I think an 11" rim is physically 12" wide. The bead is 11"
While your rims are on your car you can just measure off your existing rims (get your rim actual specs) to get your capable additional space.
Here is a good program - website for tires and rims estimations
http://www.rimsntires.com/specs.jsp
Cool program... With specs put in and 18x10.5 and 15mm offset it says "Package is 25 mm (1'') closer to suspension/brakes components. Make sure that you have enough room for that. If not, consider lower offset, narrower tires or using spacers."
Wheels: Wheel 1 Wheel 2
Wheel size 15x8 ET 0 18x10.5 ET 15
BackSpace 114 mm (4.5'') 161 mm (6.3'')
47 mm (1.9'') deeper.
Rim Lip 17 mm (0.7'') wider
Typical Weight ~ 18 lbs / 8 kg ~ 35 lbs / 16 kg
Tires: Tire 1 Tire 2
Tire size 275/60-15 295/45-18
Section width 275 mm (10.8'') 295 mm (11.6'')
Sidewall 165 mm (6.5'') 133 mm (5.2'')
Tire diameter 711 mm (28'') 723 mm (28.5'')
12 mm (1.7%) taller.
Rim sizes 15x8 to 15x10.5 18x9 to 18x11
Circumference 2233 mm (87.9'') 2270 mm (89.4'')
Revs per mile 720.9 708.9
Speedometer Assuming that this is OEM tire and speedometer readings are correct >>> When speedometer reads 60mph (96.6km/h) actual speed will be 61mph (98.2km/h): 1.7% faster.
Typical Weight ~ 39 lbs / 18 kg ~ 39 lbs / 18 kg
Wheel+Tire Weight ~ 57 lbs / 26 kg ~ 74 lbs / 34 kg
Clearance: Package 1 Package 2
Suspension Assuming that this is OEM wheel/tire and there is no any problems with clearance on both sides >>> Package is 25 mm (1'') closer to suspension/brakes components. Make sure that you have enough room for that. If not, consider lower offset, narrower tires or using spacers.
Fenders 100% clear (leaves even more room than OEM)
Wheelwells Will fill out the wheelwell by extra 6 mm. Acceptable for most cars.