thanks but meanwhile i've found out in SP-wiki^^
my code so far:
if frame==1;setVar("acc",0.5);end;
if key("a")==1;setVar("acc",0.6);
elsif key("s")==1;setVar("acc",0.7);
elsif key("d")==1;setVar("acc",0.8);
elsif key("f")==1;setVar("acc",0.9);
elsif key("g")==1;setVar("acc",1);
elsif key("r")==1;setVar("acc",0.4);
elsif key("e")==1;setVar("acc",0.5);end;
getVar("acc")-0.5
with a,s,d,f,g for gears, e for empty and r for reverse