Cars know when it’s time for an oil change

I have a Chrysler 200 that is new enough to keep me informed of how much oil life remains.

Can I trust it?

It’s been 6,000 (easy) miles and 15 months since my last oil change (synthetic), and my car is saying that 25 percent of the oil’s life is left. I’m inclined to believe it since I think that automakers are overly conservative regarding oil change intervals.

Should I change the oil when the car says 5 percent left? 10 percent? Or do you recommend a mileage or time interval?

I’d trust it, Jeff. If we just do an “order of magnitude” check, synthetic oil can easily last 7,500 to 10,000 miles before needing to be changed. So if you’ve gone 6,000 miles and have 25 percent left, you’re on track for an oil change at 8,000 miles.

That’s right on target. In case you’re interested, the oil life monitor in your car is not actually “testing” your oil. It’s not taking a sample and sending it out to the lab while you sleep, dreaming about a new Honda. The oil life monitor is measuring the conditions that affect the life of your oil. It plugs them into an algorithm and constantly produces an estimate of how much longer your oil should last. From the car’s computer, it collects information on things like the number of starts (individual trips), the engine temperature variations (driving conditions) and the number of miles you drive.

Over the years, engineers have created algorithms that are pretty darned accurate in predicting when your oil is spent. Remember, they have incentive to make sure you change your oil on time.

I’d say when you get down to 10 percent, it’s time to make an appointment for an oil change. It’s not an emergency at that point. Your oil is still fine.

The owner’s manual for our 2015 Toyota Camry recommends a tire pressure of 35 psi for all the wheels.

I check the pressure monthly, and there may be a loss of 1 to 2 psi on a couple of tires. I crank up the compressor and after a few tries, I finally get exactly that 1 psi in there.

In the process of adding air, I’ll sometimes go over by half a psi or 1 psi, which I then bleed off.

Do I need to do that? What’s the acceptable range for over and under inflation of tires?

With tire inflation, you can muck around and get close enough, and still live a full and happy life. Of the two ways to miss your mark, underinflating your tires is the bigger danger.

You probably remember the Ford Explorer/Firestone debacle from the turn of the century. Firestone made tires for the Explorer that, when underinflated and subject to lots of heat, like on Texas highways, basically fell apart and led to high speed rollovers.

While the quality of those tires themselves certainly played a role in the epidemic, all tires can be vulnerable when they’re underinflated. Underinflated tires put a larger rubber contact patch on the road, create more friction, and therefore run hotter. And heat can cause the tire’s belts to separate and come apart.

Out of that whole Ford/ Firestone disaster came a new safety feature called “Tire Pressure Monitoring Systems” or TPMS. Now every tire has its own builtin gauge and a way to communicate with the car’s computer. And if the pressure in any tire drops about 10 percent below its recommended level, an idiot light on your dashboard comes on.

So, on the bottom end, 10 percent is your lower limit. If your Camry recommends 35 psi, 31.5 psi would be the absolute lowest you’d want to let it go before adding air.

You have more flexibility on the upper end. As long as you stay below the maximum tire pressure listed on the tire’s sidewall (which is different from the recommended pressure), you can overinflate your tires by 10 percent or even more without too much concern. For instance, if 35 psi is recommended, and the maximum safe pressure listed on your sidewall is 44 psi, you can safely put 38 or 40 psi in your tires.

You can even go to 44 psi. You’ll experience a harder ride, and you may have welts on your head from hitting the roof when you go over bumps, but you won’t be creating a blowout danger. You may even experience sharper cornering and increased fuel economy, too.

So, the bottom line is that when filling your tires, the recommended tire pressure is the best compromise between handling, comfort, fuel economy and safety.

But it’s certainly fine to go over the recommended inflation by a psi or two.

And going over is always better than going under.