I got worse fuel mileage with a grill block
 

I put a cheap grill block on my jeep(plastic sheeting and duct tape.) I wasn't able to do a formal A-B-A test on it, but it was mostly a proof of concept to see if the vehicle would over heat. I left it on for a trip to Montana recently. On the first two legs, I posted 14.9 and 16.1 mpg. This is the worst hwy mileage I've posted since I began hypermiling the jeep. Both of these were level stretches of freeway with some rolling hills. I removed the grill block for the rest of the trip and mileage improved to 18.4 on the next leg. The next leg was windy mostly two-lane, and a had an approximately 3 mile 6% grade in it. I know this isn't a very scientific test, because the roads changed between runs, but I would think the third leg would be worse for economy than the first and second. Also, the fourth leg was very similiar to the first and second and I again was over 18. I have two hypotheses for these results:
-The grill block decreased airflow too much, so the fan worked more and the extra drain on the engine canceled out any aerodynamic savings.
-Either myself or the road introduced enough variation that the test is just invalid.

Has anyone here had experience with grill blocks on 97-2006 Jeep Wranglers(TJ series)? If you have, what effects did you notice on fuel mileage? I'd like to gather some more info on this vehicle before I run more tests of my own. If I cant get anything, I'll probably run the block again with a larger opening too see if I get different results.

I think that's it, in a nutshell.

Not saying the idea that over-blocking the grille couldn't reduce mileage, but the driving you did isn't a "test".

There are a number of Jeep owners who have grille blocks. Hopefully they'll chime in with their experiences.

honestly I dont think a grill block on a jeep will do much beside overheat the engine. if you have the 4.0 I6 especially, that engine runs hot. the front of the jeep is essentially a brick, the holes in the grill might even allow enough air to pass through to make it a smaller brick aerodynamically

Hello,

A headwind / tailwind could easily explain the change? Do you have a picture of your grill block?

I didn't have a noticeable change in winds. No I don't have a picture, but it was just clear plastic sheeting stretched across the grill with duct tape:) holding it on. I cut out the plastic over the center grill slot for airflow.

Nope.

I had blocked approximately half the radiator on my Thunderbird in an initial temperature test for aeromods/trying to get it to OT faster. I discovered I couldn't reduce the airflow that much due to the clutch fan engaging frequently and dropping the mileage like a rock.

No data for me or you for sure but I could hear the fan in my situation.

My dakota quad cab looks a lot like the jeep to the on coming air. I took off my clutch fan and totally blocked the upper grille. This netted me about 1-1.5 mpg. I'll probably have to install an electric fan after this winter, but so far I have had no overheating problem.--It's a 3.7L

I've got all but about 1 of the openings blocked right now. Great for winter time warm up. So far I haven't had any problems with it getting too hot. Summer time I had 4 of the openings blocked and the only time it kicked in the fan was when we saw temps over 95F with a lot of stop and go in town driving(then again what car won't). Haven't seen a drop in mileage. I've done several mods at once so I'm not sure how much it improved it. But I do remember after putting them in I noticed it pick up speed faster going down hills. So I guess my butt dyno says its helping.

I did a simple cardboard grille block on a '97 dodge ram (which has a HUGE grille) and saw 2mpg, or about a 10% improvement. I did the same thing to my Chevette, which is shaped like a small brick and saw no improvement. So I think it depends on the vehicle. Grille blocks help some more than others.