presblfc

This is a horribly basic question.. but I know nothing about cars...

I have a 2009 C class with the 17 or 18 inch AMG wheels... I bought new 16' snow tires from tire rack... do I keep the new snow tires the same tire pressure used for my AMG wheels? .... which was like 32 in front and 39 in back... or do i need to find out a different tire pressure for my new snow tires?...

Thanks...

Sportstick

Yes, and be sure to keep them at least at the correct pressure...it is better to be a bit above than below. As it gets colder out, the pressure will drop 1 psi per 10 degrees Farenheit, so get a tire gauge and check them first thing in the morning before driving too far, which warms up the tire and raises the pressure. Tire pressure should ALWAYS be checked on a "cold" tire, not driven for many hours so it has cooled down from its last ride. If you need to add air, drive gently to the nearest gas station, hopefully within a mile or two.

Tire pressure is determined as part of the overall handling and load characteristics of the car to provide the best handling. Even if you change your 3 season tires to another model, you would also maintain the MB recommended pressure.

As pressure increases, the part of the tire on the ground (the contact patch) narrows slightly increasing the relative pressure per square inch, which is helpful during the winter. The increased pressure in the tire also increases the downward force. Conversely, it is not helpful to reduce pressure and widen the contact patch, as that essentially reduces and spreads out the downward force, and reduces the pressure per square inch. To experience this personally, stand up and experience how your right foot feels when you lift your left foot...the right takes all (instead of half) the pressure of your body weight, pressing down on the floor more, but it would be more difficult for someone to slide that single foot along the floor. More pressure per square inch improves traction, which is why long ago folks started tossing heavy bags of sand in the trunk!

Hope this helps make it more understandable.

GHAZAN

Should you check when hot since that's operating temperature...I don't get the point of checking when cold if that's not how they perform.

Sportstick

No, do not check when hot, as the data is variable and therefore, of little use. How fast, how far, what ambient temp, how much load in the car the tires carried at speed, are all variables which make a hot reading of little use and potentially misleading. The 32 psi you have one day after a highway run may really be 28 psi the next when driving in the city. Tire pressure recommendations are created based on a cold tire reading, which creates the most consistent set of baseline conditions and then allows for inflation due to usage. A "hot" tire will always show a higher than recommended psi after use. That is the expectation and considered normal. Note the sidewall of the tire shows a higher maximum inflation than the recommendation. If you do any further reading or research, you will always find that pressures are based on a cold reading.

masterstudent

I agree with the tire pressure reading cold. You could also use Nitrogen in your tires instead of air, that way less variations (but that's debatable). Keep the same pressure for winter tires.

RLE

I guess I'm the only one wondering why you bought 16" tires when you say you have 17" or 18" wheels. Which is it?


Pressures for 17s for example, differ from 18s.

GHAZAN

but don't all the professional racing series keep tire warmers with hot pressures dialed in....for ideal situations but for your novice DD cold is safer?

Sportstick

No, I think there may be a significant misunderstanding here. This has nothing to do with novice or being a daily driver. Cold measuring is more accurate to create a base tire pressure setting, which will expectedly inflate with the heat of usage. Tires are designed and labelled with a cold pressure. That is the spec posted in the owner's manual and on the label on the door. This is simply a matter of realizing the spec is for a not-recently-used cold tire and following that direction...not really a matter which can be debated. To do otherwise would be not to follow the spec, and very likely have under-inflated tires once they do cool down and/or the car is in a lower speed or temperature environment.

For example, if the front tire spec is 32psi, and you only achieve that when the tire has been run at highway speeds for prolonged periods, the cool inflation will be significantly below the spec, perhaps in the high 20s, which is severely under-inflated as you drive around town. This really becomes a matter of following the instructions which came with the car and tires.

I am not sure how to make this any more understandable....anyone else?

JimPap

Tire pressure is such a tough thing to get right on this stupid car.

presblfc

Thanks for the information Sportstick.

C300S 6MT

^^^ +1 Return those 16" tires. They won't fit.

jctevere

I'm guessing that he meant to say that he bought tires to accomodate 16" rims he will be using for winter? Either way, I agree with Sportstick, the recommendations on the door jambs are "cold tire pressure" meaning, they suggest this when the tire is cold. If you have your tire pressure set to factory spec when it is warm, then that means that it is not where it should be when warm, or more simply, not conforming to factory spec. However, in all actuality, I never run factory spec on my tires. I keep front tires about 2-3 PSI lower for pothole survival and ride comfort, and rear tires about 2 PSI lower.

Sportstick

2-3psi below spec will soften the ride and may make you survive potholes more comfortably, but it increases the chance that your rim will not. As the inflation pressure drops, the sidewall flexes even more, which allows the edge of a pothole to penetrate further, possibly reaching the rim edge. The problem for a rim is not being shaken by a hard bump, it's being hit by the edge of the pothole through a compressed tire. At the risk of ride comfort, raise inflation if combating pothole damage to rims is a priority. The other benefits are improved fuel economy and quicker steering response.

hitokiri89

What's the correct tire pressure for the 18 inch wheels on the 350 ?

Peabody

Correct tire pressures are located inside the door jam on the driver's side.

hitokiri89

Thanks