Bill F

I have larger tire on my car and I read that you should reduce tire pressure when running larger tires. I even found a site that converts the recommended pressures for different size tires on a car.

Had my oil changed at a specialist shop ( not a fast lube) and the owner said yes reduce pressure, and his mechanic when checking the tire pressure increased pressure. It seems a little erratic on the highway so was going to reduce pressure.

Reduce to factory or a little lower because of the tires???

Opinions are welcome!

habbyguy

Personally, I'd never leave a wrong-size tire on a GLK (or any other AWD vehicle). Having rebuilt a transfer case, it's something I wouldn't want to repeat for the want of the correct replacement tire.

FWIW, I've had good luck buying single matching tires on eBay, taking care to get one with similar wear to the one I'm replacing.

calder-cay

Agreed. And this is only a personal opinion... I'd not swap out the factory rims and tires. The GLK looks great from the factory.
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But I understand, about 30+ years ago, I might have considered aftermarket rims and tires.
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Ours is not 4wd., so specs are different, but I'd still stay stock.
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Anyway, I'd adjust the tire pressure based on the rim and tire size that's on the vehicle, not what MB specifies.

Mmr1

Is one tire different ? Is it AWD or rear wheel drive ?

jadatis

I can help you with determining the new pressure.
but nowadays often 35,36,41 or 42 psi given as recomended pressure.
This then is the referencepressure of Standard load or XL tire.
Then if calculated for max technical carspeed and GAWR's, it might come to in the 20s psi for the OEM tires, as carmakers usually calculated in earlyer days.
then with the new tires with higher maxload at same pressure, as I think you mean, the pressure can even be lower.
so give all the data of old and new tires and your GAWR's and I will calculate.
but better would be for real axleloads in your use, and your max used speed, wich you wont go over for even a minute. Then most likely even lower pressure completely safe, so tires wont overheat driving the speed constantly, for wich pressure is determined.

Need from tires next.
1. Maximum load or loadindex
2 kind of rire to determine the referencepressure
i think here only standard load AT 36 psi or XL/reinforced/ extraload AT 42 psi, but if LT C-load AT 50 or 55 psi upto E-load AT 80 psi, on LT they give referencepressure on sidewall, on Personscartires the max allowedcold pressure between 44 and 51 psi, and an ocacional XL 60 psi.
3 Speedcode to determine the referencespeed, (for wich maxload is calculated) , not that important , mostly Q or above with refspeed 160kmph/99 mpg.

Can make a pressure/axleloadcapacity list with build in max reserve, at wich comfort and gripp is still acceptable.
then give if wanted in Kg or LBS , psi or bar/kPa.
And max speed you use and wont go over for even a minute.

Then your task is " ONLY" to determine the axleloads in your use 99% acurate, the most tricky part, and your responcibility.

Bill F

I had made a small mistake when posting. I said tire but it should have been TIRES, all 4 are the same.

New tires are a little larger, but with a higher load index and a higher speed rating. But no I don't think I will ever hit top speed LOL.

Mmr1

Larger or taller, i would fill them to what feels best , not below maybe 28 and not near the max. Low pressure overheats faster on highway,

Silver Shadow

I have 255/45/20 which corrected the speedometer error. 100kph on the speedometer is actual GPS 100kph.
I use the tire pressure recommended on the fuel door sticker... 33 front and 33 rear. Full load pressure is written on the driver's door sticker... 35 front, 39 rear.

Mmr1

Erratic on highway is more likely steering and suspension being worn out

Bill F

Was not erratic until service mechanic set front tire pressure to 46 psi, rear tires set to 48 psi.

I lowered the psi and car is fine.

MBKLUE

Yikes! Disturbing!

John CC

How are the tires "bigger"? (Moderately) larger diameter tires won't necessitate a change in pressure. Wider tires will, because you need a wider (larger) contact patch. Since the load on the tire isn't changing, the pressure will need to be reduced slightly to allow the contact patch to be bigger.

Over the longer term, you can measure the tread depth in the center and on the edges. If the center wears faster, lower the pressure. If the edges wear faster, increase it.

jadatis

i already assumed all tires.
Are your new tires same loadrange as old too. So for instance both Standard load or both XL/reinforced/extraload.
So please give all the specifications asked in my former post.
sizes would be nice too for old and new.

Mmr1

Did they set the rear to 48 cold? So after a highway drive they could be 52 -54! If so that was negligent . I drive 33-35 all 4 tires, you only need more in the rear if your loaded up heavy on the highway .

calder-cay

Here's the sticker on our (2014 Base) fuel door. We have 235, 45, R20's (Pirelli Scorpion Verde) on stock rims, and fill to "normal load" psi.

Might be different for a 4wd.

Although this website has a tire / rim calculator (under Tools menu), I don't think it shows suggested tire psi.

habbyguy

I believe that the contact patch of any tire is largely a function of the weight on that tire divided by the pressure in the tire. Yeah, there are other variables (sidewall firmness and tread flexibility) but assuming that's not hugely different across most tires, the contact patch will pretty much take car of itself at a given inflation pressure.

That is, if you have two otherwise identical tires - one a 265mm wide tire, and the other a 225mm tire, and you inflate both of them to (say...) 35psi, the contact patch on both will be pretty much the same overall area.

In the simplest analysis, the weight of the car (figure 4,400 pounds for a typical GLK with some gas and an average driver) puts about 1,100 pounds on each tire. Divide that by the inflation pressure (figure 35psi average) and you get a contact patch of (theoretically) about 31 square inches. Figuring our tires are close to 10" wide, that means the front-to-rear contact patch is around 3" long, which sounds about right. I should mention that the contact patch won't be perfectly rectangular due to the tire deformation, but you get the drift (subtle tire-related pun).

That said, the contact patch on the wider tire will be wider (side to side) because of the wider width of the tread, but shorter front-to-back. The contact patch on the skinnier (225mm) tire will be narrower side-to-side, but longer front-to-rear. That will have some effect on overall "feel" I suppose, but within the realm of tires that will actually fit on our GLKs, I can't imagine the difference would be significant for anything other than a spirited track day session (like anyone tracks their GLK...). ;-)

Mmr1

My 13, 41lb

John CC

That's one way to look at it. The other way is if the wider tire is intended to have the same (say) 3" front to rear contact patch. So the desired contact patch is bigger, and you need to reduce the pressure for the same load. In other words, if you know desired contact patch area and load, you can calculate the required pressure.

Bill F

Tires are wider than original,

Bill F

I set the pressures to match the fuel door prior to a long trip and performed great. I guess with the wider tires and higher speed and load range I could have gone a little lower. Trip was over 5000 KM's and car tracked perfectly on highway and cornered on mountain switchbacks with no problem. Car also got way better fuel economy than expected 8.7 l/ 100 KM's, not bad high speed , high winds in prairies, real mountains and some city driving.

bought car last July and it had brand new Chinese tires on it, impressed how well they work, quiet and not bothered by potholes.

Silver Shadow

If you like the Chinese tires wait until you get real tires. Mine had new Chinese tires when I bought it. They seemed very good also. A year or so later I bought Continental DWS 06 Plus. The handling and drive quality were transformed to a whole different level. I quickly realized how poor the cheap tires actually were.

Bill F

When I bought the car I factored in the cost for new tires so will probably replace next year. Took a few sharp corners in the rain in the Rockies and the car leaned and everything in the cabin went flying but the tires held with no slippage. The one thing I worry about is if a tire went on a trip it would be impossible to buy the tire anywhere.

Will most likely replace with Nokian All Weather