Honda civic (fuel tank capacity = 52L, if i am not wrong) means you can easily cover 500 kms in a single run.
Route:
Lahore to Multan through M3 & M4 (approx 330 kms)
Multan to Sukkur through M5 (approx 410 kms)
Sukkur to Hyderabad through N5 (approx 320 kms)
Hyderabad to Karachi through M9 (approx 160 kms)
Guide:
I suggest you to take 2 stops. Leave Lahore at 4 AM, you can easily reach Multan around 8 AM, leave Multan at 9 AM and reach Sukkur around 1 PM, leave Sukkur at 1:30 PM and reach Karachi around 8 PM. At both stops, you can take a rest of 30 minutes, refresh yourself and other passengers, refuel your tank. i am attaching Google Maps Links of both the locations for your help.
- Multan (https://goo.gl/maps/gM7HTnRLekwcZAMu7)
- Sukkur (https://goo.gl/maps/JrNouHmHjPLFsuo86)
at both places you can easily find fuel, food and neat washrooms.
It is safe to travel between Sukkur and Karachi in night but i still suggest you to travel this portion in daylight.