[tasdrouille]

Just stumbled on an article on the 67 Olds Turnpike Cruiser

FTA:

THE TURNPIKE engine includes another feature that contributes a great deal to fuel economy. This is the new "Climatic Combusion Control" air intake system, which is option No. K-50, available at $33.70 on all Olds V-8 engines. The system employs vacuum-operated valves in the air cleaner housing to mix warm underhood air with hot air from a muff around the exhaust pipe to maintain a constant inlet air temperature of approximately 100º F. Warm air arrives at the carburetor within seconds after a cold start. This, for all practical purposes, carburetor air temperature remains constant. The system offers several advantages, including close control of exhaust emissions to meet antismog laws. What helps overall fuel economy most, however, is that the carburetor can be calibrated for the constant 100º intake temperature. Normally, the carburetor must be jetted for the lowest underhood temperature in normal operation, which can be as low as 20 or 30º F in northern states. Tests who the difference in overall fuel economy is 0.75-1 mpg. Olds decided to include the air temperature control system in the Turnpike Cruiser Package to derive that last ounce of fuel economy.

That was a 20 mpg car.