If you have 215 and he has 205 and all other numbers are the same, for instance:
215/60R16 -vs- 205/60R16, then here'* what you're looking at:
The 60 above, describes the wheel-to-tread tire height as a percentage (the aspect ratio) of the previous number (which is the tread width in milimeters). So for the numbers above, your wheel to tread distance would be 60% of 215mm, = 129mm, whereas his would be 60% of 205mm, = 123mm. The difference, is 6mm in the radius of your tire/wheel system, or 12mm of diameter, which is about a half an inch.
That isn't so much, so you could probably get away with it, but I would personally never go smaller than the factory tires. It'* acceptable to get bigger diameter rims and a lower aspect ratio to compensate and give you little to no change in total wheel+tire system diameter, and you gain handling in doing so. Consequently you lose handling going the other direction. So to go from 215 to 205 would generally have a decrease in handling, although it may be small in your case, and your total diameter will be slightly smaller. For more radical changes it will also throw off your speedometer.
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