Yeah, you have to purchase the full racing box and push the gear ratio's to wide (but not too far!) I find that for my Esprit; '35' seems to be the optimum with a few knoches to the left on 6th. This should give you more top end.
Fiddle around with it on the test tracks to see what the best settings are for you and then you shall be on your way. Well this is what I have found Im new to it all to!
Thanks MentalMetal