what tyre pressures on 225/40/18

lee2475 · 22-02-2006, 21:15

hi all
just fitted some new rims tonite,(just got in)whats the best tyre pressures??they are 225/40/18..on a s14a..they were up 40,so im now running at 32 all round,not sure if thats too much??any ideas would be great,cheers lads..lee

Jon-s14a · 22-02-2006, 21:28

32 is what i run at unless fully laden, then 34

Adam0bmx0 · 22-02-2006, 21:35

Would it be the same for a 265/35/18 rears and 235/40/18 fronts?

Gona put my new alloys on at the weekend

Adam

lee2475 · 22-02-2006, 21:50

thanks jon,ill leave it at that then,appreciate your quick reply bud!!

Piplis · 23-02-2006, 07:21

I´m using 42,2PSI front and 40.5PSI back. Nice and responsive ride that keeps rims safe from potholes. A little slippier on wet but I don´t mind. Showy driving beats fast lap times

JuZ · 23-02-2006, 07:32

The tyre pressures are determined by the weight of the car not the size of tyre so you should be running at 32psi

having said that I usually set mine to 34

ReTeP · 23-02-2006, 16:58

Not quite Juz, This is a question thats bugged me ever since fitting fat 16's on my 205 gti so i just worked it out... First Imagine the tyre compresses under weight by a percentge of its wall height and the rims are correct for the width...

For wider rims + tyres (same wall size = same rim diameter) you need lower pressure to get the same effective stiffness (tyre 'squish) hence comfort. This is because the pressure is acting over a larger area - wider rim thus more force.

But for lower profile tyres you want higher pressure to get the same %compression under the cars weight. Possibly more to stop rims getting dented on potholes.

Both are linear relationships (well the wall depth isn't but can be sensibly linearised for small changes) hence for matched tyre+rim sizes (and equal rolling radius) a good approximation would be:

P= tyre pressure
RW= Rim width (inside rim eg. 8 inch) can be subbed for tyre width if you dont have streched/fat look but not ideal
TW= Tyre width (on tyre side eg. 225)
TD= tyre depth (on tyre side eg. 50)
1= original manufacturers spec
2= your new shiny wheels

P2 = P1*(RW1/RW2) -to account for wider rims
P2 = P1*(TW1*TD1)/(TW2*TD2) -to account for smaller tyre wall

Combine these and you get: P2 = P1*(RW1/RW2)*(TW1*TD1)/(TW2*TD2)
Simplified:

P2 = (P1*RW1*TW1*TD1)/(RW2*TD2*TW2)

This works for mixed units as they are fractions and the reason the tyre depth is found from width multiplied by depth is that the depth rating on tyres is actually a percentage of width. A worked example:

S14:
P1 = 32psi, RW = 6.5 inch, TW = 205, TD = 55.

New wheels:
RW = 7.5 inch, TW = 225, TD = 40

P2 = (32*6.5*205*55)/(7.5*40*225) = 37 psi

Only a quick formula i worked out but serves as a pointer. Realistically compressed volume ratios need to be matched but the results wont be far wrong

. That means my UK 300zx rims should be run about 28 psi and i always used to run my 205 GTI's tyres too hard - no wonder it was sooo bumpy!!!

Adam0bmx0 · 23-02-2006, 17:34

Quote:

Originally Posted by ReTeP

Not quite Juz, This is a question thats bugged me ever since fitting fat 16's on my 205 gti so i just worked it out... First Imagine the tyre compresses under weight by a percentge of its wall height and the rims are correct for the width...

For wider rims + tyres (same wall size = same rim diameter) you need lower pressure to get the same effective stiffness (tyre 'squish) hence comfort. This is because the pressure is acting over a larger area - wider rim thus more force.

But for lower profile tyres you want higher pressure to get the same %compression under the cars weight. Possibly more to stop rims getting dented on potholes.

