[Juwan320]

Ok so I have a 340i xdrive and am going for some big power. I want to drop the rear wheel size to 17 and keep the fronts at 18 to be specific 18x8.5 in the front and 17x9.5 in the rears does anyone know a calculator that allows me to find the tire and sidewall I’d need to get within that 1% if the wheels are not the same size?

[White435]

https://tiresize.com/comparison/

[johnung]

Originally Posted by Juwan320

Ok so I have a 340i xdrive and am going for some big power. I want to drop the rear wheel size to 17 and keep the fronts at 18 to be specific 18x8.5 in the front and 17x9.5 in the rears does anyone know a calculator that allows me to find the tire and sidewall I’d need to get within that 1% if the wheels are not the same size?

I’m not sure about the concept of dropping rear wheel size? Is this for drag racing?

I can’t comment on the viability of staggered wheel diameters on the 340ix, but BMW does it on some M cars with 20”+ wheels. Here’s what I do know.

BRAKES are your first issue, whether your 17” wheel of choice will even fit on the rear. There are only two possible rear brakes that came on the 340ix.

1) If you have Blue Brembo 2-piston 345x24 rears there is little chance that 17” wheels will fit over them. (I have a set of rear 330 calipers, if you need them)

2) If you have Grey single piston Bosch calipers they should fit over BMW 17” wheels but all bets are off with aftermarket 17” wheels. You’d have to test fit a wheel on your car or template your rear brakes and send the template to the wheel manufacturer to get assurance that their wheel will fit. It’s important to allow a minimum of 3mm of clearance between brake components and the inside barrel and spikes of the wheel. Otherwise road debris can wedge between wheel and brakes and damage them both.

XDRIVE is your next issue. Do NOT use any Internet calculators! They are all based on sizes on the tire sidewall which are not accurate enough. Those numbers are just ballparks that vary between tire models and even manufacturing plants for the same tire.

The XDrive specification says that the difference between the front and rear tire diameters has to be within 1% of each other or you risk damaging your $4,000+ XDrive Transfer Case.

Use the actual measured tire diameter that TireRack publishes in their Specs tab for each tire. The stock tire diameter for the F3x is about 26.0”. The math creates an easy Rule of Thumb.

If front/rear tire diameters are within 0.2” or less, then that’s within the XDrive 1% spec. However if the front/rear tire diameters have a difference of 0.3” or more, then that risks damaging the expensive XDrive Transfer Case.

TIRE SIZE & WHEEL OFFSET are your next challenges to make sure that they fit properly within the wheel well without hitting.

Hope this helps.

[Juwan320]

Originally Posted by johnung

I’m not sure about the concept of dropping rear wheel size? Is this for drag racing?

I can’t comment on the viability of staggered wheel diameters on the 340ix, but BMW does it on some M cars with 20”+ wheels. Here’s what I do know.

BRAKES are your first issue, whether your 17” wheel of choice will even fit on the rear. There are only two possible rear brakes that came on the 340ix.

1) If you have Blue Brembo 2-piston 345x24 rears there is little chance that 17” wheels will fit over them. (I have a set of rear 330 calipers, if you need them)

2) If you have Grey single piston Bosch calipers they should fit over BMW 17” wheels but all bets are off with aftermarket 17” wheels. You’d have to test fit a wheel on your car or template your rear brakes and send the template to the wheel manufacturer to get assurance that their wheel will fit. It’s important to allow a minimum of 3mm of clearance between brake components and the inside barrel and spikes of the wheel. Otherwise road debris can wedge between wheel and brakes and damage them both.

XDRIVE is your next issue. Do NOT use any Internet calculators! They are all based on sizes on the tire sidewall which are not accurate enough. Those numbers are just ballparks that vary between tire models and even manufacturing plants for the same tire.

The XDrive specification says that the difference between the front and rear tire diameters has to be within 1% of each other or you risk damaging your $4,000+ XDrive Transfer Case.

Use the actual measured tire diameter that TireRack publishes in their Specs tab for each tire. [...]

Yes it is for drag racing and I do understand i have to switch to a small brake kit I have everything all figured out and dialed in I just need specs that will work with essentially a 1in taller sidewall in the rear

[visionaut]

A 225/45-18 Front will match nicely with a 285/40-17 Rear for XDrive. Similarly, if you wanted a wider Front, a 255/40-18 will also pair nicely with that 285/40-17 Rear.

There’s other Rear possibilities - just find other 17s in the same OE 660mm diameter (800 revs/mile) as the above two front tire specs.

I like willtheyfit.com for wheel/tire fitment calculations. Once you find a ‘theoretical’ combo that works there, you can then look at specific tires and see their exact tire specs. Redo the X-drive calculation checks a final time with the exact data.

[Juwan320]

Originally Posted by visionaut

A 225/45-18 Front will match nicely with a 285/40-17 Rear for XDrive. Similarly, if you wanted a wider Front, a 255/40-18 will also pair nicely with that 285/40-17 Rear.

There’s other Rear possibilities - just find other 17s in the same OE 660mm diameter (800 revs/mile) as the above two front tire specs.

I like willtheyfit.com for wheel/tire fitment calculations. Once you find a ‘theoretical’ combo that works there, you can then look at specific tires and see their exact tire specs. Redo the X-drive calculation checks a final time with the exact data.

Thank you for that my guy I really appreciate it. Putting a motor together no problem as me about tire specs I turn into forest gump lol

[visionaut]

Glad to assist. FWIW, you might actually also run a SQUARE set-up, if desirable: 255/40-18 Front with a 255/45-17 Rear. But that 255 might be stretched slightly on the 9.5” rear width…

But given it’s for Drag, square isn’t ideal. You’re gonna want skinny upfront and a meaty rear.

[johnung]

Originally Posted by Juwan320

Yes it is for drag racing and I do understand i have to switch to a small brake kit I have everything all figured out and dialed in I just need specs that will work with essentially a 1in taller sidewall in the rear

Just avoid the internet calculators, don’t go over that 0.2” measured diameter difference and you’ll be golden. TireRack measured Specs is an awesome reference.
Good luck with your project!

[Juwan320]

Originally Posted by visionaut

Glad to assist. FWIW, you might actually also run a SQUARE set-up, if desirable: 255/40-18 Front with a 255/45-17 Rear. But that 255 might be stretched slightly on the 9.5” rear width…

But given it’s for Drag, square isn’t ideal. You’re gonna want skinny upfront and a meaty rear.

Well to make it even weirder I was to have a squared set of 18s that I can just run on the street but when it’s party time I can just throw on the rear set and it’ll all match up perfectly in a perfect world

[visionaut]

Since you’re Xdrive, the traditional ‘all the meat in back’ likely doesn’t apply (unless you X-Delete). I’d think the 255 front (vs. the 225) might be pretty good to hook-up, and gives you the option of the 255/45 or the fatter 285/40 in back, given your traction.