Tires: tires, inch rims, rim width

Question
can i mount a 255/40/18 on a 18x8 rim safetly. bf goodrich web site says rim width for that size tire should be 8.5-10. i was told by tire distributer that they mount 255/40/18 tires to 8 inch rims all the time. i am buying a wheel and tire combo from dtd. 225/45/18 were original eqipment and i wanted to increase tire width slightly and maintain or increase overall diameter slightly which this size does without buying a wider wheel.

Answer
Jimmy,

CAN you mount a 255/40R18 on an 8" rim?  Sure!!
SHOULD you mount a 255/40R18 on an 8" rim?  No!!

Using a narrow rim causes the tread face to arch and increases the footpront pressure in the center of the tread.  This will cause the vehicle to feel a little directionally unstable and may result in the center of the tread wearing out first.