When I started the game again I decided to use the MX-5 race as a quick(!!) money earner. What I did was got the cheapest MX-5, won the Spider & Roadster cup which gets you the more powerful 1.8i version. I then used that (with stock normal tyres). I pitted every 6 laps and still won. It's not that hard I just kept the tyres with grip in them. I won the race by about 30 seconds over the car with super-softs that pits every 3 laps.