Tyre pressure advise please

G

Gebbly

Member
 Hersham, Surrey
My Z4MC came from @Simon 3.2M so I trust the decisions that were made about the car. The tyres that were fitted were 235/40 ZR18 at the front and 275/35 ZR18 at the rear. I have since needed to replace the tyres but have stuck with those sizes. I understand that these are not standard sizes as came from BMW and the sticker in the drivers door frame detailing tyre pressures does not include these sizes.

Can anyone suggest what the tyre pressures should be with those sizes please?
 
If its not listed on sticker in the door then there isn't a manufacturer recommended pressure.

Therefore I'd start with the standard pressure for 18s as listed and then adjust for tyre brand (sidewall stiffness) and personal preference. Personally I run my weekend cars as specified, but my daily runs 2psi over spec.
 
I’m not a E85 SME but I’d suggest 32/34 34/36 ..usually best to try a few changes ..don’t go below 30 or above 36 imho
 
Simon, are yours higher because they are on a MC or would you have that pressure on an se z4 E85? Mike.
 
Thanks for the help everyone. I had taken a punt at 32f, 33r before making this post. I'll pop out this morning and increase to 34f, 36r as advised and also make sure I write this down :)
 
Mine is quite sensitive. Not the same car as yours but i run 30 on the front and 33 on the rear. If the front drop just a couple of PSI then i find it tramlines really badly. When i had the rears at 35 i found they wore out in the middle instead of evenly across the width. At 33 they wear as I'd want them to. So it really does need a bit of experimentation.
 
Gebbly said:
Thanks for the help everyone. I had taken a punt at 32f, 33r before making this post. I'll pop out this morning and increase to 34f, 36r as advised and also make sure I write this down :)
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Let us know how you get on please:thumbsup:
 
Will do. When I took the pressures before pumping them up they were down a little (the fronts were about 28-29psi). I had noticed it would sometimes tramline as @Rockhopper suggests. So perhaps now the pressures are up a bit it will stop that tramlining.
 
I keep an analogue dial type tyre pressure gauge in the boot of my other car. I was browsing around for another analogue dial gauge to keep in my Z4MC boot but I keep finding gauges and then reading their reviews and seeing comments about inaccuracy. Can anyone recommend a reliable, accurate analogue tyre pressure gauge?
[Edit]
Annoyingly reviews of pressure gauges focus on ease of use and if it has things like lights. All I am interested in is will it give me an accurate reading.
 
I've been using a draper analogue pressure gauge with flexi hose for years to adjust tyre pressures on trackdays. Still working perfectly.
 
Gebbly said:
I keep an analogue dial type tyre pressure gauge in the boot of my other car. I was browsing around for another analogue dial gauge to keep in my Z4MC boot but I keep finding gauges and then reading their reviews and seeing comments about inaccuracy. Can anyone recommend a reliable, accurate analogue tyre pressure gauge?
[Edit]
Annoyingly reviews of pressure gauges focus on ease of use and if it has things like lights. All I am interested in is will it give me an accurate reading.
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I would love to know how the reviewers know how accurate their gauges are, or anyone really.
Unless you have a fully calibrated gauge to compare against, it is all a lot of guesstimation.

I don't even think tyre garages have their gauges calibrated regularly, as tyre pressures are not deemed to be super critical. Somewhere near is good enough for almost everybody (except car forum members of course). ;)

With 'normal' air in tyres, they can change by a good few psi depending on the weather and other factors (like heat in the rubber). Unless you have a fully calibrated gauge (which will need recalibrating regularly), fill the tyres with dry nitrogen and check the pressure every single day it is all a bit pointless IMHO.
 
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Rockhopper said:
When i had the rears at 35 i found they wore out in the middle instead of evenly across the width. At 33 they wear as I'd want them to.
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Really?
Two psi (which is a 'puff' of air at 30 ish psi) makes all the difference? :unsure:
 
I found this interesting test
He does a pretty good job of testing. Its crazy that 2 identical units from the same manufacturer can read so differently. And some are so far off the mark.
When we are talking about differences between 32, 34, 36 psi I'm not sure if I have confidence in these gauges to be that accurate. One of the worst offenders was apparently 9psi too low.
 
Pondrew said:
Really?
Two psi (which is a 'puff' of air at 30 ish psi) makes all the difference? :unsure:
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What do you reckon the contact patch with the ground is? Conservative estimate? 20 square inches? Thats an extra 40 pounds applied.
 
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