The man is nuts. At 40 PSI, the car probably rides like it's on rocks. There is a way to reset the tire pressure monitoring system and then it will use the current pressures as the default. A lower pressure will give you a softer ride. You can either go by the pressure on the placard by the door or the one in the flap of the gas cap. Maybe 40 is good if you're heavily loaded or planning on going 100+ but for every day driving, a lower tire pressure will be better. Who do you think really knows what the dynamics of the car would be? The car manufacturer who knows the center of gravity and all specs of the car? Or the tire maker who doesn't know what car it's going on so they only give you max pressure reading? Or some guy running an alignment shop?

 

That seems to be a trait of a lot of tire shops. I have to tell mine not to put 40 lbs in. I think some of this is because most people will not check their air pressure.

 

You have a lot of leeway with tire pressure, as guys at the race track know. Heck, even the Owner's Manual gives you a 8 psi range for the rear tires. My alignment shop encouraged me to increase the pressure of my W211 front tires from 30 to 40 psi, which he said is the recommendation for the 4Matic version of the same car. He thinks the lower pressure is just for a smoother ride, which some drivers prefer. Frankly, I can't tell the difference (neither can my 80 y.o. mother.) After 30k miles, he was right, because my steering is more agile, gas mileage is better, and the rims don't dent anymore.

You can re-calibrate the TPS warning system through the dash menu after increasing the pressure.