I'm all for "combinded cycle" use of fuel, if there is a local need for heat, but don't see how this enters into the question as to which is better (more efficient) fuel for an ICE. Running an ICE car on pure NH3 was demonstrated some decades ago, but with the old (non-fuel injection) carborators that was tricky to do* and only few farmers did it. (They had tanks of NH3, and paid no tax on their car fuel.) To make your statement true, you need to claim the heat produced as an efficiency credit. I don't think it true if the CH4 is used to run an electric generator to charge a battery and the heat just discarded. Then using the CH4 as the car fuel is surely more efficient than losing ~ 2/3 of the chemical energy when generating electric energy for the battery. You could do the same "combinded cycle" with NH3 as the burnt fuel. Again it does not address the question as to which (NH3 or CH4) has the least losses of energy to drive a mile down the road. * NH3 tends to explode rather than burn.