Remember to factor in mileage and time for maintenance

Dear Car Talk:

I have a 2020 Honda CR-V with 18,000 miles on it. The maintenance minder says I need to have the rear differential fluid and the brake fluid replaced now. This seems excessive to me.

What interval do you recommend as the appropriate time to have these fluids changed? — Dave

That’s not as simple a question as you might think, Dave. Recommendations like these are based on science. But it’s not an exact science.

There are two factors involved: mileage and time. Mileage — which is a rough measure for how much use the fluid gets — matters a little more to me. The more a fluid is used, the hotter it runs, the dirtier or more contaminated it gets, and the closer it gets to the end of its useful life.

The other factor is time. Time does matter, because some chemicals or chemical products do degrade over time. Or, in the case of brake fluid, if it’s already holding all of the moisture it can keep in suspension, you don’t want excess moisture sitting in the brake lines and corroding them from within.

Honda comes down on the side of time here. Your maintenance minder comes on after three years of driving. Honda probably assumes you’ll be at around 36,000 miles by then, like the average driver. But they obviously believe time is a factor as well, and they may have data to back that up on their cars. I don’t know.

So, my answer: I don’t think there’s any great rush to change either of these fluids. I doubt anything terrible will happen if you wait another year or even two, given your low mileage. On the other hand, I’d lean toward changing the brake fluid sooner rather than later. Is it probably a little early? Yeah. But it’s not an expensive job (probably $150 at an independent mechanic, possibly a bit more at the dealer), and if you’re planning to keep the car for a long time, it’s the safest thing to do for your brake system.

I’d be willing to wait longer on the differential fluid. These cars have an issue where, when the differential fluid degrades, the differential will “chatter” on sharp turns. Honda’s solution is to call for changing the differential fluid before it has a chance to degrade. But I can’t remember seeing a CR-V with a chattering differential with such low mileage in my shop. We tend to see it at closer to 75,000 miles. So, I’d be comfortable waiting another year or two to do that.

And, of course, if you ever notice wheel chatter on sharp turns, then I’d take it in and get it changed, and that will fix it. But neither of these are emergencies, Dave. You don’t need to cancel your dentist appointment tomorrow in order to rush into the shop for brake and differential fluid.