'Farther' v/s 'Further'

Is this sentence correct?

John can run further than Jerry.

It's actually incorrect because of the improper usage of the word 'further'. The correct word in this case would be 'farther'.

Rule: 'Farther' refers to actual measurable distance whereas 'Further' refers to degree, quantity, time, or some such other quality that cannot or is not being precisely measured.

Examples:

1. John promised his boss that there would be no further delays
   
   
2. The map says the city is farther than we expected.
   
   
3. The students discussed further after the teacher left.
   
   
4. He is running farther everyday.