audiblitz

Hello

I would like to know if anybody can recommend which tire pressure I should use for 245/40/18s?

Thanks

uberwgn

if that's an OE size, you'll find all that data in the Owner's Manual.

JohnEnglish

Look on the driver side door pillar or read the manual.

audiblitz

thank you.

jadatis

Tire-pressure advice is made with a formula that I once got hold on and went running with, using the actuall load on the tire, maximum load of tire, reference-pressure of tire ( further Pr also saw it called maxloadpressure) wich is not the maximum pressure of the tire.
Pr has to be highened up with a system for higher speed then the reference-speed of 160km/99m/h for H tires and 220km/??m/h for Y speedrated tires.

If the picture is right that johnenglish gives your car has a maximum technical speed of below 210km/h, otherwise the H speedrated tires are not allowed. For that reason the Y tires could have an even lower advice then the H tires with the same Loadindex of 93.

Reason for this even higher pressure is, that the system for determinating the maximum load of tires is wrong for low aspect ratio tires ( the 40 in 245/40R18)and the tire-makers know that but dont want us to know. A 40% AR tire should have a maximum load of about 20% lower then what is written on it, and in their advices they try to compensate for that.
I also daubt if they calculated the maximum load of the Y tire for 220km/h reference-speed, this would also give a lower Loadindex , and its the same as the H tire in the picture.
So you see that there are a lot loose ends in tires maximum load calculations and pressure advices.
I have been busy with it since 2008, and slowly discovered it all.

End conclusion is that you have to use high pressures for the savety of the tires, especially the sidewalls. Comfort and gripp are bad then, but savety first.
If you give me the GAWR's of the car and the tire specifications written on the sidewall ( use the picture of JE as guideline)I can give you advice for fully loaded in wich I include the lower maximum load. If you can give the empty weight and weight division , and the normal loading ( persons and load) I can estimate the normal axle loads and give and advice for that .
Maximum speed you drive can be handy too.

audiblitz

Thanks for the information and recommendation. The only load on the car is me (200lbs). They are winter tires and I only put about 2000 miles a year on them, so I don't care about the tire wear or gas consumption.

Tire sidewall is shown below: They are 245/40/18 Michelin Alpin PA3 winter performance tires.



I haven't been able to find the GAWR. Base curb weight of car is 3847 lbs and it is 55:45 distribution.... so 2115 : 1731 lbs?

jadatis

Then I still dont have enaugh information , but I will give and advice based on the info.
Googled your tires an found on tire-rack only XL tires, so I asume they are. So Reference-pressure ( Pr ) = 2,9 bar/42psi in Europe ( using the highest so never to low).

http://tires.tirerack.com/tires/Mich...pin%20Pa3%20Xl
Maximum load is sertain thoug so 730kg/1609 lbs= Lmax in the formula.

The curb weight I also translated with google to Dutch to be the empty weight with driver and half a tank of fuell ( translated back from duch drivingready). I will make the axle loads you give front 90% of the weight to calculate with and behind 85% for reserve.
So front 2115 lbs/0,90=2350lbs to calculate with is La ( load actuall) in the formula. Behind 1731/0,85= 2036 lbs =La.
Then we calculate with the European formula = Pa=Pr*(La/Lmax)^1,25
Pa=Pressure actual so the adviced pressure.
Pa= 42psi * ( 2350/2*1609)^1,25=28,35 rounded up 29 psi front.
Pa= 42*(2036/3218)^1,25= 23,69 rouded up 24 psi behind.
this is the advice they would give before 2000 and for up to the reference speed of Vspeedrated tire tirerack says= 99m/h.
Thats fast enaugh but if you want to go faster I have to recalculate.

Then the to high calculated maximum load of here 1609 lbs a tire comes up. A 40% AR ( aspect ratio) tire deflects acording to the graphics I was sent about 27% of the free flexible part of the sidewall, when putting referencepressure and maximum load on it.
A 80% AR tire only 20%. You would expect the AR40 tire also only to may have that 20%, so it should deflect only 20/27=0,74 times that much, and I calculated such a tire to only may deflect 15% to get the same bending of the rubber as a 80% AR tire.
Then it even gets wors 27/15= 0,55 times as much deflection allowed. I found a relation between the deflection and the length of the tiresuface on the road, and becaus width of the surface stays the same for a radial tire, the relation is the same for the total surface and deflection. That is Deflection=D Surfacelength = L , then L1/L2= ( D1/D2)^0,5 , ^2 means square and power ^0,5 means root. This gives if you asume it may have 20% deflection 0,74^0,5= 0,87 x surface so maximum load aproximately, wich needs a higher Pa of 1/0,87=1,16^1,25=1,21times the calculated pressure
Front 28,35*1,21=34,22 rounded up 35 psi
Behind 23,65*1,21= 28,54 psi rounded 29 psi.
for the 15 % it means 0,55^0,5= 0,741 , 1/0,74=1,35^1,25=1,45 times the calculated advice pressures so.
Front 28,35 * 1,45= 41,10 rounded up 42 psi.
Behind 23,69 * 1,45= 34,51 rounded up 35 psi.

This all for savety of the tires, so you wont get damage to them.
Comfort and gripp can be totally gone then , but these kind of tires are put on the car more for the looks.
Pick your own choise of the 3 advices depending of who you trust.
Maybe practice is milder then my 15% but the 20% deflection it sertainly should have.
Sorry the story got to long already.

Monsoon

Or......you could assume that a company that has won LeMans ten times probably knows a little bit about tire inflation pressures and you could just use the recommendations on the back of the fuel filler door.