Slowly return to original speed before speed boost

Hey guys, I’m making a racing game and I have already programmed a speed boost for the car to accelerate immediately. Of course, after speeding up, it keeps the speed I added and I have to reset it back to its original speed before going into the boost.
Right now I have a delay which executes a new physics velocity back to the original before the boost. Problem is it executes previous velocity in one frame and the car just jumps from very fast to much slower immediately.
Wanted some ideas on how to tick the speed slowly back to the original over one or two seconds.
I’m sure you’ll have some answers

Best