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. Cars and trucks are usually within 2 to 3 percent off, but motorcycles are often off by as much as 10 percent, and it’s always on the high side. Guess what? This inaccuracy affects the odometer as well, and that directly affects your fuel-economy calculations. 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: 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: Not exactly. Like many manufacturers who have antilock braking systems, Audi specifies that you use its 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. This uses compressed air to pressurize brake fluid in the reservoir at a few psi and really push it through at high speed.

But this doesn’t work in every case. In the end, you may need to go to the dealer.