You cannot change your GTL for 2X fuel. This is managed by the server. Test online for fuel consumption figures.
Actually you can, but you have to edit your player (.PLR) file, using Wordpad or similar program.
If you scroll down, you'll find these parameters:
CURNT Fuel Consumption Multiplier="2"
PRACT Fuel Consumption Multiplier="2"
QUICK Fuel Consumption Multiplier="1"
FRACE Fuel Consumption Multiplier="1"
CHAMP Fuel Consumption Multiplier="1"
MULTI Fuel Consumption Multiplier="1"
RPLAY Fuel Consumption Multiplier="1"
CURNT Tire Wear Multiplier="4"
PRACT Tire Wear Multiplier="4"
QUICK Tire Wear Multiplier="1"
FRACE Tire Wear Multiplier="1"
CHAMP Tire Wear Multiplier="1"
MULTI Tire Wear Multiplier="1"
RPLAY Tire Wear Multiplier="1"
As you can see, both tyre wear and fuel consumption can be adjusted, for various events. Mine above are modified for some of my offline testing.
Btw, I followed the sound recommendations when installing the Merc, and haven't had any problems (although it sounds just like the 300SL that comes standard with GTL)
Cheers,
Erling