Arduino Serial Output Gone Wild!
km is never reset but continously incremented. 

km = km + circ;


This leads to an overflow of the value after some time.

When you declare int km;, then int  on an Arduino Uno is int16_t, it thus has a maximum positive value of 2^15 - 1 = 32767. Since circ is constant 1854, after executing the ISR a total of CEIL(32767 / 1854) = 18 times, km becomes greater than that maximum positive value. Thus an integer overflow occurs which takes you back to the maximum negative value of the int16_t. (See https://en.wikipedia.org/wiki/Integer_overflow) 

You might want to use uint32_t or similiar from stdint.h if you want to cover a greater value range. I think however your underlying algorithm is flawed, since even then km will  overflow after some later time. You must reset it at some point or only store the number of revolutions, which is a much smaller number. If you want to get the traveled distance then, you take the number of revolutions and multiply it by the circumference.

Also you should not not use any Serial printing within in interrupt service routine! And you should also declare your used interrupt variables as volatile. Read this article for more info.