If you want to try a tb spacer I have one for a Vortec. Just cover shipping, should be $6 if it fits in the small Priority Mail box.
The biggest improvement I saw with my LS Swapped 94 Z71 was smaller tires. I have 285/70-17 BFG 10 plys and they killed mileage a solid 2 mpg. I swapped them onto my 08 Crew Cab for a few months and it did 2 mpg as well.
Winter mileage takes a big hit due to winter blend fuel (yes fuel compositions change by the season) and cold starting. My dd commute is 18 miles and the difference in mileage summer-winter is about 1.5-2.. yeah I measure it every tank.
If you do highway a lot, a tonneau cover helps too.
Big tires look good but kill mileage.
All said and done, going back to stock tires, summer fuel and tonneau took my 08 Sierra Denali from 13.5 mpg- 17 solid. Yeah the big 6.2L... highway with the tonneau made a huge difference.
An LS swap would be cool, I did it twice, but it does not improve mpg much.
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