A typical gasoline automotive engine operates at around 25% to 30% of thermal efficiency. About 70-75% is rejected as waste heat without being converted into useful work, i.e. work delivered to wheels.
A typical diesel automotive engine operates at around 30% to 35%. In general, engines using the Diesel cycle are usually more efficient.
Sub-critical fossil fuel power plants, that are operated under critical pressure (i.e. lower than 22.1 MPa), can achieve 36–40% efficiency.
In modern nuclear power plants the overall thermal efficiency is about one-third (33%), so 3000 MWth of thermal power from the fission reaction is needed to generate 1000 MWe of electrical power.
Sometimes you can find comparison between combustion engines and electrical motors, or between conventional power plants and solar power plants. In these cases, comparisons are made often for incomparable characteristics. Sure electric vehicals have efficiency of about 70%, but in this case we are not talking about thermal efficiency, because an electrical motor is not a heat engine.
In electric cars, energy undergoes the following conversions: from stored chemical energy to electrical energy and from electrical energy to mechanical energy. But in this case, most of its chemical energy stored was produced by another heat engine (e.g. coal power plant) and you must include it.
In gasoline cars, energy undergoes the following conversions: from stored chemical energy to thermal energy and from thermal energy to mechanical energy. Automotive engines are heat engines, that can consume primary energy source.