Apparently, the Jaguar Series 3 E-Type, famous for its 5.3 liter V12 engine, was also made available with the six cylinder 4.2 liter XK engine.
According to the Jaguar Daimler Heritage Trust, only 2 or 3 were ever made, and none delivered to customers:
IIRC, the early Series III sales literature said the 6 and 12 would both be available in the Series III. Apparently early on Jaguar decided selling the same car with two engine options wasn’t feasible from a cost and revenue standpoint.
My guess is the disparity in performance was such that they figured no one would buy the 6 given the reasonable premium in price of the V12.
One that got bad mileage; one that got REALLY bad mileage.
In 1971 at $0.35 a gallon and the nation awash in gasoline nobody cared. Two years later? Everybody cared.