Is it worth getting bottom grill blocked?

Why do you want to block at those temperatures?
I think @Mendel Leisk blocks about 50% of his grille during winter. With my Gen 4 the active grille shutters handle it for me.

 

Some people say it helps a little with aerodynamics.
I think one purpose in the Gen 4 is to permit the engine to heat up quicker, reducing the warmup penalty.

According to Fuelly, 45 mpg is about average for a 2010

2010 Toyota Prius MPG - Actual MPG from 1,262 2010 Toyota Prius owners

 

Grill blocking when it is under freezing is safe. Above that, I would like to have coolant temps for both the engine loop and the inverter loop if possible, so you do not go over the 'normal' temps. (My engine loop stays at 197 F or under normally)

 

Lower speed is good, longer commutes are good, higher outside air temp is good to about 85 F. Most important is driver skill, read up on Pulse and Glide, but be sure you are not annoying other traffic.

Regenerative Braking recovers 30% of braking energy, organizing your life so you don't brake, saves 100%. Panic Braking saves 0%.

Both grill blocking and engine block heaters can improve MPG, but they have the most effect when it is cold outside. Lighter (usually smaller) wheels and tires save more MPG the faster you go. Low rolling resistance tires are easier to find in smaller sizes as well. 40 in front, 38 in rear. While I never recommend turning off the A/C as it cools the battery, window tinting to the legal limit can save gas.

 

I grill blocked last week because temps finally dipped below 20s for cold soak starts and with 50s for daytime highs. Couldn't block directly behind the front license plate but everything else on the lower grill was blocked. Most of my daily commute are only 20 min (19min 35 mph or slower + 1 min 55 mph) therefore ICE spends most of it's time off with EV kicking in. I've yet use the heater + defrosters since testing the prius before buying it during the summer so I won't have to worry about scangauging for real-time temps. Grill blocking makes no difference when I'm on the highway because ICE is always running keeps the engine oil operating temp and its afternoon or evening when on highway.

Depends on your driving conditions such as stop-and-go, majority freeway, 5-10 minute wait in locked traffic, pulse and glide- grill blocking might not make a difference.

This link helped me out the most, and I use the first post table for reference when to unblock without monitoring temps.
2010 Prius Grill Blocking strategy | PriusChat

 

What's your daily commute in terms of driving speed, distance and duration? Tire size and PSI?

I have 15s" 44/42 with majority of residential streets + a small downtown driving like a grandpa, prius gets the 56-70mpgs. When I drive with judiast priest 'breaking the law', prius gets 45-50mpg.

 

I was running the lower grill completely blocked and one spring, I drove to the city and it got in the low 80's. I was running the Torq app and coming up a long hill out of town, the temp never got over 190 so I wouldn't be afraid to block the lower grill in your case.

 

Correction: Grill blocking does make a difference on the highway.

The benefits of a grill block are twofold: first, as you and others here have noted, is to reduce warm-up time by limiting cooling airflow. But, limiting that airflow also keeps it out of the aerodynamically "dirty" (high drag) underhood area, sending it around the streamlined body instead. This is why, in the last several years, manufacturers have significantly reduced the size of grill openings on new cars; many cars now have quite small openings compared to the area that looks like grill space and would have been open grill a few model generations ago. A secondary benefit of limiting airflow into the engine bay is typically a reduction in front lift.

I run a partial lower grill block year-round, and 100% in the winter. It's not wise to do this without a Scangauge, Torque app, LED wired to the cooling fan circuit, or some other means of monitoring coolant temperature.

 

I have a strong suspicion that the 17's might be a problem. We had a 2010 that we always got over 50 mpg [whatever the original smaller rims were]
And our newer 2015 i can't even get 49 mpg, and it came with 17's. I often only get 46
It's funny but those few MPG"s drive me nuts. Of course the 2010 had narrower tires, and my newer tires i got narrower than the stock, still no good.

 

My strategy:

Consistently below 0C: 100% lower block (sometimes just stick with 50%, get lazy, it's safer too)

As long it stays under 10C: 50% lower block

Any time under heavy load, say going up a ski mountain, NO block

50% lower equals one 1.5" OD foam x 4' insulation tube, pushed onto a grill slat and held on with Velcro tape near each end.



And to hold heat in the engine bay, short pieces of the same tubing, which I leave in year round.

 

Are you calculating your mpg manually or using the car? The outer diameter of your tires may be different than stock making the car’s calculations inaccurate.

 

That explains it all.

You can remove spare tire and kit plus rear row seats if that makes a difference

 

Different OD of tires throws manual calc off too. I alternate between 195/65R15 snow tires and 215/45R17 that the car came with. The latter have a slightly smaller OD, so that going to change the odometer reporting, and so on.

I think ultimately, the differences are small enough to be "lost in the noise".