What is the best speed to maximize a vehicle's fuel efficiency?

Dear Car Talk

Dear Car Talk | Dec 01, 2002

Dear Tom and Ray:

I was amazed that the GM Web site could not answer my simple question: What is the best speed to achieve optimum miles per gallon for my '01 Pontiac Montana minivan? It even suggested that there's nowhere I can go to get that piece of information. Come, now. You mean NO ONE has that information? Wouldn't it be a simple matter of, say, the government hooking your car up to a testing machine and coming out with not just the best speed, but a line chart showing how higher and lower speeds make the vehicle progressively less fuel-efficient? I'd love to know, since I have a bit of a lead foot. But I'm also a cheapskate, so this information might help me modify my bad behavior, if you know what I mean. Thanks. -- Gene

TOM: Fear not, Gene. We have the answer. We know the exact speed at which your vehicle gets the maximum fuel economy.

RAY: Yeah. Zero. When it's parked in the garage, with the engine off. You can't do any better than that.

TOM: When it's moving, it's a little harder to determine the exact speed. It will vary depending on roads, the weather and whether the driver accelerates gently or drives like a knucklehead (hint, hint).

RAY: But if you're looking for a pretty good estimate, here it is: What you want is the spot where the engine is moving slowly, yet the wheels are turning quickly. And that spot -- the sweet spot -- is the speed just after it shifts into its highest gear.

TOM: In other words, your Montana has a Fourth gear, which is Overdrive. Let's say it shifts from Third to Overdrive at around 45 miles per hour (again, this depends on whether you're accelerating, going uphill, etc.). Right after that shift is where the engine will be turning its slowest while, comparatively, the wheels will be turning their fastest.

RAY: Now, to get back to your original question -- the EXACT speed at which you get your best mileage might vary from that by a small amount because of the way the manufacturer has set the gear ratios and engine performance, but as we say, that's a very good estimate.

TOM: The reason you get poorer fuel economy as you go faster than that is largely due to wind resistance. Wind resistance increases as a square of your speed. So, believe it or not, at 65 mph, your wind resistance is more than double what it is at 45 mph (e.g., 65 squared vs. 45 squared)!

RAY: Of course, you can't always drive at 45. At that speed, you'd be a danger to other people on the highway. But basically, just remember that the faster you go on the highway, the more gasoline you're wasting just breaking through the wind.

TOM: Is there a "breaking wind" joke here that I missed?

RAY: Let's just end the column now, shall we?

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