Full Size Spare Tire?
 

I've got an extra 18" wheel that matches the wheels I already have on my 01 X5. The tires I have currently on the X are 255/55 R18 they have a 29" diameter. I've been doing some searching around and it looks like 235/60 R18 tires are almost the same diameter (29.1") but are an inch less in width, which would allow the tire to sit down in the spare well and let the rear floor sit down flush over the tire. My question is: will the extra .1" really affect driving, differential, or transfer case when installed? I would like to drive at speed even when the spare is on. I ran the math and at 80 MPH the 29.1" tire would be making 3.25 extra turns every minute vs the 29" tire. Let me know what you guys think. :thanks:

Just google to find a simple tire size calculator. If the difference is greater than 3% (+/-) then you will have an issue. Stay within that 3% margin and you'll be fine.

Tire Size Calculator - Tire & Wheel Plus Sizing