Our company builds large industrial engines, including dual-fuel turbocharged 4-cycle engines primarily used for power generation (electrical generator drive service. ) These engines are able to switch back and forth between natural gas with diesel pilot injection and straight diesel under full load/full speed conditions. In dual fuel mode, the engines operate on ~96% methane (natural gas) and ~4% diesel; this is only possible because the engines have air/fuel ratio controls and, when in dual fuel mode, control (throttle) the air to optimize air/fuel ratios with the methane primary fuel - this is done via butterfly valves in the air manifolds controlled by pneumatic or electronic actuators. At that point, the engines are operating as Otto (not Diesel) cycle engines - the diesel fuel is used only to ignite the air/natural gas mixture.
Under full diesel operation, the natural gas supply is shut off, the air butterflies go to the full open position, the A/F ratio controls are effectively disabled and the engine operates on the Diesel cycle with the diesel fuel racks taking control.
In dual fuel (gas/diesel) mode, these engines typically run at a best fuel rate of ~6300 BTU/BHP-hour.
In straight diesel mode, the best fuel rate is ~6100 BTU/BHP-hour.
My point is that the total BTU requirement is very close to the same (there ain't no free lunch), and actual fuel rates are slightly better on straight diesel than in gas/diesel mode.
In dual fuel mode, since only 4% of the total BTU is supplied by diesel, the apparent fuel mileage (looking only at diesel and putting it in Ram truck terms) would jump from, let's say, 15 MPG to over 300 MPG. That, of course, is meaningless - one must look at total BTUs consumed.
Rusty