Racelogic do one/used to do one.

They actually had a white Sapphire Cosworth or an E30 325i Sport in their advert for it for a long time.

The white Cossie used to be owned by the guy (Mick) at Hi-tech rolling road in the West Mids I think(?)

I have heard variable reports about how effective it is though - try a search with Racelogic?

Toodlepip