There are two BMWs available - the 3.2 litre Alpina and the 3.5 litre Schnitzer. The car was generally known as a BMW 3.0 CSL - perhaps where you have got the reference to 3.0 litres, but the racing versions had bigger engines - just like the racing version of the Capri RS3100 had a bigger engine than 3.1 litres.
The speed quoted in the game is the speed with the default gearing, rather than the true maximum speed of the car with ideal gearing and a long enough road.
Simbin published a guide to the cars in GTL in three parts in pdf format at the GTL website. Its very usefull
scroll down a bit.