QDear Tom and Ray:

I was amazed that GM's Web site could not answer this question: What is the best speed to achieve optimum miles per gallon for my 2001 Pontiac Montana minivan? It even suggested there's nowhere I can go to get that information. Come, now: No one knows? 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 also 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.

-- Gene

ATOM: We know the exact speed at which your vehicle gets the best mileage.

RAY: Zero. When it's parked, with the engine off.

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 and 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 fourth around 45 mph (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.

TOM: You get poorer fuel economy as you go faster than that largely because of wind resistance. Wind resistance increases as a square of your speed. So, at 65 mph, your wind resistance is more than double what it is at 45 mph (65 squared vs. 45 squared).

RAY: Of course, you can't always drive at 45. On the highway, you'd be a danger to others. But just remember that the faster you go on the highway, the more fuel you're wasting just breaking through the wind.

Dear Tom and Ray:

I recently purchased a 1999 Toyota Avalon XLS. It's a great car, but the owner's manual calls for H-rated tires. From what I understand, such tires are for speeds up to 130 mph. I never drive over 75 (okay, maybe 80 sometimes on the highway, when no one is looking). H-rated tires cost more and are harder to find, especially with a high-mileage warranty. My dealer and tire salesmen say I should use only H-rated tires, but mechanics say the high-speed tires aren't necessary. What problem would I have if I used tires with an S (112 mph) or T (118 mph) rating?

-- Wayne

RAY: I don't think you'd have any problem. But our lawyer, J. Cheever Loophole, says we can't recommend that you do that.

TOM: Although you don't plan to drive 120 mph, the car is apparently capable of going that fast. And because it's possible to drive the car that fast, Toyota has to equip it -- and recommend that you equip it -- with tires that will stay intact at that speed.

RAY: So you could buy lower-rated tires, and you'd probably be fine as long as you never approached their limits. But if your lead-footed mother-in-law borrowed your car one day and decided to "see what she could do," she might never come back.

TOM: And what would be the downside? Ah! The car wouldn't come back, either.

RAY: Well, do what you think is best, Wayne. But we recommend that you stick with the H-rated tires, just to be safe.

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(c)2002 by Tom and Ray Magliozzi and Doug Berman