I have a '98 base. 3.73 ratio with trutrac differential and 2.5" exhaust. srsly, the difference mechanically between the base &LSC is the gears and exhaust. 3.73s puts cruise rpm in the sweet spot for torque and I have not found any change in economy. the engine has two knock sensors and with knock sensed, the PCM will back off timing in a somewhat draconian manner and then advance it 2deg and listen for knock and keep doing that until it hears knock come back then back off 2deg. this tuning happens extremely fast. with very low torque at cruise, the timing is quite advanced. NGS mach2 tester has a nice readout on ignition advance. I am at 6800' altitude and run 87 octane on the city loop, but up into the rockies, I run with the premium. just finished running BG-44K and found the idle somewhat smoother, no change on power output. thinking about pulling the intake manifold and checking the IMRC system for function and cleaning.