[Konstantinos]

Yes, you can. By getting and changing the vehicle velocity.

For example:

pawn Код:

new
    Float: vx,
    Float: vy,
    Float: vz,
    Float: vs = 1.2 // kind of speedboost. 1.0 is normal (off), under 1.0 goes slower than the normal speed.
;
GetVehicleVelocity( GetPlayerVehicleID( playerid ), vx, vy, vz );
SetVehicleVelocity( GetPlayerVehicleID( playerid ), vx * vs, vy * vs, vz * vs );