It's not that I don't believe you, but your experience seems to be an anomaly which leads me to believe that perhaps something else is going on with your Vortec. I have no experience with TBI's so I wont comment on that.
The fact of the matter is, your car will run rich until it reaches operating temperature. With the lower temp t-stats your vehicle doesn't reach the necessary temperatures, thus it uses more gas by continuing to run rich for the entire duration of your drive.
Unless you have some serious power adders, or have you vehicle tuned to run on lower temps, a lower degree thermostat is not needed and is killing your mpgs.
This is well documented on the LS1 community and on this forum as well.
I really wish the myth of a 160°F thermostat keeping a Vortec or even TBI engine running rich in cold start enrichment mode would vanish.
~45°F cold start captured on a wideband. I am in closed loop long before either thermostat opens. The thermostat temperature will not make the warmup any shorter or longer. Long term fuel trims are enabled by 140°F on the stock tune. Short terms become active long before that. It varies depending on the IAT temps but the factory L31 Van tune goes into closed loop at something like 80°F as long as the 02s have adequate activity.
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I have always run this engine on a 170°F thermostat and it has always given great fuel mileage.
The times I have run it on the dyno in back to back pulls, the first pull is always measurably more powerful and the engine looses power each pull as the coolant temps rise. The last run the fan clutch kicked in and lost a total of 15 ft/lbs and about 10 hp compared to the first run. Run 2 was down 5-8 tq and matching HP as well.
The tuning is set to pull a large amount of timing with both elevated coolant and IAT temps. At 188* it is pulling 3° of timing. At 203° it is pulling 5°. At 120°F the IAT table starts pulling timing. The timing pulled there is equally aggressive and pulls another 4-5° by 150°. In hot weather with the a/c blasting in town or stuck in traffic it is completely possible to have -10° of timing at WOT from a stop. Feels like the engine dies before the truck starts to roll. Running with the timing retarded makes the engine run even hotter which makes it pull even more timing.
