New car mileage

Thy claimed the light weight bumper brackets and hood support as well as a 1bbl and a recurved distributor got it mid 30's ...lies. Even the new cars state "actual mileage will probably be less, only use them for comparisons". @mpgmike would know. Buddy had a first gen Duramax (M.A.N. injectors?) shortbed and he swore he could get low 40's unloaded on the highway. Motor hydro-locked twice from cracked injectors, 2 warranty blocks.