An idle question about regular oil changes on a car


Published: Saturday, January 13, 2007 at 6:01 a.m.
Last Modified: Friday, January 12, 2007 at 11:45 p.m.
Dear Tom and Ray: You guys are great. Thanks for all your knowledge and insights. My question: My vehicle's manufacturer suggests changing the oil every 5,000 miles. If I were averaging a speed of 60 mph for that 5,000 miles, it would take just over 83 hours to reach that mileage. I have installed an hour meter on my vehicle so that I know to change my oil every 83 hours. Does the manufacturer figure in idle time (caused by heavy traffic and such)? I drove an 18-wheeler for 11 years, and idle time on a big rig is astronomical. That is when I came up with this crazy question. Am I actually crazy, or overprotective, or do I simply have too much free time on my hands?
Dale Tom: All of the above, Dale.
Ray: Actually, you're not crazy. Five thousand miles is the manufacturer's estimate of when the oil will need changing, based on the average person's driving and idling time. But it's just that: an estimate.
Tom: And you're right. The time your engine spends idling certainly does contribute to the breakdown of the oil. Which is why vehicles that idle a lot, like 18-wheelers, taxis and police cars, get their oil changed more frequently than your car does.
Ray: If you look in your owner's manual, you'll see that they change the estimate if you use your car abnormally.
Tom: Does living in your car constitute using it abnormally?
Ray: Yes, but not with regard to oil life. Your owner's manual probably lists two different oil-change intervals. One is for ''normal'' duty, the other is for ''severe'' duty. Severe duty is defined as operation in extremely hot or cold weather, towing or using the vehicle like a taxi (stopping, starting and idling a lot). And the severe-duty oil change is more frequent, to account for that extra wear and tear.
Tom: Even better than those estimates, a number of cars' computers now actually calculate when the oil needs to be changed. They do it based on an algorithm that takes into account idling time, engine temperature and driving conditions. So your idea is perfectly logical, Dale. It's just another way to estimate oil life.
Ray: But you might be overdoing it a bit. It would take you 83 hours to reach 5,000 miles IF you were driving at 60 mph. But you're not driving at 60 mph all day, every day. If you were, you'd have a hell of a time getting out of your driveway without wiping out your mailbox and your neighbor's prized Uzbeki tulips.
Tom: Assuming you do a normal mix of highway and city driving, it would probably take you more like 100 to 125 hours to reach 5,000 miles.
Ray: I'd recommend that you go with 100 hours, Dale.
Tom: Based on what algorithm?
Ray: Based on the number 100 being easier to remember than 83.
  • Dear Tom and Ray: We purchased a new 2005 Jeep Liberty for our daughter last year and have faithfully taken it in to the dealership where we bought it for routine maintenance. At the time of the last maintenance checkup (11,784 miles), the repair shop performed rear and front differential service, at a cost of $240. They said it should be performed every 12,000 miles. I've never had routine differential service performed on any car I've ever owned. Is this legitimate?
    Federico Tom: Hmm. How to put this gently, Federico? Did the service manager have binoculars hanging around his neck? Because he definitely saw you coming.
    Ray: Most cars never need differential service. That's a component that, under normal driving conditions, should last for the life of the car without needing any attention.
    Tom: If you do what manufacturers call ''severe duty driving,'' then Jeep does recommend changing the fluid in both differentials every 12,000 miles. But severe duty means you're doing things like extensive off-road driving, using the vehicle as a taxi, driving mostly in extreme hot or cold temperatures, or regularly towing around a couple of Angus bulls.
    Ray: If that describes your daughter's driving, then the dealer is doing the service that Jeep recommends. But if your daughter is like 99 percent of America's drivers and uses the Liberty to go to school or work, or take road trips with her friends, then the dealer took you for a ride. And you should go back and ask for some money back.
    Tom: For future reference, if you look in the back of your owner's manual, the recommended services for each mileage interval are listed there. So you can see for yourself what the manufacturer actually recommends at 12,000 miles. Look under Schedule A, which for Jeep is normal use.
    Ray: If your daughter's Liberty doesn't do severe duty, and you don't get satisfaction from this dealer, remember that you can take the car to any mechanic you like for its regular service - it does not have to be done at a dealership. Simply hand them the owner's manual, point to your mileage interval and say, ''These are the services I'd like you to perform.'' Keep the receipts, and it will have absolutely no effect on any free warranty work you need to have done by the dealer.
    Got a question about cars? Write to Click and Clack in care of this newspaper, or e-mail them by visiting the Car Talk Web site at www.cartalk.com.
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