was wondering what tyre pressures should be running on my sc300 18's.

 

was wondering why you spelt it "tyre"

anyways, the recomended pressure will be on the side of the tire.

 

He's in Canada so it's tyre. The same goes for Europe, etc...

I'd start at about 40psi. What width/profile is the tire?

 

so thats why celine dion talks weird. hmph

 

The pressure stamped on the tire sidewall is the maximum pressure. It is not the recommended pressure. It is unsafe to inflate your tires to maximum pressure, especially if you live in a warmer climate.

 

Here is the proper calculation for tire pressure. It's a bit more tricky than what most people think.

Each tire is marked with a Load Index number
here are the load index and the lbs they represent


LIN lbs
71 761
72 783
73 805
74 827
75 853
76 882
77 908
78 937
79 963
80 992
81 1019
82 1047
83 1074
84 1102
85 1135
86 1168
87 1201
88 1235
89 1279
90 1323
91 1356
92 1389
93 1433
94 1477
95 1521
96 1565
97 1609
98 1653
99 1709
100 1764
101 1819
102 1874
103 1929
104 1984
105 2039
106 2094
107 2149
108 2205
109 2271
110 2337

the load index number is what the tire should be inflated to when max load is reached.

Let's assue the following.
we'll take a Bridgestone Potenza S-03 Pole Position 265/35/18 93Y with max pressure rating of 51lbs as an example
1992 SC400 with factory curb weight of 3575lbs as an example and assume that it has a 50/50 F/R and 50/50 S/S weight distribution and driver weighs 200lbs

265/35/18 is the size of the tire
93 is the load index rating
Y is the speed rating
51lbs is the maximum the tire should be inflated to if the tire reaches full weight capacity. This means if the car weighed 5732lbs and had a 50/50 F/R and 50/50 S/S weight distribution your tires should be inflated to 51lbs.
93 = 1433 lbs max load
3575lbs + 200lbs divided by 4 is 945 lbs
945lbs/1433 = about .66 x 100 = 66%
66% of 51psi = 34psi cold

If you car is equipped with 265/35/18 93Y F/R and have a weigh distribution of 50/50 F/R and 50/50 S/S then you should be putting 34psi when the tires are cold.


this is the very calculation that the car manufacturers use when putting the PSI label on their cars. One information I have not been able to confirm is what % of max vehicle load they base the factory pressure on.

If anyone can provide factory SC300 or SC400 tirewall reading I can calculate what the factory PSI rating is based on.

hope this helped and hope someone can give me the info on the factory tire sidewall reading.

 

When you say "cold" do you mean cold outside temperature or cold air inside the tires ie tires haven't moved.

 

cold as in cold air inside the tires (i.e. tires haven't moved) cold.

 

LOL, no we spell it TIRE as well. W'elps! I'm going to back to my igloo now j/p!

 

Hehe well I never said ALL Canadians spell it wrong.

 

Awesome information VS!!

I'm of the type that prefers to hear the why as opposed to "it just is"!!

Great stuff!

 

I don't have any math or science to throw at you but here's my 2 cents. I've use performance tires over 20 years. My butt and my experience has taught me a lot. I do agree that auto manufacturers recommend a certain psi based on weight but they also take into account the tire they put on the car when produced. When we change our tires to performance set-ups, manufacturer recommendations mean less. I learned a long time ago the lower the profile (aspect ratio), the higher the pressure should be. Never the max stated on the tire though. Tires heat up when in use causing the air to expand. That's not good. My recommendations take into consideration comfort, longer tread life, handling, load capacity and safety. For my wheels with the fat set up....38-40 psi in the winter and 36-38 in the summer.

 

your butt is correct. The manufacturer recommended tires pressure is for comfort driving. If you are taking aggressive corners you have to take into account for the weight distribution during those hard corners. Manufacturers do take into account for the pressure difference under direct sunlight and heat generated by driivng which can differ by as much as 10psi's in extreme cases.

The calculation is for comfort driving. The more aggressive you get the more you have to add pressure to the tire to support that weight so you don't end up destroying the tires.

but I do agree with you in your pressures. Your butt has served you correct in the above example if your driving style is above "normal comfort" then you should be adding more air. manipulating a tire pressure by 5psi to properly dial in your driving style on 85-95 load indexed tires are common.


and remember it's better to be 5psi over pressurised than 5psi under pressurised.

 

i feel the same way. It took me nearly a week of aggressive searching to dig up this info.... my google skills stink.

 

Speed Rating Symbol Load Index and Load in Lbs
RATING SYMBOL SPEED (km/h) RATING (mph) LOAD INDEX LOAD (lbs) LOAD INDEX LOAD (lbs)
B 50 31 65 639 85 1135
C 50 37 66 661 86 1168
D 55 40 67 677 87 1201
E 70 43 68 694 88 1235
F 80 50 69 716 89 1279
G 90 56 70 739 90 1323
J 100 62 71 761 91 1356
K 110 68 72 783 92 1389
L 120 75 73 805 93 1433
M 130 81 74 827 94 1477
N 140 87 75 852 95 1521
P 150 93 76 882 96 1565
Q 160 99 77 908 97 1609
R 170 106 78 937 98 1653
S 180 112 79 963 99 1709
T 190 118 80 992 100 1764
U 200 124 81 1019 101 1819
H 210 130 82 1047 102 1874
V 240 150 83 1074 103 1929
W 270 169 84 1102 104 1984

These ratings are from Europe