Originally Posted by Torque
Did you try it out? I'm not gonna do it but I'm curious.
Yes. I used to do many years ago when I was living up north near the Canadian border. In the winter time, it would take 10 miles to warm an engine fully, and at highway speeds the engine would drop 20-30F below the 190F design temperature. This was a pure gasoline large engine 4x4 truck.
And I have tried it 2 weeks ago on the RAV4. So far I blocked only the grille between the headlights, not the lower grille in the bumper. The lower is much larger actually. This helped with MPG greatly.
There is a 20 miles route that I take regularly, with about a 35mph average speed. In the warmer weather I got a consistent 44 to 44.5 MPG. When the temps dropped to around the freezing mark, and I started using cabin heat, MPG dropped to 35 to 35.5 MPG on the same route. With the upper vents blocked I'm back to 40-41 MPG.
But when I look at coolant temperature through OBDII, the engine still does not make enough heat from propelling the vehicle, as coolant sometimes drops into 150-160F range. This is telling me that more grille area can be blocked.
Everyone's mileage will vary, as they say. It depends how much grille area is block, what the outside temps are, the length of trip and type of driving. But you will never know until you try.
I would be very curious to see what improvement you get in the much colder climate than I am. You can Google "vent blocking prius". There is a lot of information about this practice.