While I don't know what the relative difference might be in actual numbers, it seems to be a very small difference. Specifically, advice from BMW dealers seems to be that if one (of four tires) is replaced with a new one, then the new one should be shaved to match the other three. Since tire tread is often measured in 32's of an inch, then allowable differences between tire diameters on an xdrive appears to be quite small. Assuming your wheels are 18"s, the difference in diameter between 245/45 and 245/40 is a nearly 1/2 inch (678 mm versus 653 mm). And a 1/2 inch cannot offset by shaving of rear tires - you'd turn them into racing slicks or, more likely, destroy them in the process. Just curious, did your X3 come with OE staggered wheels or were these added later?

 

I stand corrected. The ride height would be a 1/2" less with 245/40 tires (vice 245/45 tires), but the difference in diameter is almost an 1 inch (25 mm), not 1/2 inch as I stated above. Unless you install rear tires of a matching diameter, the size difference between the front and rear tires would likely damage the drive line.

 

Tire Rack has pretty much every brand, model, and size of tire made.

TireRack.com has a blurb on matching tires on AWD:

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18&

Subaru wants the diameter of the tires to bet within 1/8th inch. For Audi, it's 1/4th inch. They don't say what BMW requires, but changing the diameter by one inch is crazy. In addition to wearing the clutches in the xDrive transfer case, you could also confuse the stability control and traction control.

 

Thank you for the link to that page on Tire Rack. Although I spend a lot of time on their site, I had not come across that page. I cannot remember the last time I bought tires from someone other than Tire Rack.

Like you, I live in one of the "summer only" parts of the country. A cold winter day here means 50F in the morning, so no real need for All Season tires.

I absolutely love the Michelin Pilot Super Sport (20") tires that came on my X4 M40i. The miss's X4 2.8i, that we bought used, came with 19" Pirelli RFT. Both cars are M-sport with the staggered setup.

Michelin has replaced the Michelin PSS with their Pilot Sport 4S which is getting great reviews. The 2.8i takes 245/45-19 on the front and 275/40-19 on the rear. According to Tire Rack, there is no such thing as a Pilot Sport 4S in 245/45-19. Bummer.