Both are linear relationships (well the wall depth isn't but can be sensibly linearised for small changes) hence for matched tyre+rim sizes (and equal rolling radius) a good approximation would be:

P= tyre pressure
RW= Rim width (inside rim eg. 8 inch)
TW= Tyre width (on tyre side eg. 225)
TD= tyre depth (on tyre side eg. 50)
1= original manufacturers spec
2= your new shiny wheels

P2 = P1*(RW1/RW2) -to account for wider rims
P2 = P1*(TW1*TD1)/(TW2*TD2) -to account for smaller tyre wall

Combine these and you get: P2 = P1*(RW1/RW2)*(TW1*TD1)/(TW2*TD2)
Simplified:

P2 = (P1*RW1*TW1*TD1)/(RW2*TD2*TW2)

This works for mixed units as they are fractions and the reason the tyre depth is found from width multiplied by depth is that the depth rating on tyres is actually a percentage of width. A worked example:

S14:
P1 = 32psi, RW = 6.5 inch, TW = 205, TD = 55.

New wheels:
RW = 7.5 inch, TW = 225, TD = 40

P2 = (32*6.5*205*55)/(7.5*40*225) = 37 psi

Only a quick formula i worked out but serves as a pointer. Realistically compressed volume ratios need to be matched but the results wont be far wrong

. That means my UK 300zx rims should be run about 28 psi and i always used to run my 205 GTI's tyres too hard - no wonder it was sooo bumpy!!!

Erm.....................................so what would i need for my tires mentioned above then?

I could work it out, but i've just had 2 harsh engineering exams at uni today

ReTeP · 23-02-2006, 18:45

Bit late for exams? All mine were early january, but hope you done well.

Well the rim width could be subbed for tyre width if things are sensibly matched but best done with rim size:

265/35/18 rears:

P2 = (32*205*205*55)/(265*35*265) = 30.1 psi

235/40/18 front:

P2 = (32*205*205*55)/(235*40*235) = 33.5 psi

I gave you results to 1dp since your an engineering student like me

, what Uni/course/year are you in? Im UWE/ Aerospace Systems Engineering/ 4 (masters) BTW

.

ReTeP · 26-02-2006, 18:24

Just to confirm 28 psi on 300zx rims is ideal, grippy and comforts the same. 30/32 psi was just a crashy ride and gave much wheel spinnage (an ITR got a sideways surprise!)

paul · 26-02-2006, 18:45

But if I run my Z rims that low it feels wollowy and tramlines more.
I run them 32-34, control wheel spin with throttle and ride is great

Paul

ReTeP · 26-02-2006, 19:08

I have 7.5 inch Z rims all round. Interesting comment though.....

If i run 32 psi I get less grip than original wheels, every bit of interior trim rattles and its a crashy ride! Must say although I had piss all grip it was well stable when powersliding

Maybe your used to a hard ride, Ah and Bristol has shite roads BTW???!

Hell I could have put my original rims at 36psi if i wanted less wallow

paul · 26-02-2006, 19:14

So what about uneven wear issues? Have they been ok?

lee2475 · 26-02-2006, 19:27

lol this has got abit involved i think ill stick to 32 and see how they wear.

ReTeP · 26-02-2006, 22:01

They sit just like the 205's before, can't see it causing uneven wear but only been on a day!

Apparently the Z guys think 27 ish Psi is best and there cars are heavier!, but i suspect its because they heat the tyres more which raises pressure.

Adam0bmx0 · 26-02-2006, 22:42

Quote:

Originally Posted by ReTeP

Bit late for exams? All mine were early january, but hope you done well.

Well the rim width could be subbed for tyre width if things are sensibly matched but best done with rim size:

265/35/18 rears:

P2 = (32*205*205*55)/(265*35*265) = 30.1 psi

235/40/18 front:

P2 = (32*205*205*55)/(235*40*235) = 33.5 psi

I gave you results to 1dp since your an engineering student like me

, what Uni/course/year are you in? Im UWE/ Aerospace Systems Engineering/ 4 (masters) BTW

.

Sorry for the late reply, so many threads to keep track on,lol

Im at Plymouth uni in my 1st year doing a Bsc in Marine and Composite Technology

I think i'll stick 32psi in and see what there like, thanks for working that out though

Adam

ReTeP · 27-02-2006, 22:56

NP

, Plymouth is a nice place to go to Uni (been there a few times), just watch out for local idiots and Navy wide boys - they dont get along and neither like students. Good luck with your course but work hard. Get food at Jake's too its well tasty

Busky2k · 28-02-2006, 02:09

I ran 40psi in the rears and the center of the tyre balded out first.

