Very clever I must say 
Now we have 42 free characters to use for another feature.
With 42 characters we can easily do any of your suggestions maybe except the middle line (but it would be really cool to get it in there).
I used 11 precious characters to make the mountains taller. It looks a bit better I think:
1 J=S-J*(J<96-S):K=J\16:FORW=0TO2:VPOKE8208+W,P(W+K):NEXT:IFTTHENRESUMENEXTELSEONERRORGOTO1:DEFINTA-Y:COLOR15,5,1:V=14:SCREEN2,3,0:FORX=0TO2:FORY=0TO31:R=16-Y\2:LINE(R-(X-1)*((SIN(Y^.8/6)-(Y<16))*6-R-2)+(X<2),X*32+Y)-STEP(Y+1,0):NEXTY,X
2 N=14336:FORI=0TO383:VPOKEN+I,VPEEK((IAND7)+(IAND8)*32+(IAND48)\2+(IAND448)*8):NEXT:SCREEN1:S$="OI1F20407FJCF7FJ70OIC02010F0J98F0J70RI0F101F2F1F19SJ8040J80TI060F09gI3":KEYOFF:LOCATE8,1:PRINT"PLEASE WAIT!":FORO=1TO71:A$=MID$(S$,O,2)
3 R=Q:Q=ASC(A$)-73:Y=VAL("&H"+A$):O=O-(Q<0):FORD=0TO-(R>0)*R:VPOKEN+I,Y:I=I-(Y>0ORR>0):NEXTD,O:X=V:R=N\2:FORI=256TO1023:VPOKE7952+I,85:O=VPEEK(I):VPOKEI,(OORO\2)\(2-(IAND4)\4):NEXT:FORI=ITO1279:VPOKE6432+(IAND63),228+8*(IAND3)-(IAND32)
4 W=2^(IAND7):J=(I\32AND1)*127\W:VPOKEI+512,JAND-W:VPOKEI+768,JOR-WAND255:J=8-(IAND32)\8:VPOKEI,(JANDI)*255\J:NEXT:FORK=1TO13:FORH=0TO31:VPOKE6464+H+K*32,-(K<9ORH0:-:D=D*D\96:PRINTUSING"SPEED:##0kmh RANK:###";S;N\128-82:-:F=B:IFC<TTHENC=V+T+TMOD99:B=(R-T>99)*(1-CMOD3)
6 S=S-(S<18ANDSTRIG(0))*2+(S>0):-:FORW=3TO4:PUTSPRITEW+1,(F*10+W*H,95),V,4*B+W+1:PUTSPRITE3+W,(W*H,127),V,4*F+3+W:NEXT:-:T=T+S-1:PUTSPRITE1,(115-(D<34)*((24-D)*B+F*10)+(E-64)*D\225,-Y*(98+D)-31),-6*(T0)*(N\4-V):SOUND8,U:SOUND9,U:PUTSPRITE0,(116-(36-X)*D\126,68+D),-(D>0),S-D\24:LOCATE6,4:PRINTMID$(Z$+"Race Over Time ="+STR$(Z/10)+"s"+Z$,NMOD42+1,S):-:NEXT:SCREEN0
Arjan: are you implying that you can actually READ this stuff? You're crazy!! 
BTW: The source is brilliant. Up till now, I've always thought what was possible in BASIC. I guess I was wrong
well, NYYRIKKI did a better job at decrypting the sourcecode I'm just suggesting some micro-optimizations...
We've done quite a few micro-optimizations. If we get a couple more of them
we'll have enough space to add for example a center line
ah indeed, I missed the last - in line 8
btw, 255 is occurring 4 times in the code
Yeah, now we have pretty good packing ratio. For example the last three letters "gI3" actually define last 31 bytes (30*0 + 1*3)
NY: so, what's new in your packing (in words)? Why was the 6-bit-per-byte packing of dvik not used anymore, btw? Or is it now compressed 8-bit per byte packing?
manuel, I removed my packing when NYYRIKKI changed the road generation.
The packing with my 6-bit version is a bit better but the code to unpack requires
more characters. So when we only need to pack the car sprite data, NYYRIKKIs
method is more efficient code space wise.
I saved another 13 characters on my lunch break 
. here is an update:
1 J=S-J*(J<96-S):K=J\16:FORW=0TO2:VPOKE8208+W,P(W+K):NEXT:IFTTHENRESUMENEXTELSEONERRORGOTO1:DEFINTA-Y:COLOR15,5,1:V=14:SCREEN2,3,0:FORX=0TO2:FORY=0TO31:R=16-Y\2:LINE(R-(X-1)*((SIN(Y^.8/6)-(Y<16))*6-R-2)+(X<2),X*32+Y)-STEP(Y+1,0):NEXTY,X
2 N=14336:U=255:FORI=0TO383:VPOKEN+I,VPEEK((IAND7)+(IAND8)*32+(IAND48)\2+(IAND448)*8):NEXT:SCREEN1:S$="OI1F20407FJCF7FJ70OIC02010F0J98F0J70RI0F101F2F1F19SJ8040J80TI060F09gI3":KEYOFF:LOCATE8,1:PRINT"PLEASE WAIT!":FORO=1TO71:A$=MID$(S$,O,2)
3 R=K:K=ASC(A$)-73:Y=VAL("&H"+A$):O=O-(K<0):FORW=0TO-(R>0)*R:VPOKEN+I,Y:I=I-(Y>0ORR>0):NEXTW,O:X=V:R=N\2:FORI=256TO1023:VPOKE7952+I,85:O=VPEEK(I):VPOKEI,(OORO\2)\(2-(IAND4)\4):NEXT:FORI=ITO1279:O=IAND32:VPOKE6432+(IAND63),228+8*(IAND3)-O
4 W=2^(IAND7):J=O*127\W\32:VPOKEI+512,JAND-W:VPOKEI+768,JOR-WANDU:J=8-O\8:VPOKEI,(JANDI)*U\J:NEXT:FORK=1TO13:FORH=0TO31:VPOKE6464+H+K*32,-(K<9ORH0:-:D=D*D\96:PRINTUSING"SPEED:##0kmh RANK:###";S;N\128-82:-:F=B:IFC<TTHENC=V+T+TMOD99:B=(R-T>99)*(1-CMOD3)
6 S=S-(S<18ANDSTRIG(0))*2+(S>0):-:FORW=3TO4:PUTSPRITEW+1,(F*10+W*H,95),V,4*B+W+1:PUTSPRITE3+W,(W*H,127),V,4*F+3+W:NEXT:-:T=T+S-1:PUTSPRITE1,(115-(D<34)*((24-D)*B+F*10)+(E-64)*D\225,-Y*(98+D)-31),-6*(T0)*(N\4-V):SOUND8,O:SOUND9,O:PUTSPRITE0,(116-(36-X)*D\126,68+D),-(D>0),S-D\24:LOCATE6,4:PRINTMID$(Z$+"Race Over Time ="+STR$(Z/10)+"s"+Z$,NMOD42+1,S):-:NEXT:SCREEN0
