X5 wheels, too wide for T5?

[YoYo]

Just bought some X5 Beemer wheels for my T5 van.
They're silly wide at 10 and 11". I never carry a heavy load, am I just going to have next to no rear traction in the arse in the wet?
Fronts are 10" wide running 245x45 rubber(19" rims), rear are 11" wide with 275x40s. Edit:- just learnt you measure rims on inside width, so the rear would be 10" wide, the front only 9". Still, too wide?
I need new rubber on the rears. What's the narrowest I can run on 11" wide rims? Should I run the wider rims on the front?
If I had 17" rims on 235x55s should I stick with the lower 40 height?
Please ignore stupid questions,I'm going to go school myself now on google.
Anyone have the same rims and want to swap their 10" ones for my 11"s?