Re: Numbers...
"Still in my perfect car."
Extremely perfect, with an engine that doesn't suffer even modest pumping losses.
The absolute maximum efficiency of an automotive engine (otto or diesel) is around 35%(*), so divide the available energy from your fuel by 3 to get extractable energy (the rest is expended driving the pistons and valvegear)
(*) This only occurs at Wide open throttle/full load conditions.
Under most circumstances a car engine is in the 2-15% range and mostly at the bottom end of that. This is why decoupling the engine from the drivetrain can give such an improvement in efficiency despite the extra losses and rolling mass inherent in any hybrid system.
It's also why "certain petrolhead media programs" *ahem*5thgear*ahem*topgear*ahem* can produce reports "proving" that XYZ small engine car is more efficient than a hybrid - it is under the kind of testing that a driving program uses (mostly openroad operation) because they don't replicate real world conditions (ie, stop/start driving, short trips, urban runabouts) where hybrids and decoupled drivetrains excel and where the vast majority of vehicles spend the vast majority of their operating lives.
(If Toyota's 10kW free-piston generators are practical then we could see a step-change in automotive efficiency, as you'd only need to activate as many pistons as required instead of having them all tied to a crankshaft, and only operate them for as long as necessary to keep the battery topped off)