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" Q: One tire on my Yukon has a slow leak. Or so I thought, until I just replaced the tire because I got tired of filling it every week. Now the new tire leaks. The guy at the gas station wants to put some of that green, slimy goo in the tire to fix it, but I want him to replace the new tire with one that doesn&amp;#039;t leak. A: Did either of you two think that maybe the tire isn&amp;#039;t leaking? Most cars have alloy rims nowadays. And unlike steel rims, a fair number of these wheels leak air. It&amp;#039;s not a bead-seat surface leak, where air can creep out between the rim and rubber, but a leak through porosities in the cast-aluminum alloy. GM has a published repair procedure for this problem: Inflate the tire to 40 psi or so, and immerse the whole thing in a conventional dunk tank. Mark the rim with a grease pencil where the bubbles form. Then pull the rim out of the water, and demount the tire from the rim. Scuff lightly with 80-grit sandpaper. Dry off and clean the area corresponding to the leak on the inside of the rim with carb cleaner or mineral spirits, and cover it with a thin layer of silicone gasket sealer. (Don&amp;#039;t use silicone tub caulk, which outgases acetic acid as it cures; it will corrode the metal wheel and may damage any tire-pressure monitoring-system sender.) Allow it to cure a few hours, then remount and rebalance the tire. That&amp;#039;s GM&amp;#039;s take. Other manufacturers have different policies as to what the warranty will do about faulty, leaky rims. I chatted with Matt Edmonds, vice president of Tire Rack, which sells a huge number of tires and alloy wheels. &amp;quot;If we find a wheel that&amp;#039;s porous and leaking, we just replace it, not repair it.&amp;quot; I&amp;#039;m siding with Matt. Any wheel that has a porous spot large enough to leak air obviously isn&amp;#039;t as strong as one that&amp;#039;s properly cast. I&amp;#039;ve had wheels fail structurally at speed, and it&amp;#039;s pretty scary. The Mechanic&amp;#039;s Heads Up Working as Designed: Heating Up Heated Mirrors  (Photograph by Getty Images) Heated sideview mirrors are a great convenience. They use a simple resistance heater glued to the back of the mirror glass to gently warm the surface and remove ice and snow, while you keep your fingers warm and dry inside the vehicle. It&amp;#039;s a good safety device. I was driving a new pickup last week and noticed an odd thing--the passenger side mirror heated up more slowly than the driver&amp;#039;s side. A lot more slowly. So, as you&amp;#039;d expect, I had to ask why.  Turns out I wasn&amp;#039;t the only one who noticed this, because GM has a service bulletin on it, covering a bunch of trucks and passenger cars from the past 10 years or so. There&amp;#039;s no problem, they say. The glass on the passenger side is thicker because the mirror is more curved--&amp;quot;Objects in mirror may be closer than they appear,&amp;quot; indeed. Thicker glass takes longer to heat up. So complaining to your dealer won&amp;#039;t help. And, in truth, the heater isn&amp;#039;t supposed to zap all the moisture off the mirror instantly. It&amp;#039;s supposed to keep the surface clear while you&amp;#039;re driving along, which it does just fine. But if I owned one of these trucks or SUVs, I&amp;#039;d track down a junkyard mirror, dismantle the heater and use it on my own truck. It would be easy to glue it to the back of the mirror right next to the original and wire it in parallel, doubling the heat. Gearing Down Q: I own a half-ton pickup with an automatic and a V6 engine. I often downshift as I leave the freeway. Would you please comment on this practice? A: Sure. I once had the privilege of spending some time with engineer and race driver Mark Donohue, back when I was a race driver and not a journalist. The topic of downshifting came up when another driver said that he would downshift coming into corners to preserve his brakes. Donohue disagreed with this practice: &amp;quot;Brakes are cheaper than engines.&amp;quot;  And I have to agree. Don&amp;#039;t get me wrong, I downshift both manual and automatic transmissions all the time when the need arises. I do it when I want the transmission to be in the correct gear while exiting the corner, either on the street or the racetrack. On the road, downhills are a good reason to grab a lower gear to keep from building up too much speed and cooking the brakes by riding them. I know. Modern cars, with low-rolling-resistance tires, low-friction engines, tall gears and superlow drag coefficients, don&amp;#039;t slow down as much as they used to. That&amp;#039;s part of the reason brake pads have such short lives nowadays. Bottom line: In spite of Mark&amp;#039;s advice to avoid downshifting unnecessarily, downshifting won&amp;#039;t hurt your truck. The automatic transmission won&amp;#039;t let you get into a gear that&amp;#039;s too low and that would consequently bend the valves. However, you&amp;#039;ll have better control on slippery surfaces with your front-heavy pickup when your foot&amp;#039;s on the brake pedal. This will make the brakes slow down with all four wheels instead of using the engine to brake just the rear tires. This goes double in rain or snow. Braking the rear wheels too much is guaranteed to cause an instant spin. That&amp;#039;s why cars and trucks have bigger, more powerful brakes on the front axle. If you see bubbles coming through the metal wheel, mark the area with a grease pencil. "^^<http://www.w3.org/2001/XMLSchema#string>
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