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Tyre Pressure Questions


Mac
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The stock tyre sizes on my M3 are 245x35 fronts and 255x35 rears, however I run 255x35 fronts and 265x35 rears. Mainly because it opens up the options on tyres and because the reviews I've read say these build tyres offer a more sure-footing and a better ride. I'm not convinced on this bit, but hey ho.

What do I do with tyre pressures? On the stock tyres pressure is 2.3 fronts, 2.4 rears. Should I just stick with that or should I be running a different set up?

Excerpt from the US manual:

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245/35ZR19 for the rears is a well-known typo in the 2008 manual, should be 255x35.

Anyways, they list 2.3/2.4 for up to 100Mph/160Kph which to be fair covers me. This raises to 2.8/3.0 if you go over that speed fully loaded.

That's clear, the only query I have is the fact that I am using slightly wider tyres :confused:

I've been running 2.4/2.5 and it seems fine, but just wondering really whether I should be carrying less/more pressure.

What's TSN's opinion?

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I can't see a tenth of a bar making much difference if I'm honest. I'd say it's more important to check and adjust regularly to whichever you choose than the difference between 2.3/2.4 and 2.4/2.5. For what it's worth I'd probably stick with the manufacturer's 2.3/2.4 although I can understand the logic of trying a tenth higher for the wider tyres.

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With lower profiles (like 35s) the pressures need to kept up. Car Manufacturers deliberately state them low cos new buyers will notice the shitty ride and won't buy the car. The cost to the punter is piss poor tyre life and less poor performance than optimal.. I've mullered 2 sets not knowing this despite keeping an eye on pressures.

My 2c... Keep the pressure high. It's how they were designed.

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I hope this doesn't sound wrong ...

... why not ask the people that recommended you change the size? If they can tell the difference in the size then I'm sure they can tell the difference in the pressures and what works best with them.

Mainly because the opinion is very subjective matey. Also, most of where this is discussed is in the US, and they have very different roads to the UK.

Now, I though standard widths were 245 front and 265 back; that is certainly what I've got and have seen on other model specific forums.

Yes, you're quite right Andrew - Sunday morning typo on my part. Mine are now 255x35ZR19s and 275x35ZR19s, against stock 245/35 and 265/35.

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You might want to factor in the summer / winter bit. Cold air, cold roads need 0.2 bar more than summer pressure.

It's a very subjective thing, how you drive, how far you drive, the types of road, the amount of passenger and luggage.

Theres also that old fashioned thing, eyeball them and check wear. If its even then it should be right, if wear in the middle then they are over and if worn on the outside and inside they are under.

I tend to go on the firm side, always have.

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Mainly because the opinion is very subjective matey. Also, most of where this is discussed is in the US, and they have very different roads to the UK.

Yup and because of the roads they have different tyre compounds and maybe even pressures. I nearly said this when I saw the US manual extract.

I'm not deliberately being picky I just don't understand why would go with their opinion on changing size but not about the pressures.

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Bugger all. 255/35=89.25mm sidewall against 245/35=85.75mm originals - so rolling radius of 2076mm against 2054mm so less than 1%.

Reads go from 265/35 to 275/35 so 2098mm against 2120 so again less than 1%.

For what it's worth, the speedo on the '3 is virtually bang on compared to the SatNav/TomTom speed - probably the most accurate I've had in any car.

Edited by Mac
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