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Abuser of Charvels
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Discussion Starter #1
Two of my tyers (both left hand side ones) looked a little flat during the day so I stopped at a servo to check on them and give them some air. At first I noticed it said max pressure was 51psi!!! And I was like, whoa, that's up there. Finally noticed it happened to say that the max pressure was for USA and Canada, whatever that's about.

In the end I settled on 35psi all round. Does that sound right? These are brand new SP9000's, standard XR 18's.

Any idea why they got to 51 in the US?
 

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Registered
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1,500 Posts
35 PSI seems about right. I run 36 in the fronts and 34 in the back on my XR8 with the 18 inch wheels and that seems to work pretty well. I run a little more in the front tyres because of the extra weight over the front wheels.
51 PSI in the US might have something to do with driving in snow maybe.
 

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Birth, Drift, Death...
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we usually put 38psi minimum in 18s, pref about 40psi, because the stiffer sidewalls put more 'weight' on the outer edges of the tyres, and you have to increase pressure in the center to compensate and get even wear.

but 'normal' commo and falcon tyre wear could be completely different.
 

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Teh Pimp
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36psi what I would use on normal cars but because the XR8s are not normal cars and they have lower profile tyres, I would actually be using a quite high psi. If you are doing highway driving, it is even recommended that you keep them at around 50-51psi!

Assuming that you don't do lots of highway driving but you live in areas with 80km/h speed limits, I would be setting the rears to something like 45psi and the fronts to 47psi or even higher! This is better for the wet (which we have been getting a lot of lately), even wear and of course she will handle significantly better.

Mum uses what I tell her and she thinks that that was the best compromise. You don't want to get in trouble in a crash for tyres not being inflated enough. Really, the more the better really unless you are doing beach driving (in an XR8?) up till the max psi.

My cars 15"s max is 44psi and yesterday I set the fronts to 40psi and the rears to 38psi. Not very comfortable but who wants to crash in comfort? She handles way better and I can hear them work as they squeal when I take corners tighter than usual. To me you guys with 18"ers setting to 36psi is a bit too low, in general, those ratings should be used for cars like mine with smaller rims and bigger tyre walls.

Remember: Overinflation is when you go over the max psi indicated on the tyres which is 51psi. Tyres are the most important thing to your car so check them at least twice a week with your own metal gauge.

Cheers.
 

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T5 Expert Operator
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Ok i'm running 17x7 wheels with SP9000 tyres. I seem to be the odd one out in that i set my rear tyre pressure to 38 and fronts to 36. Reason being is that i feel the XR6 with a neutral even tyre pressure all round under steers abit too much for my liking. Therefore putting a tad more in the rear gives it abit of oversteer bias.

What i'm amazed at however is people running there cars with over 40psi. Believe me even with an XR6 running 38psi in the rears results in eaasy wheel spin in the dry.
 

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Copyright �2003
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I run 36psi all round on my 18's.

A word of warning, if you notice your tyres have lost pressure, take them off the car and repair them immediately. Don't do what I did and think it would be ok to drive them a couple of klms down to the servo to pump them up. I found out the hard way that the rims will rest on the inner wall of the tyre and cause them to chew the rubber down to the wire. The tyre pressure when I got to the servo was 16psi and it didn't look that low.

When I took the tyre in to have the nail removed and repaired was when the problem was noticed. That short trip cost me over $400. :thdn:
 

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Falcontastic.
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I used to run 36psi allround in my 17x8 Tickford's(Dunlop's) but I started to notice a quick wear on the inside edge's,so after some discusion's with the boy's at Herrod's and a wheel allignment they advised me to run 40psi all round,I'm hoping this will slow the wear down that these tyre's are bad for.
 

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Birth, Drift, Death...
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Ok i'm running 17x7 wheels with SP9000 tyres. I seem to be the odd one out in that i set my rear tyre pressure to 38 and fronts to 36. Reason being is that i feel the XR6 with a neutral even tyre pressure all round under steers abit too much for my liking. Therefore putting a tad more in the rear gives it abit of oversteer bias.
the reason most people have more pressure in the front than the rear is because the front tyres work through a lot larger 'range' of load than the rears - the rears have to supply drive and a small amount of brake effort, and then some cornering load, all while having 0 degrees camber (for a live axle car anyway)

the fronts sit at a degree or so of neg camber at straight ahead, and do most of your braking and nearly all of your turning (unless you are sideways, in which case NONE of this theory applies!) they move through different angles over bumps and drops etc.

so the front tyres work the edges of the tyres harder than the rears in general.

you put more pressure in the fronts to try and get the middle of the tyre to do more work, by making it contact with the road harder than it otherwise would. if you can get as much tyre on the road as possible in all situations you get even wear and max grip.

with a car that has IRS i would probably use the same pressures front and rear, as the rear wheels are generally cambered a little too, but it depends on the way you drive. (this is assuming that your wheel alignment specs match your driving style/conditions)

have you tried 38 all round or 40 front 38 rear? or were they 36 all round and you just put more pressure in the rear?
 

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Rollin said:
have you tried 38 all round or 40 front 38 rear? or were they 36 all round and you just put more pressure in the rear?

I've tried a few combo's from going a neutral pressure all round to harder up front and to what i'm using now. Your right on the theory but it seems the car corners better or more specifically turns in better as on exit one can use the gas as we are all familiar with.
 

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Birth, Drift, Death...
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interesting...it must be something to do with the angles that ford designed into the rear suspension on the XRs, or maybe just your specific car. never heard of a car handling better with more pressure in the rear (cept a porche maybe?)

make no mistake, i believe you, and im not dissing your wheel alligner person, just commenting on how strange it is.
 

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Abuser of Charvels
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Discussion Starter #14
I checked the pressure on my XF before it goes to it's new home. It runs Yokohama 235/60/15 and it's max tyre pressure is 35, which is what it is set at. Does this mean I should be at 51 on the ZR 18's? They are not much higher atm and are a much lower profile. This is very confusing.
 

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ESP Man
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888 Posts
What is important is to check/set your tyre pressures cold.

Generally, the bigger the diameter the higher the pressure you need to run. For an XR8 which are a heavier car than most, you would run slightly more in the front if the boot is empty.

As a starting point, 40 up front and 38 rear would the minimum I would run and trial and error with higher pressures until you are happy with what suits your driving style and comfort. Anything above mid forties will give a hard ride and be slippery in the wet
 
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