I would try a narrow beam US sensor pair, this model would probably be best... your accuracy will still be off though...
I also just thought about using those. As you say though, due to the cone sensing, they are not very accurate for beam sensing.
Presumably you could account for a bit of that though, if it was from one to the next then presumably the cone sensing is the same so unless the car moves significantly closer or further away it would be pretty accurate...
Any better ideas for a speed sensor? My idea would cost me well under 10$ so price is also a part in this.
Would cost considerably more than $10 though, $100 ish for the new maxbotix sensors?
Mowcius