I’ve been driving my 2012 Volt for 5+ years, and Voltstats tells me my lifetime ev% = 72.6%, and my lifetime MPGcs = 37.02. Math tells me that if my car had been tuned to increase the MPGcs by 5-8 mpg, my lifetime gas consumption could have been reduced by 35-53 gallons over 5+ years, or ~7-10 gallons annually. IOW, those who achieve a high ev% when driving a Gen 1 Volt have little to gain by small increases in the MPGcs.
I also note that my 37 MPGcs is an average of great variation. Most local trips are short drives on battery power. Non-vacation-driving use of ICE is mostly restricted to the final mile or two when I do exceed battery range around town, or via EMM episodes, i.e., low MPGcs numbers achieved using small amounts of gas to run a gas generator, not a car’s engine. On the other hand, last year on a long vacation trip, my 2012 Volt gobbled up only 12.2 gallons of gas as I drove 569 miles eastward in sunny weather through Nebraska, Illinois, and on into Indiana (46.6 MPGcs!), and then the same car’s performance deflated my enthusiasm by using 13.8 gallons to drive only 398 miles as I drove westward and crossed through Wyoming as wind gusts up to 35+ and 50+ mpg were blowing (28.8 MPGcs!). Would fine tuning a Gen 1 "gas generator" minimize the hit on MPGcs arising from the aerodynamics of driving at freeway speeds with prevailing winds?