It's been a little while since we've had a pet peeves thread. I'll start:

Friends who start their car in the dead of winter and crank the blower motor to high, apparently unaware that the temperature of the resulting air from the vents is dependent solely on the temperature of their car's engine, NOT THE FRIGGIN' VELOCITY OF THEIR BLOWER MOTOR. So you sit and chatter your teeth in the torrent of cold air, and if anything, the heater core takes a bit LONGER to warm up with the fan running full throttle. Sheesh...

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)