My 01 CCSB 1500 HD 3.73 gear was about 12-13 city and highway. This was with FIPK, headers, cat back exhaust and nelson hot tune. My 05 ECSB LM7 on 17" wheels is 16 on the money. The 05 has the display for MPG and I dont know what gear the 05 has.

I drive em hard though. The company buys gas. It really depends more on how you drive.

IMHO I would get the 6.0