Tyre sizes

[944 LIL]

Oli

I spot you're online

My tyres are 235/45/17

Are you suggesting 215 (two sizes down). Falken only seem to do 40 or 45 - which would you suggest.

Your Carbibles link is fascinating if a little OTT for a novice like me.

Will these fit?

 

[James_G]

Colin Firstly, I've just read an interesting article in this week's Autocar about the perils of Far Eastern tyres. It makes sobbering reading, and they mention Wanli []. Re tyre sizes another thread elsewhere mentions that 225/45 R17 on a 7.5j rim (which is what yours are IIRC) was a good place to be (Peter E said this I think). Don't take my word as gospel of course, see what your local tyre emporium has to say. I have always had good experience with Merrow Tyre & Alignment in Farnham (no affiliation etc. ) and they have a proper alignment machine and know how to use it, so you can have that inconsistent wear pattern resolved at the same time. Let us know what you decide on.

 

[944 LIL]

Another biff question, but what's a 7.5j rim? how do I tell from the wheel / tyre

 

[Fen]

Broadly 7.5j means 7 1/2 inch wide. If you have original Porsche wheels then the size might be cast into the rim by the valve hole, with the offset being on the other side of the valve hole. '86 might be too early for them to do that however; I know Fuchs don't have it, but as they are forged maybe that is the difference.

 

[944 man]

Fuchs have will always have a 901/911 or similarly prefixed part number on the inside of the centre. If you post the part number then I will tell you what the size and offset is.

 

[zcacogp]

Colin, I've just seen this! Between James and Fen you're in safer hands than any I could offer you. But, broadly, narrower tyres on the front of your Lux will be a good thing. As will getting rid of the Korean Ditchfinders currently on there. Particularly as I glance out of the window at the 4 inches (and growing) of white stuff lying around. However ... I'd think that a 215 is probably a good choice. And in that size, I'd be looking for at least a 45 profile. (In the tyre size number WWW/PP/RR, WWW is the tyre width and PP is the height of the sidewall as a percentage of the width. Therefore, if you are looking at reducing the width but want to maintain the rolling radius - i.e. the height of the sidewall - your new sidewall height needs to be a higher percentage as the width is less. Therefore if you currently have 235/45 and go down to 215, you need the 45 to ideally increase slightly.) James says that 225/45/17 is a good size, on recommendation from Peter Empson. My only comment on this would be that Peter is putting a lot more power down to the road, and therefore will get his tyres significantly warmer than yours will. Tyres have a temperature window within which they should operate, and if they don't reach this window then you won't get optimum performance out of them. Wider tyres need more power to make them warm up, and you can have too wide a tyre fitted to a low(er) power car. If Peter Empson is running a particular tyre width then I'd be looking for something a bit smaller ... just my $0.02's worth. Do ask if you have any more questions, but I suspect I will be less helpful than the two who have already posted here. Oli. ETA: He's not running Fuchs, chaps. They're Boxster (-alike) 17's with the correct offset.

 

[idwoodvale]

Hopefully relavent. Does anyone have any hard figures on tyre presssures when increasing the tyre size. I have (or will when I get it back) 17" 225/45 on the front and 255/40 on the back. Someone asked this question a few years ago and I suggested that I was having no trouble with wear when running standard pressures. In hindsight when changed the tyres they were clearly (and unsurprisingly[]) over inflated as the centre of the rears were as Hammond would put it 'a little bit' completely bald.[&:] This is for a Lux obviously

 

[Fen]

Try something in the 32-34 range both ends for 225 & 255 road tyres on 17s. I think I settled on 33 all round but others adjust that slightly.

 

[idwoodvale]

Standard for mine is 29 front and 34 rear for I think 215 on 16s. I was assuming I'd have to adjust down from that for wider tyres or am I doing the maths the wrong way? ... car weight now spread over a greater area therefore less pressure required? The turbo front pressures are higher aren't they. Around 32?

 

[zcacogp]

Standard size tyres on D90's on my S2 are at around 38 front and rear. Yes, that sounds high, but is (I think) the recommended pressure. (Actually, it may be 36 ... in which case mine are also at 36.) The tyre guide link I posted up there ^^^^ suggests that a very good way of finding the best pressure is to look for the maximum pressure stamped on the side of the tyre, and inflate to 90% of that. I do know that a lot of people have their tyres underinflated (probably because they leak a small amount of air over time and most people don't check them oftn enough!) Oli. ETA: The link wasn't above, it's here: http://www.carbibles.com/tyre_bible.html

 

[idwoodvale]

Cheers, I'll have a look. Was wondering what on earth a carbibble is.... I see it's a Car Bible.[]

 

[944Turbo]

I run 32 F&R, A top tip I think originally from Andrew Sweetenham is to put a chalk or crayon line across the tyre and go for a short drive, see which bit wears off first, Tony

 

[idwoodvale]

ORIGINAL: 944Turbo I run 32 F&R, A top tip I think originally from Andrew Sweetenham is to put a chalk or crayon line across the tyre and go for a short drive, see which bit wears off first, Tony

More good advice thanks. I'll definitely try that in the summer when I (hopefully) have the car on the road. Oli.... great reference you posted. Very interesting read (my wife would think I'm sooo sad[8|]). Just how much you can know about a tyre is surprising. Only just reached the section on tyre pressures though and have run out of lunch hour so will have to finish later.

 

[mik_ok]

The tyre guide link I posted up there ^^^^ suggests that a very good way of finding the best pressure is to look for the maximum pressure stamped on the side of the tyre, and inflate to 90% of that.

That sounds rather silly. [&:] The pressure required varies with weight on the tyre ~ a Lotus Elise and a Volvo Estate may use the same size rear tyres - follow this approach on the Lotus and it'll be vastly overinflated and will wear the centre of the tread only. Lotus forums discuss "ideal" pressures in the mid 20's - which would be hopelessly soft for our volvo above.

 

[TheFiend]

[link=http://www.performancetyresdirect.co.uk/converter/table14b.html]http://www.performancetyresdirect.co.uk/converter/table14b.html[/link] I find this website useful for comparing tyre sizes []

 

[Hilux]

Standard size tyres on D90's on my S2 are at around 38 front and rear. Yes, that sounds high,

I think it is (personally) especially when you consider that your tyres will gain up to 2 or 3 psi when warm. 33 all round on my Turbo (29 ish on track when cold) seems to work well and a lot of people on here seem to be a tad either side of that figure. On 700 - 800kg kit cars I`ve owned/built its been around 17 psi and dropped to 12 -14 on track []

 

[Fen]

I agree with Mik. I haven't looked at the Car Bible site for some time thought I thought it was pretty good info, but that advice is not good. The pressure on the side is a maximum working pressure isn't it? That being the case you probably wouldn't want to run 90% of it at all, but regardless the weight of the vehicle makes a massive difference to the ideal pressure in any specific application. Anyway, it's absolutely correct that normally as the width increases that the pressure drops, by 29 in the front on 16" tyres is too low as a starting point.

 

[Hilux]

29ish becomes a stable 33 ish when warm on mine at least, 33 ish cold became 37ish warm and the car was very skittish and thats been on warm and cool days.