Hi what are the correct psi for front and back tyres on a wrx year 2000 classic with 225/35/18 please.

 

Std pressures for early classic running 205/55/15s are 35F;34R (not sure about MY97-00 cars, with their 205/50 R16 tyres). So, Becuse yours are lower profile, they normally require a higher pressure, so perhaps add 2-4psi in both F and R...?

 

40psi front
38psi rear

 

im running the exact same size tyres and i run 32 psi front and rear and they feel and handle really well and the tyre wear isnt uneven either.

 

same sizes here and I'm using 34/32.

40/38 is way over the top imo.

 

Tyre pressure is relative to the sprung weight of the car, NOT the wheel or tyre size, so standard pressures apart from any preferred variation for specific handling requirements.

 

Two words: Boyle's Law.

If mass and temp are constant, then the pressure of the gas (air in this case) within the tyre increases as the volume decreases i.e. they're inversely proportional to each other.

So, lower profile tyres (and inspite of any increased width) tend to hold, to a lesser or greater degree, a lower overall volume of air (depending on whether they're of +1, +2 or 'rubber bands'!, etc. upgrades).

Therefore, to retain a similar amount of tyre 'springiness' as standard (assuming, of course, similar like-for-like rubber compounds), the pressure needs to be appropriately increased (compared to the OEM pressures). Rubber band type tyres have virtually no sidewall lol, so will feel stiff/crashy as hell, regardless.

Have you ever checked the pressures of a bicycle's (very low volume, relatively speaking) tyres?... In the region of c.70psi!


...But this is the most relevant point... and is ultimately the reason for experimenting with tyre pressures.

 

You're a better man than me that can work out the mathematical internal volume of different tyres. Given that the wheel size has gone up by 2", there has to be a reduction in profile to ensure they fit the arch and suspension components hence a reduction in volume. However, there is also an increase in width giving an increase in volume.

Overall I get it to about a 20% reduction in internal volume. Whether that warrants a 20% increase in pressure is a bit beyond me. It would mean 32psi raising to 38 or so I only ever run any wheel/tyre at that sort of pressure on track. It would be very skittish on road at that level. Also, given the physical increase in rigidity of the tyre wall through its reduced height, but probably not reduced in thickness, a relative reduction in pressure to offset this might just set you back near the original 32psi

My initial statement was a direct quote from a representative of a professional tyre specialist company whom supplies tens of 000's of wheels and tyres both retail and trade.

 

I'm not talking about absolutes, or exact calculations, etc. Just mentioning that there is a direct relationship between gas volume and pressure (regardless of however insignificant, or otherwise), that's all.

But I agree that what IS important is to experiment with tyre pressures front and rear to try to optimise the handling for the car's suspension set-up/its sprung weight and the confidence the driver has in the overall feel/behaviour/basic handling trait of it.