Today: The Difference Between Highway and Off-road Miles?

Dear Car Talk

Dear Car Talk | May 13, 2014

Dear Tom and Ray:

Greetings from Doha, Qatar. Six months ago, I bought a used 2006 Hummer H3 (now with 115,000 miles), which my wife uses five days a week to take our son to school. Traffic in Doha is notoriously bad (and a bit dangerous), so these daily school runs are pretty stressful. But we may have a safety valve. We can trim the commute (and avoid a lot of traffic) by taking a three-mile off-road stretch. This isn't boulder-strewn terrain, it's just mildly rocky dirt.

I encourage my wife to take the H3 on this stretch at every opportunity -- but she's concerned that 60 off-road miles every week (to and from school twice a day, five days a week) is too much for the car and it will "fall apart" prematurely. We're only going to be in Doha for another 18 months, and she wants the vehicle to last at least that long. I say it's a Hummer -- and that this kind of driving is a Hummer's existential purpose. I don't see the car falling apart for several years to come. What do you guys say? Can she take our H3 off-road on every school run?

-- Mike

TOM: I think the real problem is ride quality in the Hummer, Mike. She's bouncing off the seat on that dirt road, banging her head against the dome light. If you had spent a little more and bought her a Cadillac Escalade, you wouldn't be hearing these complaints.

RAY: You're actually both right about the H3, though. Sometimes when you see a used-car ad, the seller will say something like "150,000 mostly highway miles." Why does he say that?

TOM: So he can unload his rattletrap of a car. Mike probably will say the same thing when he sells this Hummer.

RAY: Sure. But it's well known that smooth highway miles take less of a toll on the car than bumpy, pothole-encrusted city miles.

TOM: When you bounce the car up and down off the road continually (which is what you're doing on a rocky dirt road), you cause certain parts to wear out more quickly -- notably, the front suspension components.

RAY: Things like the tires, the struts, the springs, the ball joints, the tie rods, the stabilizer links, the steering rack and all the bushings and couplings down there get pounded when you drive that way. Those parts will wear out sooner than they would have otherwise. Even on a Hummer.

TOM: All that shaking and bouncing also causes other stuff in the truck to "loosen up." This ultimately results in what we call "rattletrap syndrome," where even if stuff is still working, the truck sounds and feels like it's falling apart because it rattles and clanks down the road, even on pavement.

RAY: In truth, it's impossible for us to know if something major will break in the next 18 months, whether you take that shortcut or not. The car has a lot of miles on it. And who knows if it would have happened anyway? All we know for certain is that the chance of having to replace suspension components goes up when you move from paved roads to rocky dirt roads.

TOM: If it were me, I'd take my chances. The H3 is based on Chevrolet's small pickup truck, the Colorado. So while it's not super heavy-duty (or unbearable to drive) like a real Hummer, it's based on a truck chassis, and should be able to take a reasonable amount of punishment.

RAY: And based on what you say, the shortcut would save your wife time and stress, and might even be safer, since it's less heavily trafficked.

TOM: I agree, as long as your wife has good cellphone service that works out there. If she ever were to break down, or just get a flat tire, you'd want her to be able to easily and reliably call for help. And make sure she has a broomstick or something in the car, so she can fend off an angry band of camels if necessary. Good luck, Mike.

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