DEAR TOM AND RAY: Please settle a difference of opinion. All things being equal, which is harder on my engine: traveling 500 miles at 50 mph, or going 500 miles at 75 mph? My other half justifies her 75 mph theory with the shorter time the engine is working. Which uses less fuel? And if you could cite research sources, that would be appreciated. Thank you for the time and trouble (and no, there isn’t any money riding on the answer, just my male ego).
- Will
TOM: Congratulations, Will. Your ego will remain not only intact, but actually enhanced by our answer.
RAY: This reminds me of the old lame joke about the guy who was almost out of gas, so he drove home fast, hoping to get there before he ran out.
TOM: The primary difference between 55 mph and 75 mph is the wind resistance, because wind resistance makes the engine work harder - a lot harder. The wind resistance is almost double at 75 what it is at 55.
RAY: Here are some citations for that fact, Will: Newton, Issac, second law of physics, 1687; Newton, Issac, air resistance, 1726; Bernoulli, Daniel, Hydrodynamica, 1738; and Euler, Leonhard, Euler equations, 1757.
TOM: Have your “other half” start with that stuff, and when she’s ready, write back and we’ll get her a workbook with some Navier-Stokes equations.
RAY: More recently, Bridgestone did a study, mostly for the benefit of truckers trying to find the ideal highway speed, and they found that at 75 mph versus 55 mph, over the long term, maintenance costs could increase by 10 percent to 15 percent, with a corresponding drop in engine durability.
TOM: They also found that tire life decreased 10 percent to 30 percent due to the higher speed.
RAY:And fuel economy definitely takes a hit due to the higher wind resistance. The same study found that when you drop your speed from 75 mph to 55 mph, your mileage improves by almost 40 percent! Here’s the link, Will, because I’m sure she’s not going to believe you, or us, with good reason: (tinyurl.com/lu37aqr).
TOM: They found that for every mile per hour you increase over 55, you lose an average of 1.6 miles per gallon.
RAY: Now, all vehicles are different, with different engines, transmissions and drag coefficients. So the “optimal” speed for any individual car might not be exactly 55 mph. But in general, the faster you go over 55 or 60, the harder your engine has to work, and the lower your mileage.
TOM: Of course, there is a cost for driving at 55 mph versus 75: your time! It takes longer to get to your destination.
RAY: And in your case, that may be a reason to go faster, Will. If you let your wife drive at 75, it’ll allow less time for her to regale you with her wacko theories.
TOM: Enjoy your victory, Will, but try not to gloat. Remember, you’re undoubtedly wrong about plenty of other things!
DEAR TOM AND RAY: I have an ’06 Toyota Sienna CE model with approximately 85,600 miles on it. Recently, I changed two of my tires and had a wheel alignment. Since that time, my tire-pressure warning light has been coming on. I took it to the mechanic who replaced the tires, and he checked and found nothing wrong. He said the tire pressures are all fine, and he said to just drive it and the problem will go away by itself. I’ve driven it for a couple of days now, and the light is still coming on. When you start the car, the light doesn’t come on right away. But then after driving five to seven miles, it comes on. The tire pressure is good.I am checking it every day. Any idea how I can resolve this matter?
- Mir
RAY: Yeah - by going back to the mechanic with a tin of warm brownies. That often gets us to try a little harder. That’s what your guy needs to do.
TOM: My guess would be that when he changed your tires, he accidentally damaged one of the tire-pressure sensors.
RAY: On the part of the valve stem that sits inside the tire is a pressure sensor with a little transmitter. That sends information about the tire’s pressure to the car’s computer.
TOM: The tire-pressure warning system on the ’06 Sienna does not tell you which tire is low, so you don’t know which of the two sensors got damaged.
RAY: But here’s what you can suggest to the mechanic. Tell him that while you’re not absolutely sure, it seems pretty likely that one of the tire-pressure sensors got damaged when he changed the tires.
TOM: Then make him a deal. Have him put new sensors in those two new tires. He can do them one at a time if he wants to. Maybe he’ll guess right the first time - he’s got a 50-50 shot - and he won’t have to do both.
RAY: And if a new pressure sensor - or pressure sensors - makes the light go off, then it was pretty obvious that he’s responsible, and you’re all set.
TOM: But if he changes both sensors and that doesn’t fix the problem, then you’ve determined that he’s not at fault - in which case, you’ll pay him for that extra work.
RAY: In that case, it may have just been a weird coincidence, and a sensor in another wheel just happened to fail after your tires were changed. But I doubt it. Good luck, Mir.
Click & Clack (Tom and Ray Magliozzi) dispense advice about cars in Car Talk every Saturday. Email them by visiting cartalk.com
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