Drive it Forever
Q. Until now, my old Mercedes diesel has been hard to beat. Built in 1982, the engine is fine after 240,000 miles, and the car gets low 20s in gas mileage, on average.
I don't drive it on the interstate much, because the gearing was for the 55 miles per hour speed limit, and it turns over 3,000 revolutions per minute to keep up. I'm wondering if a different rear end or transmission would work to increase highway mileage by lowering engine rpm with a taller ratio in top gear.
As it is, first gear is darn low and it will patch rubber in seconds, so I'm thinking that a new rear end is the cheaper, quicker, and less risky move. Mercedes tech support will have no part of it, of course.
A. You're right - making your Benz turn fewer revs at speed should improve economy, although it won't be magic. But hunting up a taller numerical ring-and-pinion set for such a vintage foreign car might be tough. The set from a similar year, gasoline-powered car might fit.
There are aftermarket overdrive adapters that go into the driveshaft, but I am not aware of anything specific to a 25-year-old sedan. Consequently you'll have to engineer the installation yourself from generic parts. These overdrives are often trouble to keep working properly, though.
So I'll suggest the simplest, easiest modification: Switch to taller tires, which will lower revs at any speed.
Your car probably had pretty tall, narrow tires to start with, though, so any economy improvement you can make will not be huge.
Q. I just bought a supercharged 2000 Buick Regal, and I'd love to find out how hard I can push down with my right foot when I go driving.
Is it bad to floor it every once in a while, like when merging onto the freeway? What parts of the engine and transmission are most likely to suffer premature wear or failure because of aggressive driving?
I occasionally notice a puff of smoke emitted from the exhaust when people hurriedly accelerate to the next intersection. What is this smoke, and why does it happen?
A. You won't hurt the engine, transmission or suspension with an occasional burst of wide-open throttle during a merging or passing maneuver. Actually, an occasional burst of high-throttle, high-rpm running will blow out some of the engine's accumulated carbon.
Be sure that the engine is completely warmed up, though. The engine is calibrated to run rich on sudden acceleration to avoid stumbles and spark knock caused by momentarily lean mixtures. Rich mixtures burn with visible smoke.
Q. We read numerous stories about the benefits of E85 fuel for cars, and I for one would like to use it, since it is becoming more available. But the service manuals for my 2003 Acura and 2007 Hyundai both state to use no more than 10 percent alcohol content in fuel.
Are there any fixes being produced by auto manufacturers or others to utilize this fuel that won't violate the manufacturer's warranty? If not, it's all hot air and useless effort, since there are hardly any cars on the road today that can employ its pollution-reducing benefits.
A. No car maker provides a kit to change a vehicle over to flex-fuel vehicle status.
E85 cars have corrosion-resistant fuel systems with upgraded plastic and rubber parts and a fuel sensor that can determine the proportion of ethanol to gasoline. There are aftermarket kits, but they do not use this sensor and can trim fuel-mixture ratios only by using the oxygen sensor, which I think isn't adequate. The car makers agree with me, or they would have done it this way and saved the cost of the sensor.
Mike Allen is a senior editor for Popular Mechanics magazine. Questions should be sent via e-mail to driveit@nytimes.com.
