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YOUR RESEARCHtire pressure?
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Run 32/40 cold
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36/44 cold is pretty easy to determine and adjust from as needed.
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On second thought, after further review, maybe I don�t need to adjust from the stock pressures I�ve been running after all.
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As a side note, I am always checking tire pressures, when cold and when hot.
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It's all about weight, if you're driving fully loaded, you want the pressures at the recommended level.
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So the manual says 36/44 (front/rear) for my '03 C2 with 18" wheels. On less than ideal roads, that results in a pretty jarring ride.
Anybody use slightly less pressure? I'm thinking maybe 35/40.
Related question - if you were about to take a 1000 drive on interstate highways, what pressures would you use?
Thanks!
I keep my 18" factory wheels at 36/39 for normal, round the town driving. If I'm going out for a twisty cruise with just me and wife, 36/41.
If I was going on a long trip with lots of luggage and the wife, 36/44.
It's all about weight, if you're driving fully loaded, you want the pressures at the recommended level.
That psi.spec is for 4 passengers, if you rarely have more than 2 adult passengers You will wear out the center of the rear tires very fast !
Front spec is ok.
A couple things to think about.
Originally, our cars came with Continental and Michelin tires. Conti's have a softer sidewall than do Michellins. Which means they will have a softer feel going over the bumps, than will a Michelin. The tire pressure recommendation in the door well are for the original tires for the car. And are typically what you want to shoot for when the tire is warmed up. It doesn't have to be dead on, but it would make sense to be close when the tire is warmed up.
So as an extreme example, let's say you are running the oem Contis and you participate in a DE. Your target pressure is 36/44. So if you start cold at 36/44, then do 10 laps on the track at speed, the pressure will rise most likely by 4-10 psi depending on how hot you get the tires, and assuming you are running air versus nitrogen. Air by the way has moisture in it which causes the expansion when hot or contraction when cold. Nitrogen theoretically has very little moisture so not much change in heat or cold.
So when you come of the track, your tire pressure may increase to 40-46 in the front and 48-54 in the rear. Which is way too much pressure because you are now riding on just the center strip and therefore have less point of contact with the road and more wear and tear on the center of the tire.
So, the track solution is to drop your tire pressure by 4-10 psi on all 4 wheels when cold, so the tires hit the optimal pressure once they are heated up. This same concept plays out on a hot summer day just driving around town or the highway. If you are running your tires at 36/44 and they heat up and pressure expands to 39/48, you are now decreasing your contact patch to just the center of the tire. The same can be said for very cold days where your 36/40 becomes 32/36 and now you are riding on the outer treads of the tire and putting your rims at risk if you hit a meaningful pothole.
Putting your rims at risk is important because if you have the Oem tire size of 225/40/18 in the front, and 285/30/18 in the rear, you have very short sidewalls. Which means that if you do not get the tire pressure up to the recommended level, you have very little rubber height on your wheel. Low pressure coupled with low profile results in a greater risk of wheel rim damage if you hit a bump. This isnt as much of a risk on track, because the track is smooth and all it takes is a few laps and the tires heat up, pressure builds and you are up to Oem target pressures.
So, in answer to your question, you know that the target pressures are 36/44 on Oem approved street tires. Your cold tire pressure will vary depending on air temperature, highway temp, type of driving you are going to do etc. So on a very hot summer day, on a windy road, you may decide to start at 34/42 as an example. On a very cold day you may decide to start at 36/44 exactly. For street tires on the street/highway, you just want to be in a range close to the recommended pressure. Most people don't pay attention to tire pressures, but when you track your car, you realize very quickly how a 1 lb difference in pressure is everything for grip.
I don't know sumi's but do know that you will get to know your tires and tire wear depending on pressures you run and make adjusts. I would not run pressures substantially under the target pressures unless you plan on them heating up to near the target number.
Face it, you are in a sports car with low profile tires, which means a bumpier ride. You can experiment with going to 285/35/18 on the rear, which will give you a bit higher profile in the back and possibly a smoother ride as well. For the fronts, I am not certain what to do. I have not met anyone who has tested out 225/45/18 to give feedback on viability.
Bottom line, don't run the tires substantially under inflated, unless you are planning on heating them up on the track, or you will have major damage to you rims if/when you hit a major bump, and you eventually will. Been there and done that.
Clearly there are many other safety issues with under inflated and over in late tires as well. But we can save them for future threads.
Hope this helps.
Page 262-263 in my manual, #WKD 991 021 12 (2012 991) has standard tire pressure charts for :
summer tires, snow tires and COMFORT pressures for 19” & 20” tires (at speeds up to 165 mph)
I don’t remember which page the 996/997 manuals had this but it is there.
Good question from both of you guys.
I wish I can tell you what/how/and why the manual suggests that.
What I can say is that my experience, which comes from the track, is that by the time my tire pressures began to exceed the target pressure, I was losing grip. So my comments suggested target = warm.
The conclusion I have drawn is that if you stay within the target band on the street that they give you of 36/44 cold, you should be ok. Your risk is that you start at 36/44 cold and it goes up substantially, when warm, you will have a tire wear issue and a grip issue.
I think you will find that if you want to save your tires and have even wear, you will get to know your tire pressures pretty darn well and make adjustments as needed.
Don't know if this helps.
As a side note, I am always checking tire pressures, when cold and when hot. In the garage, on the track, in my driveway. Tires are the only thing between you and the street.
As a side note, every tire model has its quirks as well. Soft rubber, hard rubber, hard sidewalls, soft sidewalls, good in rain, bad in rain, good in cold weather, bad in cold weather.
Get to know your tires and their quirks. You want the confidence knowing what the tires are going to do when you ask them to do it. Same with brakes!
Best explanation may be that you need a starting point with pressures. 36/44 cold is pretty easy to determine and adjust from as needed. Most folks have no clue as to how their tire pressures change throughout the day as they drive around. And in most cases, it doesn't matter as long as you are at the recommended pressure of 36/44.
Where it does matter is on the track, or I suspect if you live in the dessert with wild swings in temps.
Nitrogen eliminates quite a bit of this conversation.
Yeah, in many ways it's trial and error on the street. You will know if you are too over inflated or under inflated by tire wear. If you have camber, it will further influence wear. Not to mention the tires you are running with stiffer sidewalls such as michelins or softer such as contis.
Good news is it doesn't have to be exact at 36/44. Keep a tire gauge in your car and measure pressures and wear, and you will come up with the perfect numbers.
Some day we can talk comparing different tires on the track, sidewalls, tire pressures and grip. It's science, a lot of trial and error, and a lot of art.
I run 32/40 cold. Hancook Ventus V12's. I'm on the 17" wheel & stock sizes. (not for long...)(he's says while wishing...) The ride is not harsh. I've never crashed into a rim going over potholes. One person in the car. My drives are typical 45-90mph, 45-60min.
I usually run sport cups and found between 32-34 fronts and 40-42 rears give the most even wear on the street. But I got a set of Toyo Proxies that are very soft rubber and not as stiff of a sidewall as the sport cups. Still messing with the tire pressures on the street. Quite frankly I will probably have to burn thru this set and get another one to understand best pressures. 36/44 is starting point and then it's all about finding the right numbers that make sense for you and your driving style.
