Very low MPG across two states (38-40 MPG)

Front grill = Black
Pipe insulation = Black

The insulation is hard to see (camoflauged) due to being a similar color to the grill. Plus, you are inside the car (mostly) and wouldn't see it anyhow.

I would recommend that you up the rear tire pressures to within (-2) PSI of the front tires. Toyota had a 2 PSI offset from the fronts to the rears.

Do realize that most cars get their best gas mileage when driven on long highway runs like yours. The Prius gets very good mileage on long highway runs, but it is optimal in slow and go driving (typical commute type of driving). Still, even the "40 mpg" cars would have a hard time hitting 40 mpg when cruising at 75+ mph....The would more likely be in the low 30's.

 

This is from the technique video (see pinned thread):

 

Last week I made a trip to Chicago from south central Oklahoma. Filled up in Missori and drove 74 mph and was getting 48 mpg. When I was low, I bought gas in Ill. And only got 39 mpg. I have been getting about 45 in oklahoma on the highway. I am thinking that the gas in Ill. has so many more enviormentally safe additives in it to knock down the millage. Burinng as much of the gas out now that I am back in Oklahoma. You have to watch out and not but E85 in the car up there.

 

Fairly heavily loaded, intentionally exceeding the (high) speed limit by close to 10 mph and you're wondering why your mileage isn't so good?

 

Yesterday, there was a fierce headwind, temp was in the 40's, the Prius was packed inside, and we had 2 bikes on back. I knew that 175-mile trip with lots of highway travel at 65 & 70 mph wasn't going to be ideal. 38 MPG was a pretty good outcome in those circumstances. How many other cars could accomplish that?

 

AC use plus speeds between 70-77 are going to lower your mpg. Look at the screen grab up the page.

 

50-60 degrees is a little cool. I think the Prius likes 65-75 degrees a little more. If you had a side wind thats bad. All the little things make a difference.

 

Sorry, you'll have to "show me" by running a two-pass, benchmark and documenting by taking two photos of the trip meter:

1. Some time with air conditioner OFF, no rain and low winds, drive to IS-64 and IS-255 and head south.
2. Set cruise control to the posted speed limit.
3. Reset a trip meter.
4. Without touching the accelerator, drive to IS-255 / Rt. 50 and the OH side of the Mississippi bridge.
5. Take first photo of the trip meter showing:
   1. Distance traveled
   2. Average speed (should be the cruise control set speed)
   3. MPG
6. Cross the bridge and take an exit to return across the bridge.
7. Set cruise control to the posted speed limit.
8. Upon reaching the OH side of bridge, Reset the trip meter.
9. Without touching the accelerator, drive to IS-64 and IS-255.
10. Take second photo of the trip meter showing:
    1. Distance traveled
    2. Average speed (should be cruise control set speed)
    3. MPG
11. Post the two photos along with date and time to get local airport weather conditions.

The two photos 'show me' speed and MPG. This route is fairly flat and should have little or no traffic. With the speed cruise control managed, we can get reproducible data. The two photos will confirm this along with giving a way to remove the latent wind and altitude changes. It is possible your car has an undiagnosed problem. This benchmark will let us know if it is in the range of expected mph vs MPG.

Bob Wilson

 

You got it!

A benchmark test holding as many things constant or factoring out any variables is how we get some trustable metrics. Then others can replicate to fault isolate

 

Keep in mind both Chicago and I think St. Louis region are EPA RFG (reformulated gasoline) which is basically a little lower energy content because EPA feels some of the higher energy content compoments of gasoline burn less cleanly. Going back to OP you would have had winter gasoline, less energy yet. USA just switched to winter gasoline now Sept 16 I think the first version hits stations in some areas.

My best recent tank came by filling up just outside of my RFG region. I have posted a map you can see how you might be able increase MPG by filling up outside RFG areas in your travels. Also RFG more expensive, so you can pay less and get better MPG outisde RFG areas.

 

I have to say I bought the Prius primarily because I wanted to take advantage of the great mileage. But to me it's a no-brainer... the faster you drive, the worse your mpg is going to get. There is no way around this as it ties into physics and the mechanics of how an ICE works. And while the Prius is a technological wonder that offers pretty much the best mileage out there, it's still a car that functions on some pretty basic and general principles. I guess I'm surprised that people wonder why they get 38 t0 40 mpg when averaging 78 mph over a long haul. I think you should be grateful that your mileage was a great as that, and bow down to the Prius for making it so! C'mon, it ain't a Tesla!

Here's an interesting article on speed and fuel efficiency. Fuel Efficiency & Speeding: The True Cost | Automatic Blog

This is a chart from the article...

 

From the article "Aggregated data from over 8000 hours of driving for 2009-2012 model years of each car."

• 9 mph - maximum Prius EV speed when cold explains first local peak
• ~42 mph - typical transition speed between hybrid mode, turns off engine, and engine on all the time explains the second local peak
• ~60 mph - typical highway speed, limited access, lower speeds have too many stop-and-go transitions. Above 60 mph, limited access with increasing aerodynamic drag

Nice chart, I'd like to get a copy of their raw data for other analysis.

Bob Wilson

 

Looking at the chart would suggest that you get better FE at 60 mph than at 50 mph. I don't have definitive proof that this is true or false since I switched to cruising at 90-100 kph (from 80) just as the temperatures dropped by about 5-10 degrees C. My gut feeling is that 50 mph gives better FE, although not by a lot if you just use CC.

 

That's a wonderful plot and great article about wastefulness of speeding, but presumably plot is not based on real MPG calc'ed from gallons gasoline pumped in. My car can easily be low 40's (MFD) at 75 MPH but you have Gen3 so maybe a tad better.

 

Note that this chart isn't for steady speed MPG. It includes a heavy mix of common engine warmup time, acceleration, and stop-and-go, not even close to evenly distributed by speed. For someone wanting to find steady-speed MPG, this chart is horribly skewed.

Bob Wilson produced a chart of steady-speed MPG for Prius, posted here:
Updated MPG vs MPH chart

 

^^^This data lead me to Glider's 2009 Tire pressure vs. MPG data.
Wow that's good data too, that would have been before I started being more active in Prius Chat, so I was not aware of it.
If I understand correctly, Glider measured about 1 MPG improvement per 10 psi higher, which is fairly minimal. Looks quite close to the theoretical estimates I've shown.

Final results on MPG vs Tire Pressure on G3 | PriusChat