Drive it forever

Q. My Audi has 160,000 miles on the ticker, so I decided to perform a brake-system overhaul. I replaced the calipers, rotors, and pads on all four corners. Now the pedal feels soft.

I performed the brake bed-in process, per the rotor/pad manufacturer's suggestion. A road test confirmed the soft-pedal feel at each stop.

However, the new brakes stop the vehicle extremely well, and the pads on all four corners slide well on the caliper carriers.

Is there something unique to the bleeding process on the Audi?

A. Like many manufacturers who have antilock braking systems, Audi specifies that you use their factory computer-diagnostic tool, which costs thousands of dollars, to cycle the ABS hydraulic pump while bleeding the brakes. Foot-powered conventional bleeding won't get the last bubble of air out of the system.

I've managed to cure some cases of spongy pedal on these systems with a pressure bleeder. But this doesn't work in every case. In the end, you may need to go to the dealer.

Q. I recently purchased a portable GPS unit. I was cruising on the freeway the other day and noticed that the vehicle speed displayed on the GPS was consistently 4 mph less than the car's speedometer.

Since the GPS uses a calculated time/distance relationship, would it be more accurate than the car's mechanical speedometer?

A. Much more accurate. I regularly use a handheld GPS unit to check the accuracy of speedometers. It's not uncommon for them to be several percent off. Cars and trucks are usually within 2 to 3 percent, but motorcycles are often off by as much as 10 percent.

Guess what? This inaccuracy affects the odometer as well, and that directly affects your fuel-economy calculations.

But wait, there's more: Overly optimistic odometer readings also affect your warranty, making it expire early by apparently getting you to the cutoff threshold sooner.

Q. Last spring the belts started squealing whenever I started my car, but then the noise would stop right away.

I took the car to the mechanic, and he applied some lube to the belts. It was OK for a while, and only made noise occasionally. When winter came, though, it started again.

I went to the mechanic and had him change the belts. Now it's worse than ever.

I've also noticed that they squeak when I use the heater or the rear defogger.

A. The belts were installed incorrectly. Go to a mechanic who knows how to properly tension an accessory drive belt.

Belt dressing, the lube you speak of, doesn't work for very long, The belt needs to be properly tensioned on a set of clean, properly aligned pulleys.

Q. Methanol is derived from wood sources. If methanol can be blended with petroleum-based fuel to form gasoline, then we will be able to have a renewable fuel source without causing undue hardship on the regions of the world that rely on corn for food.

Can methanol be blended with gasoline?

A. Yes, it can. There were a number of M85 - 85 percent methanol/15 percent gasoline - cars on the market in the late 1980s and early 1990s.

M85 never caught on. The mileage wasn't very good, and methanol was more expensive than gasoline, especially per mile driven.

It's cheaper now, but there are no M85 cars to be had. I saw an abandoned M85 pump at a gas station in California last month, in fact. And most methanol isn't made from trees. It's made primarily from natural gas.

Mike Allen is a senior editor for Popular Mechanics magazine. Questions should be sent via e-mail to driveit@nytimes.com.