Originally Posted by ScottC
Most GOOD car systems, like the one in your Mercedes don't use GPS as their primary navigation system, they use a tacho sensor and gyro's. The GPS is only used to determine the start and end point should it have lost that.
Well, and periodically update the INS. In a city, yeah, the INS is better, but not by that much--the gyros used in the systems have come leaps and bounds up in accuracy and down in price in the last 10 years, but (relatively speaking) the accelerometers are still kind of crappy. In terms of sheer position, a WAAS enabled 12 channel receiver will get closer. Moving, it's probably a wash.
I used to miss a real INS, particularly in places like Manhattan and the loop in Chicago (eg, concrete canyons). Then I bought an amped external antenna for my 60C. Outside of tunnels, it's all good in the car.