Mercedes-Benz 2011 GL-Class Operator's Manual page 362

Wheel/tire combinations
360
i The prescribed tire pressure is also given on a yellow label on the emergency spare wheel.
"Minispare" emergency spare wheel
R19 Wheels
Offset
Tires
Tire pressure
51 Use of snow chains not permitted.
51
GL 450
GL 550
4.5 B x 19 H2
1.58 in (40 mm)
T 165/90 D19 119 M
or
T 165/90 R19 119 M
420 kPa (4.2 bar/61 psi)