[ucsbwsr]

Additionally I have played around with running larger tire sizes. My X5D came with OEM Style 335 19x9 255/50 square. Rolling diameter is 29.1" which means the motor is at 2,300rpm at 80mph.

I was 100% set on running style 612M wheels on my E70. I love the look, they are light, and strong (forged). The downside is limited tire choices. They came off an F85 X5M with the stock Michelin PSS 325/30 in the rear. This is a 28.7 rolling diameter so as I sit now at 2,300rpm I am at 78mph, a little down from stock.

I am a big fan of upsizing the aspect ratio of tires since it offers better ride quality and a beefier look, WIN-WIN in my book but tire clearance and rubbing become a consideration. AFAIK BMW x-drive systems detect wheel slip based off wheel speed sensors so you need the rolling diameter variation to be <1% between front and rear.

Stock X5M tire sizes are 285/35/21 F and 325/30/21 R. My goal was to stuff 285/45/21 F and 325/40/21 R. Unfortunately that didn't pan out up front with lack of clearance with the upper ball joint/spindle. :/

Here are some pics on what I test fit:
STOCK 285/35


285/45


This issue


I came to the conclusion that I needed to settle for a more modest 275/40 F 315/35 R.
But now I run X-Delete and my X5 is in RWD all the time, I live in San Diego so there are not negative repercussions for doing this, I can flash back to AWD in about 90 seconds if needed. The reality is if I ever NEED AWD it will be for a mountain trip to the snow and I will have an extra set of wheels with aggressive all season or dedicated snow tires so no worries about my X5 being in AWD with the 21" X5M wheels.

This is important as it allows me to toss the rolling diameters out the window for my street setup. Where the 285/45 didn't fit up front, I don't see there being an issue with it's matching 325/40 in the back so I am thinking of running 285/40 F 325/40 R. This will give me added cushion I am looking for but also offer a larger rolling diameter out back. Let's see how that would look:

Rolling diameter jumps to 31" which means at 2,300 rpm I will not be at 84mph (+4pmh compared to stock and +6mph compared to my current setup). Conversely going 78mph would now only require 2,125rpms (down from 2,300).

Swapping 3.64 gearing for 3.15 would really stretch the legs of the X5D, especially with taller tires. Here is some info:

325/30 + 3.64 + 2,300rpm = 78mph
325/40 + 3.64 + 2,300rpm = 84mph (78mph = 2,125rpm)
325/30 + 3.15 + 2,300rpm = 90mph (78mph = 1,975rpm)
325/40 + 3.15 + 2,300rpm = 98mph (78mph = 1,850rpm)

My goal is the bottom setup. Once my X5D is deleted and tuned I don't foresee there being any acceleration issues as it will be making gobs of torque and I am really optimistic about the fuel efficiency with the longer gears and larger tires. The drivetrain would be under greater stress from the longer gearing + greater torque so I would keep an eye on that stuff.

Evan