Now I run 35psi all round, works good!

Smoothound · 17-04-2007, 10:17

Time to awaken this thread

I'm also running 225 40 R 18.

I checked my tyres last night and they're worn in the centre considerably more than the outer edge : Over Infalted.

BUT, I've been running at 29PSI.

So, what the hell should my pressure be?

Confused.

Prior · 17-04-2007, 10:40

Quote:

Originally Posted by ReTeP

P2 = (32*6.5*205*55)/(7.5*40*225) = 37 psi

.

Shoot me down in flames if im wrong here, but ive worked that out at P2=2345200/67500= 34.7 psi not 37 psi, what am i doing wrong?

ive calculated mine which equals P2=(32x6.5x205x55)/(9x40x235)=28psi does this sound right?

22-02-2006, 21:15	#1
lee2475 Member Join Date: Feb 2006 Location: cornwall Posts: 750	what tyre pressures on 225/40/18 hi all just fitted some new rims tonite,(just got in)whats the best tyre pressures??they are 225/40/18..on a s14a..they were up 40,so im now running at 32 all round,not sure if thats too much??any ideas would be great,cheers lads..lee

22-02-2006, 21:28	#2
Jon-s14a Guest Join Date: Dec 2005 Location: Here, there and everywhere Posts: 1,308	32 is what i run at unless fully laden, then 34 __________________ I'm a simple man, I like my beer cold, my women hot, my car clean

22-02-2006, 21:35	#3
Adam0bmx0 Custom User Title Join Date: Oct 2004 Location: Cornwall/Devon Boarder Posts: 5,239	Would it be the same for a 265/35/18 rears and 235/40/18 fronts? Gona put my new alloys on at the weekend Adam __________________

23-02-2006, 07:21	#5
Piplis NON-MEMBER Join Date: Jun 2003 Location: Finland Posts: 177	I´m using 42,2PSI front and 40.5PSI back. Nice and responsive ride that keeps rims safe from potholes. A little slippier on wet but I don´t mind. Showy driving beats fast lap times __________________ Just waiting to get snow... 272hp/350Nm@0.8bar, 272hp/258lbs/ft@11.76PSI Running @1bar/14.5PSI Last edited by Piplis; 23-02-2006 at 07:24.

23-02-2006, 07:32	#6
JuZ Short arse Join Date: Oct 2003 Location: West Sussex Posts: 7,543	The tyre pressures are determined by the weight of the car not the size of tyre so you should be running at 32psi having said that I usually set mine to 34 __________________ My Photography

22-02-2006, 21:50	#4
lee2475 Member Join Date: Feb 2006 Location: cornwall Posts: 750	thanks jon,ill leave it at that then,appreciate your quick reply bud!!

23-02-2006, 16:58	#7
ReTeP Member Join Date: Jul 2005 Location: Preston Posts: 337	Not quite Juz, This is a question thats bugged me ever since fitting fat 16's on my 205 gti so i just worked it out... First Imagine the tyre compresses under weight by a percentge of its wall height and the rims are correct for the width... For wider rims + tyres (same wall size = same rim diameter) you need lower pressure to get the same effective stiffness (tyre 'squish) hence comfort. This is because the pressure is acting over a larger area - wider rim thus more force. But for lower profile tyres you want higher pressure to get the same %compression under the cars weight. Possibly more to stop rims getting dented on potholes. Both are linear relationships (well the wall depth isn't but can be sensibly linearised for small changes) hence for matched tyre+rim sizes (and equal rolling radius) a good approximation would be: P= tyre pressure RW= Rim width (inside rim eg. 8 inch) can be subbed for tyre width if you dont have streched/fat look but not ideal TW= Tyre width (on tyre side eg. 225) TD= tyre depth (on tyre side eg. 50) 1= original manufacturers spec 2= your new shiny wheels P2 = P1(RW1/RW2) -to account for wider rims P2 = P1(TW1TD1)/(TW2TD2) -to account for smaller tyre wall Combine these and you get: P2 = P1(RW1/RW2)(TW1TD1)/(TW2TD2) Simplified: P2 = (P1RW1TW1TD1)/(RW2TD2TW2) This works for mixed units as they are fractions and the reason the tyre depth is found from width multiplied by depth is that the depth rating on tyres is actually a percentage of width. A worked example: S14: P1 = 32psi, RW = 6.5 inch, TW = 205, TD = 55. New wheels: RW = 7.5 inch, TW = 225, TD = 40 P2 = (326.520555)/(7.540225) = 37 psi Only a quick formula i worked out but serves as a pointer. Realistically compressed volume ratios need to be matched but the results wont be far wrong . That means my UK 300zx rims should be run about 28 psi and i always used to run my 205 GTI's tyres too hard - no wonder it was sooo bumpy!!! Last edited by ReTeP; 23-02-2006 at 18:35.

