tyre profile significance

[rossa123]

I have a porsche 986 - the current tyres are front 225/40/18 rear 265/35/18

The profile of the tyre makes a difference to rotations etc and my query is would it be better to have more or less rotations per mile - my example may help :

................265/35/18...225/40/18...235/40/18
diameter.........25.3.........25.1..........25.4
circumference....79.5.........78.8..........79.8
revs/mile........797.5........804.3.........794.4
speed............20...........19.8..........20.1
speed............60...........59.5..........60.2
speed............80...........79.3..........80.3


The closest match to the rear tyres would be to put the 235's on the front - does it really matter ?
your comments welcomed -

[marshalla]

Do you want a notionally higher top speed, or to be able to get to top speed a very small fraction of a second faster ?

[rossa123]

my thoughts were does the difference unbalance/unsettle the car, so is it important to get as close as possible to a match

[rossa123]

I have a porsche 986 - the current tyres are front 225/40/18 rear 265/35/18

The profile of the tyre makes a difference to rotations etc and my query is would it be better to have more or less rotations per mile - my example may help :

................265/35/18...225/40/18...235/40/18
diameter.........25.3.........25.1..........25.4
circumference....79.5.........78.8..........79.8
revs/mile........797.5........804.3.........794.4
speed............20...........19.8..........20.1
speed............60...........59.5..........60.2
speed............80...........79.3..........80.3

Does a difference unbalance/unsettle the car?
The closest match to the rear tyres would be to put the 235's on the front - does it really matter ?
your comments welcomed -

[littleredrooster]

If the current tyres are manufacturer standard sizes, why do you want to change? What do you want to achieve?

[conkerman]

No significance at all. Changing the sidewall height will have a small effect on the gearing if fitted to the driven wheels.

As for keeping front and rear whells spinning together. Does not matter. Use the recommended sizes, its what the car was developed with after all.

[Orangecurry]

So what sized wheels do you have?

If you say 18 I'll shoot you.

....and yes the 265/35 is slightly 'bigger' new than the 225/40, but the rears wear faster too. Porsche have thought of all this.

[Trev450]

The op states they are 18's in his opening sentence unless of course you are referring to the width.

[Orangecurry]

Yes - I was.

[Kawasicki]

It's two wheel drive only, so revs/km or overall diameter differences shouldn't make any difference...

Are your sizes what Porsche originally fitted? If so, changing the tyre with the aim of getting equal overall diameter might not be too smart. The different front to rear sizes were done not only for traction, but also to give calm/measured/precise steering.

[rossa123]

I raised a query because some places have said the front should be 235 and not the 225 that our on the car - other places have said 225 is correct ?!?

[conkerman]

What does your handbook say and what size (width) wheels do you have?

[jon-]

You'll be changing the rake of the car, and the roll centres with it.

Do this in a big way and the car will feel very different.

With those sizes and for normal road driving, I'm not sure you'll notice anything.

[paranha]

Have 987 Cayman, F 235/40/18-265/40/18.

[Garvin]

You have a range of diameters which vary by 0.3" or 0.15" for the radii which is all of 3.8mm. The tyre tread starts out with ~8mm of tread and the vehicle is quite happy to drive along down to 1.6mm, a difference of 6.4mm on the radius. This is a long winded way of saying it doesn't matter one iota as long as the wider tyres don't foul the arches on full lock.

[meemperor]

https://www.1010tires.com/Tools/Tire-Size-Calculat...

HTH

[Matt Seabrook]

Kawasicki said:

It's two wheel drive only, so revs/km or overall diameter differences shouldn't make any difference...

Unless you have ABS or PSM then it could

[Kawasicki]

Matt Seabrook said:

Kawasicki said:

It's two wheel drive only, so revs/km or overall diameter differences shouldn't make any difference...

Unless you have ABS or PSM then it could

Absolutely...

[rossa123]

conkerman said:

What does your handbook say and what size (width) wheels do you have?

I haven't got the handbook nor the car with me to measure the width - I think generally people are saying it doesn't matter too much -
When I have the chassis number I will speak to Porsche and see what was fitted originally in the factory -
thanks

[rossa123]

Thanks for your input -

According to Porsche the 986 from 1999 - 2002 had 265/35/18 at rear and 225/40/18 at front.