Just put on new tires for my 4matic E550(2009), sized 255/40/R18 99V, and was wondering what is the recommended PSI for these tires. the stock 245/40/R18 takes 33 for the fronts and 35 for the rears so would +2 front and rear be sufficient or is there a better amount to put in them? Any help is appreciated

 

Because 1 size wider, you can do with a bit lower pressure, OEM probably has loadindex 96.
But those recomended pressures are calculated for maximum permissable axleloads and maximum technical carspeed.
So in dayly use, only driver and a little load, and not going over 160 kmph/99mph, you can do with lower pressure, and most likely front higher then rear.

Will make you a cold pressure/axleloadcapacity list with build in maximum reserve, at wich comfort and gripp are still acceptable. First have to google if standard load or XL/reinforced / extraload.
For this I give 90% of calculated axleload for the pressure for 160kmph/99mph.
Then you " ONLY" have to determine the axleloads 99% acurate, the most tricky part, and your responcibility. And I think you can control yourselfes not to go over 160kmph.


​​​​​​Googled and found its XL// so used next.
LI 99/ maxload 775kg/1709 lbs AT 42 psi upto 160kmph/99mph.

So maximum reserve build in, so you dont need to do pre- or after-calculations, and given per axleload. In front of cold psi lbs and behind in kg.

Axleload kg/ cold psi/ Axleload lbs
1448 lbs/ 18,0 psi / 656 kg
1517 lbs/ 19,0 psi / 688 kg
1586 lbs/ 20,0 psi / 719 kg
1655 lbs/ 21,0 psi / 750 kg
1724 lbs/ 22,0 psi / 782 kg
1793 lbs/ 23,0 psi / 813 kg
1862 lbs/ 24,0 psi / 844 kg
1930 lbs/ 25,0 psi / 875 kg
1998 lbs/ 26,0 psi / 906 kg
2066 lbs/ 27,0 psi / 937 kg
2134 lbs/ 28,0 psi / 968 kg
2202 lbs/ 29,0 psi / 998 kg
2270 lbs/ 30,0 psi / 1029 kg
2338 lbs/ 31,0 psi / 1060 kg
2405 lbs/ 32,0 psi / 1091 kg
2473 lbs/ 33,0 psi / 1121 kg
2540 lbs/ 34,0 psi / 1152 kg
2607 lbs/ 35,0 psi / 1182 kg
2675 lbs/ 36,0 psi / 1213 kg
2742 lbs/ 37,0 psi / 1243 kg
2809 lbs/ 38,0 psi / 1273 kg
2876 lbs/ 39,0 psi / 1304 kg
2942 lbs/ 40,0 psi / 1334 kg
3009 lbs/ 41,0 psi / 1364 kg
3076 lbs/ 42,0 psi / 1395 kg/referencepress
3142 lbs/ 43,0 psi / 1425 kg
3209 lbs/ 44,0 psi / 1455 kg
3275 lbs/ 45,0 psi / 1485 kg
3341 lbs/ 46,0 psi / 1515 kg
3408 lbs/ 47,0 psi / 1545 kg
3474 lbs/ 48,0 psi / 1575 kg
3540 lbs/ 49,0 psi / 1605 kg
3606 lbs/ 50,0 psi / 1635 kg
3672 lbs/ 51,0 psi / 1665 kg/ often only given on sidewall maximum allowed cold pressure

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