23-02-2006, 18:45	#9
ReTeP Member Join Date: Jul 2005 Location: Preston Posts: 337	Bit late for exams? All mine were early january, but hope you done well. Well the rim width could be subbed for tyre width if things are sensibly matched but best done with rim size: 265/35/18 rears: P2 = (3220520555)/(26535265) = 30.1 psi 235/40/18 front: P2 = (3220520555)/(23540235) = 33.5 psi I gave you results to 1dp since your an engineering student like me , what Uni/course/year are you in? Im UWE/ Aerospace Systems Engineering/ 4 (masters) BTW .

26-02-2006, 18:24	#10
ReTeP Member Join Date: Jul 2005 Location: Preston Posts: 337	Just to confirm 28 psi on 300zx rims is ideal, grippy and comforts the same. 30/32 psi was just a crashy ride and gave much wheel spinnage (an ITR got a sideways surprise!)

26-02-2006, 18:45	#11
paul Guest Join Date: Mar 2003 Location: Chelmsford/London Posts: 5,878	But if I run my Z rims that low it feels wollowy and tramlines more. I run them 32-34, control wheel spin with throttle and ride is great Paul

26-02-2006, 19:08	#12
ReTeP Member Join Date: Jul 2005 Location: Preston Posts: 337	I have 7.5 inch Z rims all round. Interesting comment though..... If i run 32 psi I get less grip than original wheels, every bit of interior trim rattles and its a crashy ride! Must say although I had piss all grip it was well stable when powersliding Maybe your used to a hard ride, Ah and Bristol has shite roads BTW???! Hell I could have put my original rims at 36psi if i wanted less wallow Last edited by ReTeP; 26-02-2006 at 19:11.

26-02-2006, 19:14	#13
paul Guest Join Date: Mar 2003 Location: Chelmsford/London Posts: 5,878	So what about uneven wear issues? Have they been ok?

26-02-2006, 19:27	#14
lee2475 Member Join Date: Feb 2006 Location: cornwall Posts: 750	lol this has got abit involved i think ill stick to 32 and see how they wear.

26-02-2006, 22:01	#15
ReTeP Member Join Date: Jul 2005 Location: Preston Posts: 337	They sit just like the 205's before, can't see it causing uneven wear but only been on a day! Apparently the Z guys think 27 ish Psi is best and there cars are heavier!, but i suspect its because they heat the tyres more which raises pressure.

27-02-2006, 22:56	#17
ReTeP Member Join Date: Jul 2005 Location: Preston Posts: 337	NP , Plymouth is a nice place to go to Uni (been there a few times), just watch out for local idiots and Navy wide boys - they dont get along and neither like students. Good luck with your course but work hard. Get food at Jake's too its well tasty

28-02-2006, 02:09	#18
Busky2k Guest Join Date: Jun 2003 Location: Adelaide, Australia Posts: 792	I ran 40psi in the rears and the center of the tyre balded out first. Now I run 35psi all round, works good! __________________ *S14.5 - 400 bhp* SXOC NVCS (VVT) Sprocket Groupbuy V.3 £270 delivered - Finished!

17-04-2007, 10:17	#19
Smoothound Member (snigger) Join Date: Oct 2005 Location: York Posts: 4,167	Time to awaken this thread I'm also running 225 40 R 18. I checked my tyres last night and they're worn in the centre considerably more than the outer edge : Over Infalted. BUT, I've been running at 29PSI. So, what the hell should my pressure be? Confused. __________________ Griff