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"SAVE FUEL, GET THERE FAST Dennis Hold, Punta Gorda, Florida I have a 2012 Mustang V6 automatic with a 2.73:1 rear-end ratio. I&amp;#039;m thinking of changing to a 3.31 or 3.55. Which of these would better improve performance without a big drop in fuel economy? My Ford dealer quotes $1200 to swap the car to the 3.31, which the tech recommended, but that seems high. What should I do? Generally speaking, installing shorter (numerically higher) gearing will improve acceleration but use more fuel. Your engine will be running at a higher rpm for any given road speed. There&amp;#039;s no way around it. If you&amp;#039;re serious about performance but want to keep fuel economy in check, the 3.31 limited-slip is a logical compromise. That ratio is standard in the Mustang GT and available from the factory in the Mustang V-6 Performance package, so it&amp;#039;s a pretty safe swap. As far as the financial hit, items like these are always going to be cheaper factory-installed, not when done later. (Consider that the V6 Performance pack costs $1995 on a window sticker and also includes different wheels and upgraded chassis components.) Blame labor costs and parts markup. The dealer has to drop the differential, swap the gears, and reinstall. The factory only installs the diff once, at the most convenient time in the assembly process. The other option\u2014and hear us out\u2014is to ditch the V6 car and get yourself a used V8-powered Mustang GT. It&amp;#039;ll cost more than the $1200 you&amp;#039;d pay for the diff, but not as much as you&amp;#039;d think. You&amp;#039;ll get gobs more torque and power as well as more bang for your buck and a likely smaller hit if you resell. If there&amp;#039;s any way you can swing it, adding two more cylinders makes the most sense.  SAVE FUEL, GET THERE SAFE Lars Jensen, San Ramon, California With regard to hypermiling, I&amp;#039;ve heard that coasting shuts off fuel flow and is thus better for saving gas than letting the engine idle in neutral at speed. What&amp;#039;s the most effective technique? A hypermiler, eh? This is a loaded subject. Chasing fuel mileage is fine as long as you don&amp;#039;t put yourself or others in danger, but shutting off your engine on the highway, taking the car out of gear, and drafting semis aren&amp;#039;t safe. Remember, too, that if you&amp;#039;re in neutral, it takes time to shift into gear so you can get back on the power. If you&amp;#039;re trying to avoid an accident, tenths of a second can matter. That also applies to creeping along so slowly that you impede the flow of traffic. That said, what you&amp;#039;ve heard is correct. Modern cars employ a fuel cut under engine braking, and coasting in gear will usually consume less gas than doing so in neutral (although the wheels will be driving the engine at that point, slowing the car more than if it were out of gear). The problem is that when this cutoff applies varies from vehicle to vehicle, and it&amp;#039;s often difficult to detect. Rest assured that the engine computer is trying its best to conserve fuel, and, like Kimi R\u00E4ikk\u00F6nen, it&amp;#039;ll work best if you leave it alone. If you insist on hypermiling, please do so responsibly. Stick with coasting and you&amp;#039;re likely to see fuel savings instead of an insurance bill. Or an obituary. Car advice, free of charge. Send your questions to fixer@roadandtrack.com. "^^<http://www.w3.org/2001/XMLSchema#string>
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