I have set the normal pressure on my ML as 38 psi, so at what pressure should the low pressure light come on?

 

By law it has to come on if the pressure drops 20-25%, and that's typically when the factory TPMS systems throw up the tire pressure warning. So in your case it should come on at ~28 to ~30 psi at the latest.

 

The fuel door placard doesn’t agree with the drivers door placard for air pressures. Can anyone tell us why?? Door says 45-48 f-r and fuel says 32 ???
2013 ML350 gas

 

Not sure why there is a discrepancy but I have always been told that we are supposed to follow the pressure listed on the fuel door. Personally I found anywhere from 35-38 Psi works the best for me.

 

Can you post a picture of the fuel door placard? There's not a single recommended tire pressure usually. The recommended tire pressure depends on the load scenario. The driver's door placard in the USA only shows what's known as the maximum load recommended tire pressure. The max load tire pressure is for when the car is loaded with the maximum number of passengers and cargo. You should find the door placard pressure as one listed on the fuel door placard. After the Firestone incidents back in the 90s, the US DOT decided that American's can't be trusted with figuring out the proper tire pressure based on the load scenario, so they decided to only show the max load tire pressure on the driver's door placard. It's the safest pressure you can run, but it leads to an unnecessary uncomfortable ride if the car is not loaded down with passengers and cargo. The fuel door placard lists all the different recommended tire pressures for the various types of tires and load scenarios. Some manufacturers put these details in the owner's manual, but MB generally puts it on the fuel door. Here for example is the one from my C63S. You can see there are even different tire pressures for driving faster than 155 mph. In pretty much all of the rest of the world, cars have had these detailed stickers forever and people are educated and understand what the different pressures are for.