral, the engine is still idling, burning gas at the rate of 2-4 gallons per hour.

Q7.    I want to buy a car that’s 100% made in the USA, can you give me a list?

A: There are no vehicles with 100% domestic content. Even quintessentially American icons like the F-150 pickup truck only have 90% domestic content, and that includes Canadian parts. Even the Crown Victoria, as American as Apple Pie, was assembled in Canada. In fact, some Asian-nameplate vehicles assembled in the U.S. have far more true domestic content than some American nameplate cars. The list is too long to reproduce here, but here’s a hint: the amount of domestic content on any vehicle is listed clearly on the window sticker when you’re shopping at the dealer.

Q8.    My [insert relative] says I can save gas if I run my tires at 50 psi to reduce rolling resistance?

A: Well, he’s technically correct. But the amount you’ll save is minor, and it’ll be at the expense of braking and cornering capability. Overly high pressures will drastically reduce the amount of rubber the tire puts on the road as the contact patch shrinks as the pressure goes up. Check the placard in the doorframe or glovebox for the manufacturer’s recommended pressures, and set them—cold—to the upper end of the spec range for the best mileage. Remember that air pressure changes about one psi for every ten degrees of outside air temperature, so check regularly as the seasons change.

Mike Allen is a longtime automotive expert and journalist. He owns and publishes SaturdayMechanic.com, a website dedicated to people who still believe in repairing their own cars. Mike spent 25 years at Popular Mechanics magazine, finishing his stint there as Senior Automotive Editor. His Car Clinic column was syndicated by the NY Times, reaching nearly 20 million readers weekly. Mike has appeared on national TV numerous times, including Monster Garage, Mythbusters, NBC Dateline, Paula Zahn and numerous regional shows. He’s also a current ASE-certified Service Technician.