Car manufacturer says 34 psi. OEM tires were 44 psi max, Will switching to 51 max psi tires result in under-inflation at that same 34 psi?

The car manufacturer's recommendation is based upon the weight of the vehicle and total weight of the passengers (including the driver).

OEM MAX pressure is based on a determined safe level of psi before it becomes unsafe (exploding) from being over pressurized.

51 MAX falls into the same category as the previous OEM pressure rating. Putting these tires on your vehicle would still require the 34 psi recommended by the car manufacturer, nothing higher.

many people are confused when it comes to the correct psi for tires; the decal on the driver's side door jam is ONLY for the tires that were equipped on the vehicle at the factory !! for any replacement tires, you would adhere to the psi for those tires, as some tire manufacturers will have a different psi rating for their tire...so - if you want to have installed tires with a psi of 51 ( and I have never heard of such a thing ), then they would be inflated to 46 psi, which is 5 #'s less than the max and the correct psi for driving.......inflating tires with a 51 psi to only 34 will result in VERY underinflated tires

No under-inflation if the new tires are the same size as the old tires

If you got bigger or smaller tires, then the pressure should still be the same if the tire widths are the same

Because the contact area changes very little going up or down 1 inches

For different width tires, the pressure should to be changed

new recommended pressure = old recommended pressure x old width of tires/ new width of tires

The new pressure cannot be higher than the maximum pressure of the new tires

ps: You can over inflate or under inflate your tires for a better ride

No. The max figure has no effect on the recommended pressure. Stick to 34 psi.

Max rating on the tire is just that, MAX rating for the tire. The RECOMMENDED psi is on the decal in the driver's door area. Unless you think you know better than engineers, follow the recommended pressure

if you got special tires, then follow what is printed on the tires - if you have those tire inflation sensors, they won't work with non-standard tires - you will never know if the dashboard warning is real or not