Conversions:
1 kWh = 3412 BTU
1 gal (US) gasoline = 114,500 BTU
The bike went 8.7 km (5.41 miles) on 0.12 kWh
0.12 kWh = 409.44 BTU (0.12 * 3412)
409.44 BTU = 0.003576 gal (US) (409.44/114,500)
5.41 mi / 0.003576 gal (US) = 1512.9 mpg (US) equivalent
You can do the mpg/L 100k conversion.
So I guess the formula for mpg (US) equivalent is:
mi. / kWh * 3412 / 114,500