I don't think the electrical system load on the engine would amount to much gas saving, a 100 amp alternator may use only 4HP of your engine's power. Outside of the starter motor you don't use anything close to 100A for the accessories. Even an electric A/C is only a 5-10 horsepower saving.
You'd see a performance improvement by reducing those parasitic loads, but that doesn't translate into a 10% fuel savings.
ICEs are incredibly inefficient, most of the petrol energy is lost to waste heat. Something like 60+% IIRC.
On my MX-5 when I ditched the AC compressor and switched to an OEM manual rack the car picked up some pep but I observed no significant difference in MPG...
IIRC the major way to move the MPG needle on these things is to increase the compression ratio, since that's directly linked to the thermal efficiency.
It's not so much that ICE engines are inefficient, it is all heat engines of any type are bounded in by the same thermal limitations (ICE, gas turbine, steam turbine, other externally heated piston engines, etc). In the physics of heat engines, there is the Carnot efficiency that is based only on the temperature of the hot source (combustion chamber in an ICE), and the cold sink (atmospheric air). Relative to this limit on all heat engines, modern ICE engines are very efficient at their peak efficiency point.
A problem specific to ICE in a vehicle application is that at the engines end up running in conditions away from their peak efficiency point due to vehicle load, terrain and traffic conditions. That is where hybrids are supposed to pick up their efficiency by letting the ICE turn off completely in the regimes that force the engine to run at low efficiency points. Such as idling where the MPG is zero.
It's not 10% all the time. It's 10% with high beams on and every conceivable thing on max. Even then you'd probably have to have a super awesome stereo :D
