I opted to block about 66% of my lower grill this November to see what impact it would have on my mileage. I've tracked my mileage on a spreadsheet and on Fuelly, so I can compare my unblocked mileage vs blocked mileage. The only variable favoring my current results are that my tires have ~35,000 on them rather than 15,000 miles. Updated with more mileage Total miles since November 15 = 4852 miles with Grill Blocking. All results below are calculated from the pump. Each fill up is around 550 miles. 11/12/2010 vs 11/15/2011 = 61.4 mpg vs 60.8 mpg 11/23/2010 vs 11/23/2011 = 56.1 mpg vs 60.2 mpg 12/02/2010 vs 12/02/2011 = 56.3 mpg vs 60.5 mpg 12/12/2010 vs 12/12/2011 = 55.6 mpg vs 59.5 mpg 12/18/2010 vs 12/21/2011 = 56.1 mpg vs 60.9 mpg 1/11/2011 vs 1/10/2012 = 51.8 mpg vs 60.2 mpg 1/17/2011 vs 1/20/2012 = 56.7 mpg vs 57.3 mpg 1/26/2011 vs 1/26/2012 = 55.5 mpg vs 58.9 mpg 2/03/2011 vs 2/03/2012 = 58.2 mpg vs 60.7 mpg November 12, 2010, I had a note on fuelly that we still were having warm weather, which would explain the 61.4 mpg. My note for November 12, 2011 was that morning temps were in the 40s (much colder). The remainder, we've been having steadily cold mornings (below 40F, recent tank was pretty much 32F +/- 2F each morning). On average, I'm seeing a 4 MPG improvement due to grill blocking (and more worn tires, I'd guess 3 MPG due to blocking, 1 MPG due to tires). *Edit* Interestingly, on my 1/11/2011 fill up on Fuelly, I had a note saying morning temps were in the high 20's/low 30's and my mileage was 51.8 mpg (my worst tank ever). Grill blocking may be even more effective than I realized, as my last tank had morning temperatures in that range. **Edit** My tank for 1/20/2012 was for same conditions as the tank for 1/11/2011. Morning temps in the high 20's (27 - 28F) to low 30's (32F) and even a touch of wet weather last night. The grill blocked fill up was 57.3 mpg vs unblocked of 51.8 mpg.