[PetrolHead]

A ride on the terrible non-existent roads to Fern Canyon in the Redwood National and State Parks has left my car with a flat and I am forced to replace a tire. I am taking this opportunity to replace the God awful Conti run flats with some Michelin PSSs on all 4 wheels.

I plan to continue using the 18" OEM 400M wheels and the car is on stock suspension. I would like to fit slightly wider tires than the OEM ones in the front to reduce the understeer and in the back to provide a larger contact patch with the road. I was thinking:

1. 225/45/18 -> 245/40/18, don't want to go beyond 245 as the front wheels are only 8" wide, the minimum requirement for 245s from what I have read
2. 255/40/18 -> 265/40/18, have a feeling that anything more than this (maybe even this) could result in rubbing

Thoughts? Appreciate any help that I can get!

[5hift]

if your vehicle is Xdrive this would be the best tire sizes as the outside diameter and revs per mile are identical if you follow the wheel width as per the specs of the tire sizes.

front: 235/45/18
rear: 265/40/18 (the problem is the recommended wheel width for this size is 9" minimum and the 400m is 8.5.)


you can do 235/45/18 on all 4 wheels, as the wheels widths of the 400m are within spec of the tire requirements. the advantages of a square set-up on xdrive further reduces understeer and allows you to rotate tires f & r. hth

Look at the specs on the link below:

http://www.tirerack.com/tires/tires....omCompare1=yes

http://www.tirerack.com/tires/tires....omCompare1=yes

[alohasurftoad]

leave the rears with 255/40/18 and increase the fronts to 245/40/18. being almost square reduces the understeer dramatically, yet it still looks good because of the slight stagger.

[.BMW.]

An alternative with slightly taller sidewalls which fills the wheel wells better, offers more wheel protection and reduce the risk for flat tires.
F: 8x18" ET34, 245/45-18
R: 8.5x18" ET47, 255/45-18

This setup can be combined with rear spacers for the perfect stance, but only consider these sizes if your car is RWD.

265/40 is not an approved size for the rear 8.5" wide wheels.

[PetrolHead]

Originally Posted by 5hift

if your vehicle is Xdrive this would be the best tire sizes as the outside diameter and revs per mile are identical if you follow the wheel width as per the specs of the tire sizes.

front: 235/45/18
rear: 265/40/18 (the problem is the recommended wheel width for this size is 9" minimum and the 400m is 8.5.)


you can do 235/45/18 on all 4 wheels, as the wheels widths of the 400m are within spec of the tire requirements. the advantages of a square set-up on xdrive further reduces understeer and allows you to rotate tires f & r. hth

Look at the specs on the link below:

http://www.tirerack.com/tires/tires....omCompare1=yes

http://www.tirerack.com/tires/tires....omCompare1=yes

Thanks. Mine is a RWD and I don't have to deal with the added confusion of maintaining similar outside diameters.

Originally Posted by alohasurftoad

leave the rears with 255/40/18 and increase the fronts to 245/40/18. being almost square reduces the understeer dramatically, yet it still looks good because of the slight stagger.

Thanks, I think this is most likely what I am going to do.

Originally Posted by .BMW.

An alternative with slightly taller sidewalls which fills the wheel wells better, offers more wheel protection and reduce the risk for flat tires.
F: 8x18" ET34, 245/45-18
R: 8.5x18" ET47, 255/45-18

This setup can be combined with rear spacers for the perfect stance, but only consider these sizes if your car is RWD.

265/40 is not an approved size for the rear 8.5" wide wheels.

Thanks, 245/45 was what I initially thought too, as I am more of a spirited Grand Touring kind of driver rather than a track rat, but those tires would result in an outer dimension increase of about 1", wondering if that would be an issue wrt suspension travel (SF Bay Area and CA overall don't have the smoothest pavement out there)?

Also would like to give a shout out to Keith from Tire Rack. Shot him an e-mail on the address in the sticky, his reply was very quick and clear. FYI he had the same recommendation as alohasurftoad.

[128GP]

anyone have any pics of 245/40 on 400m/???

[2016340iMSport]

Hope these help, two of the front and 1 of the rear.

[silverF30xDrive-LA]

Thanks 2016340iMSport for the photos. I have a 2013 335 xDive M Sport with the OE 400M wheels (18x8) with stock 225-45R18 tires. Have you had any rubbing or handling issues with your 245/40 tires? Thanks, Lee

[apexmoti]

What kind of tire pressure is everyone using on this setup?
255/40/18 PS4S
245/40/18 PS4S

[Johnnydeep]

I have a question. Is anyone running spacers with stock tire set up on 400's? If so what size spacers? Pics?