rate of change

I was driving from Montreal to Halifax this summer. The speed limit on the highway in New Brunswick and Nova Scotia is 110km/h, but I've heard that this is + or - 10 km/h, therefore cops don't stop you if you are driving up to 120km/h.

Anyway, I was driving and driving (it's a 13h drive). At one point, my whole family was sleeping, so I had nothing better to do but to think. I thought of many silly things, but one of them was math related. As you might realize, I wanted to get to Halifax as quickly as possile, therefore was trying to figure out how much time I would save if I went 5km/h faster than I was at the time.