hot air from vents into intake?
 

I don't know how many people have covered this idea:

My assumption is that in the cold winter months ( it being -10 degrees F outside) that you could route the hot air from your cars climate control/air vents into the vehicles intake to get better fuel efficiency in winter?
Maybe you could use the hot air from the vents close to the pedals.

My average in summer when its warm is around 35 mpg, but in winter it drops to around 22 with the cold climate on the highway.

Maybe to run a small hose through the firewall and into the engine compartment?

Hi booploops and welcome.

Your idea has some merit but, depending on your vehicle, there are easier ways to pick up warm air.
What many do (me included) is to draw air from behind the exhaust manifold heat shield.
The benefits of doing it this way are:
The exhaust manifold heats up quicker than the cooling system, so you have warm air to help while the engine is warming up.
Once the cooling system thermostat is open, the air is preheated passing through the radiator, then warmed up further by proximity to the hot exhaust. This may (or not) result in warmer air for the intake.
You don't have to cut a 2" hole in your firewall.

My June (mid winter) average last year, just sucking air from behind the radiator was 41.2mpg. This year's June average was 42mpg sucking from the manifold, despite colder weather.

The idea here is to have a convenient "adjust-a-temp" system using existing heat exchangers in the car aka heater core. theres probably some easy, painless way to get hot air from the cabin to the intake.

its pretty cramped by my headers, cant access them from the top of the engine compartment.

Radiator heat isnt going to cut it at -28c unless I build a boxed heat exchanger using coolant lines. at that temp my vehicle barely turns over.

There is.
Put a heater core in your intake and control it with an adjustable thermostatic control valve.
The cars intake is very noisy, you will not want it sharing your cabin air system at all.

Or do this:
http://ecomodder.com/forum/showthrea...ake-27400.html
This way you wont have to wait for coolant to warm up to get heated air.