I started the game again and noticed that when you buy the Speed Step upgrade you automaticaly gain 4X multiplier relative to the maximun speed, even when you aren't at 4 steps. Note that I didn't buy Coin Multiplier. I think this is intended since at some point it was removed the decay of the multiplier, but then the Coin Multiplier Upgrade is useles.
#The actual speed don't affect coin multiplier
dry scarab
static knoll
The active Speed step increases your running speed, while the maximum unlocked Speed step multiplies the coin value that you get from enemies at all times