running staggered tires on a fwd based vehicle is a waste of time and money as it will not help with performance or handling. X5M is a rwd based vehicle so wider tires in the back are needed to improve handling, since you are pushing with the rear wheels. On a Volvo it would not work since your rear wheels only activate once front wheel spin is detected by the system
It will look good however i am not sure if you can get the two different sizes to match up perfectly since once the system is activated everything has to rotate at the same speed and if your rear and front axles rotate at different speeds either the AWD will refuse to activate or create slip and either break something or damage the clutch.
Id stick with 4 tires maybe run some wheel spacers in the rear to achieve that wide look next to the rear fenders.
I had new tires put on yesterday. Got in on the Discount Tire Labor Day sale and will be getting back $220 in visa gift cards on a set of 4 Pirelli Scorpion Verde Plus tires. Tires were $838 with the lifetime balance/rotation and certificates from Discount Tire (got them for 1/2 price). Total after rebate will be $618 for the set.
I live in Minnesota and with winter coming up any day now, was told to stick with the original size instead of going to the wider 255 tires. The narrow tires will be better for snow traction. If you live in a northern area with snowy winters.
I live in Montana and we average 168-190 inches of snowfall per year. I have the standard 19" Vulcanis rims with 255/50R19 tires. I love the width of the tires in the summer, and they are fine in the winter. I am currently running (and about to replace) Toyo Proxees ST2's. I do not recommend the tires for the winter. They are terrible on ice and in slush. I am going to either add a large spacer or push out from the hub to accommodate 255/60R19's. I want the extra ground clearance for the winter and wider stance for stability. As far as widening the tires, to achieve anything useful, I would widen all four tires, but just for the summer. You would have to push out from the hub (like jacked up trucks, but not as drastic) to make room for the tires to have full turning radius. In the winter, the skinnier the tire, the better because you end up with more weight on each tire to push into slush and ice. I have heard of people running up to 275 width on the XC90's. I don't know how they did it, but it is possible.
Any snow to test the new Toyos out in? The 19" rims really limit my options for good winter tires. I was looking at BFG KO2's, but the closest they come is 18" rim size. For anyone looking at winter tires with 18" rims, these are a dedicated winter tire with amazing reviews and recently replaced the original BFG KO's.