Physics question (vehicle, windshield, spark plug, fuel)

[VTsnowbird]

Driving down the road at highway speed, all windows closed. Would I improve or reduce my gas mileage if I opened the inside vent to let fresh air into the car?

PLEASE NOTE - this is not a question about using AC vs. rolling the windows down. Don't even go there.

The air is already entering the engine compartment and presumable swirling around in some way. Is it helping the aerodynamics if I let it into the interior of the car?

[Bitey]

No.

[kwong7]

I'd say it's negligible. Most cars have an intake vent near the windshield/hood and a equalization vent in the trunk; this is why your ears aren't pained when you close the door with your windows up. Your cabin air should not come from your engine compartment.

[M3 Mitch]

Strictly from an aerodynamics standpoint, on most cars the inside vent air comes from that little grille right between the hood and the windscreen. As you go down the road a high pressure area forms there as air hits the windscreen.

I think theoretically you might get better mileage with the vent open, although I would not hold my breath to actually measure the increase at the pump.

This is likely the same order of magnitude as the decrease in MPG from burning your headlights. It's there, at least theoretically, but it's so small it would be hard to measure.

As a practical matter, I would operate the cab vent based on comfort, and operate the headlights based on need to see or be seen, with little or no thought to MPG.

If you want better practical MPG, increase the air pressure in your tires (up to a point, most passenger car tires I would go to 35 PSI and quit), maybe open up your spark plug gap a bit, perhaps most importantly both for fuel atomization and fuel pump life, change the fuel filter on time. If you have a manual, try to open the throttle a bit more and shift a bit shorter in RPM, again don't over-do this, but it can help a lot. That and in general anticipate slowdowns and stops, etc. etc. If you have non-ethanol gas available, it will improve MPG and be kinder to older fuel systems, although your cost per mile will probably go up due to the increased cost per gallon.

I'm assuming a fairly modern car with closed loop control fuel injection. Say 1982 or later.

[bfrabel]

No. The air has nowhere to go, therefore it will just slow you down/create more drag.

[RayinAK]

The air vent that is between the hood and the windshield is always open, so it would not make a difference relating to fuel consumption. However there is a gate that's located closer to the cabin air filter and blower housing that can be closed by placing the airflow lever or switch to "recirculate," in which case the fresh air that enters thought the vent in front of the windshield does not reach the cabin filter (it stops at the gate).

When the lever or switch is moved from "recirculate" to "fresh air," the gate is open and the air just flows though the cabin under pressure (if the vehicle is moving forward at high enough speed, or if the blower is turned on), and exits the cab out a exhaust vent (s) toward the rear of the cab, and sometimes in the trunk. This exhaust vent prevents back flow air by the use of a lightweight plastic curtain or flapper. If the blower is off and the car is not moving forward, this curtain or flapper collapses to the close position. If you turn the blower's fan on, then the intake air enters the cab at a higher rate of pressure, and exits though the same exhaust vent.

To the OP: the fresh air entering the cab does not go through the engine compartment, but though the vent openings between the hood and the windshield.