Speed control
Speed is a decision you make continuously, not once. The limit is a maximum for good conditions, not a target: in an urban area, in rain, in fog or in traffic, the safe speed is below it.
You can read the speed of the vehicle in front from its brake lights. When they come on, the gap is closing - react to the light, not to the vehicle getting visibly nearer.
Fuel use follows speed too, and not in the direction most people assume. Crawling in a low gear burns more fuel than travelling steadily at a higher one, because the engine works harder for every metre covered. Smooth, steady driving is both cheaper and safer than repeated acceleration and braking.