First up, I have a confession.
I've had my car for 8 months and I didn't check the tyre pressures until today. My feeble excuse is that I assumed (foolishly I know) that the dealer would have set the pressures correctly during PDI, and that the tyre pressure monitor was keeping an eye on them.
They were between 33 and 34psi front and rear. The fuel filler flap says 35 front and rear when partially laden, and 38 front, 45 rear when fully laden. So they were a little lower than the partially laden recommended figure, but not too bad. Reading the manual (please don't let any other men know I've done that!) tells me that the tyre monitor only warns of "a significant loss of pressure", whatever that might be. But clearly not a pound or two. So lesson 1 learnt is not to rely on the tyre pressure monitor to keep pressures at their best.
But what is best? With partially and fully laden pressures for the rear tyres given as 35 and 45psi respectively, that's a massive difference (28%!). I can go from one extreme of loading to the other a few times a week. Do I really have to reset the pressures for every trip? Of course that would be inpractical.
I've found a tool on the web that gives an idea of the effect of incorrect pressures -
Tyre Inflation - Mercedes-Benz Tyres | Inchcape Mercedes-Benz Inputting the 28% difference that I referred to above told me that this would lead to a 2% increase in fuel consuption and a 16.8% decrease in tyre life if I set the pressures to the upper limit and left them there. Conversely, if the upper limit was the correct value and I set the pressures to the lower limit (22% down), fuel consumption would rise by 3.5% and life reduce by 32.5% - even worse.
So, I've decided that what's best for me is to go for something inbetween but heading for the higher values. I've gone for 37psi front and 40psi rear.
A little more reading of the manual
crazy
tells me that this is OK, or does it?:
The tyre pressure values given for low loads are minimum values which offer you good ride comfort characteristics. However, you can also use the values given for higher loads. These are permissible and will not adversely affect the running of the vehicle.
The values on the tyre pressure information label (fuel filler flap) for the vehicle in "partially laden" and "fully laden" conditions apply when driving at maximum speed. Despite the values stated on the tyre pressure information label, for the vehicle with
"partially laden up to 210 km/h" status, the values can be reduced as follows, without a reduction in safety:
-
30 kPa ( 0.3 bar/ 4 psi) for all models
This does not apply to vehicles towing a trailer.
This improves the ride comfort of your vehicle; however, the fuel consumption may increase slightly.
I think that's saying that pressures between the two limits are safe, and you can even safely drop 4psi below the bottom figure as long as you don't exceed 210kph.
It's sort of making sense.
0.3 bar/
