Pre-season testing for the latest batch of Formula 1 cars is just around the corner. Since this is the second season of more-or-less stable technical regulations, we can expect the car concepts to converge.
The aerodynamic and tyre changes for 2017 were fairly dramatic and they brought roughly what was predicted in terms of lap time improvement. This year, optimising that package will be the order of the day.
During the season, there is always a fair of amount of development and most teams will try to bring around one-tenth of a second of improvement to every race. That should add up to a two-second gain over the season.