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: (www.bridgestonetrucktires.com/us_eng/real/magazines/ra_special-edit_4/ra_special4_fuel-speed.asp).
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!
— Get more Click and Clack in their new book, "Ask Click and Clack: Answers from Car Talk." Got a question about cars? Write to Click and Clack in care of this newspaper, or email them by visiting the Car Talk website at www.cartalk.com.