Question 40903
THE 16 IN TIRE WILL ROTATE ONLY 15/16 TIMES THE 15 INCH TIRE THEREFORE THE WEAR OF THE 16 INCH TIRE IS ONLY 15/16 TIMES THE 15 INCH TIRE. 
ANOTHER WAY OF EVALUATING THIS PROBLEM THE 16 INCH TIRE WILL HAS 16/15 OR 1.0667 MORE TREAD AND TRAVELS 1.0667 TIMES THE 15 INCH TIRE THUS THE 16 INCH TIRE ROTATES ONLY 15/16 OR .9375 TIMES THE 15 INH TIRE. 

SUGGESTION ALWAYS BUY/USE THE BIGGER TIRE FOR LONGER WEAR.