Tyre pressures for aftermarket wheels/tyres

[Conquistador]

Can't really find a definitive answer to this online, so thought I'd ask here.

I recently got some different wheels for my new car but have no idea what the correct tyre pressures should be with after-market wheels and tyre sizes.

I assume the speed rating and load rating are all important but obviously it isn't now as simple as looking in the manual or the door sill for the correct tyre pressures. Pumped them up to 30psi for now.

Old size was a rather puny 205/50/16", new size is 235/30/19" on the front and 265/30/19" on the rear with BBS LM-R rims.

[PsYcHe]

I'd put that up to nearer 40, but that's a personal thing.

[ainarssems]

It's hard to say for sure. One way to tell is to measure tyre temperature, not sure how well it works on radial tyres but give it a try. Get yourself infrared thermometer and measure tyre temperature after a bit of 60+ mph driving in straight line for couple of miles. If middle of tyre is hotter then sides decrease pressure, if sides are hotter then middle increase pressure. This method can also be used for wheel alignment.

30 psi does sound little for 30% sidewall, you risk of damaging rims if tyre completely compresses on one side when hitting pothole.

[oldnick]

bit baffled by this

wasn't the car originally fitted with the same size tyres at both ends ?

if so the 235/30's are fine , your speedo will under read by 2% which is just fine , presumably the sensor is off the front end , will normally over read by 5-7% oe

but why the 265's? they will be 5% out ...presuming this is 4wd I wouldn't consider this difference even if it didn't affect the balance of the car ; presuming you have the same width rims front and rear one size is liable to be outside the permitted width for the tyre anyway

with regard to pressures , get your axles weighed in your normal running condition , then contact the tyre manufacturers , give them the loads and they will quote you pressures from their ETRTO charts as a base pressure to work up from

[Conquistador]

Originally Posted by oldnick

bit baffled by this

wasn't the car originally fitted with the same size tyres at both ends ?

if so the 235/30's are fine , your speedo will under read by 2% which is just fine , presumably the sensor is off the front end , will normally over read by 5-7% oe

but why the 265's? they will be 5% out ...presuming this is 4wd I wouldn't consider this difference even if it didn't affect the balance of the car ; presuming you have the same width rims front and rear one size is liable to be outside the permitted width for the tyre anyway

with regard to pressures , get your axles weighed in your normal running condition , then contact the tyre manufacturers , give them the loads and they will quote you pressures from their ETRTO charts as a base pressure to work up from

Nick, should have mentioned it's a BMW Z4, not an A8 or S8 any more, hence why the tyres are slightly fatter at the back for the deep dish alloys.

The speedo being slightly out was my other concern but I'll address that immediately afterwards.

These are the new wheels:


These are the current wheels, with run flat tyres. Handles horribly:

[ScottD3]

put 35psi in.
Look at the tread and see if it sits flat and even on the road.
If it is, then its a good thing.



I normally run 40psi on all my tyres, be them 205\55\16 or 255\45\18.
The tread seems to sit more even on the road.

[oldnick]

Originally Posted by Conquistador

Nick, should have mentioned it's a BMW Z4, not an A8 or S8 any more, hence why the tyres are slightly fatter at the back for the deep dish alloys.

The speedo being slightly out was my other concern but I'll address that immediately afterwards.

These are the new wheels:


These are the current wheels, with run flat tyres. Handles horribly:

all is clear !
speedo will still over read but by 2% less as stated [ try comparing against your sat nav on a straight flat motorway ]

what was the oe tyre size at the rear ?

modern radials will be flat on the road whether or not inflated to the correct pressure

[mannyo]

Will the different rotational speeds between the 235/30/19" on the front and 265/30/19" on the rear not upset the ABS and traction control, normal advice is to get as close to the same overall circumference all round. 235/30 will have significantly less sidewall than a 265/30, some 18mm less in the diameter. remember the /30 is a percentage of the tyre width and not a fixed size, 235/35 is much close in diameter to 265/30 with a difference of 5mm.

[Conquistador]

Sorry, yes front size is 235/35 19" rather than 30%.

Original size was 205/50 16".

I've got 35psi in them at the moment and all seems fine visually and handling wise.