It can run for 75 kms.

Logic:

i) A bike needs two wheels to run. So initially with two tyres on run a distance of 25 km. Then remove one and replace with the stepni.

ii) Then run for another 25 km. Now remove the second one and replace with the first one removed at 25 km distance

iii) Now it can run for another 25 km.

Thus adding all the 3 above, we get 75 km.