Influence of speed, temperature, and weight on MPG

OK folks, I tried to insert the data chart in the post as opposed to an attachment but had no success.

Anyway, the chart attached (open in a second window) is MPG data collected after ten 365-mile-long trips. Some important info about the trips and chart:

-These are one-way trips in the Midwest, fairly flat, under different conditions inbound versus outbound; therefore, it did not make any sense to combine them in 5 round trips.
-The gasoline used was always 87 octane (Shell or Speedway).
-This is just a preliminary data; I plan to do a more complete analysis after collecting 100 trips in one year! (Yes I drive a lot!).
-TEMP was the temperature observed by the Prius thermometer during the majority of the drive. In cases where there was a significant change in the temperature from departure to arriving, I just used a calculated mean temp.
- MPH was calculated my Garmin (more precise than the Prius MPH gauge). Average speed varied from 55 to 67.7 MPH, but most often above 65 MPH. I got unprecedented traffic in my first trip which caused the MPH of 55. I usually cruise at 70 – 80 MPH using cruise control.
- The letters L, M, and H by the MPG line means light, moderate, and heavy weight, estimated to be <300 (L), 300-500 (M), and >500 (H) pounds including my weight.
Numbers in the x (horizontal) axis refers to 10 different trips, hereinafter labeled as T1, T2, etc, for simplification.
- The texts above the X axis (above the trip numbers) indicate when the heater was used and when there was significant rain and respective time percentages. 70F 100% means the heater was on all the time set at 70F. L Rain 60% means light rain during 60% of the trip.

Conclusions:

The obvious: FE in the Prius is really similar to a category 5 hurricane, like someone pointed out elsewhere in this chat. Everything must be perfect for maximum performance. The combination of higher average speeds (MPH), higher weight, and lower temperatures cause a very significant drop.

Surprise: The data from Trips 2-4 (T2-T4) suggested that temperature itself caused a MPG drop regardless of the speed and this can be aggravated by weight. I thought this was a surprise because these are essentially 99.9% HWY-only trips, therefore, engine warm-up has little impact. Just the cold air by itself results in a hit.

TEMP, TEMP, TEMP...: T4-T5 reinforces my point above, i.e. just the increase in temp itself was sufficient to improve the MPG despite of the increased weight and higher speed (MPH). Look at T8-T9 now. Despite of the lower speed (same weight), there was a drop on the MPG caused by lower temperatures, so folks I'm convinced that temperature is the major factor considering the speed range I drove so far. Note that, during colder air temperatures (T2, T3, T4, T8, and T9), the heater was often turned on for at least 50% of the time. I believe, the heater may have contributed to the MPG drops seen at those trips as it drags heat (energy) from the engine. T3 was poisoned for low MPG: Lowest temp recorded, heavy cargo weight, heater on and rain...

Unknown: Look at T5-T6. What does explain the steady MPG between the two trips in the presence of higher speed and lower temp? It should have a pretty significant drop on my T6 MPG but it didn’t. I doubt the lighter weight compensated the higher speed and lower temp, so, I looked at my records for those 2 trips and the only thing I changed was driving in NORMAL mode instead of ECO. Right between T5 and T6 I changed from ECO to NORMAL and I'm still keeping it in normal. Could that be a factor? I doubt it too… so not sure how to explain it… maybe wind and humidity? I did not record those 2 parameters…

 

Yeah, I'm literally 5 blocks from the HWY in each end, so I really think engine warm-up is not impacting my MPG a lot. I think Bob Wilson posted some data in the past (may have been in the gen 2 forum) showing that the colder the temperatures the lower the MPG because of increased air drag, or something similar.

Anyway, it will be interesting to see what's going to happen to my MPG when I start doing those trips under freezing. Most of my way stays under freezing for most of the winter.

Nov has been so far warmer than Oct. Fall and Spring in the Midwest are perfect to test the influence of temperature on MPG because the temp oscillate a lot as you can see in the chart.

 

Thanks. Yes, I thought about grill blocking to see the impact but I don't have a device to monitor the temperature, like the scangauge. So, with these hwy-only trips, it could be dangerous if engine temperature is not monitored. Besides, I'm not sure how much a grill block would help. The block helps warming up the engine faster but it does not interfere with air drag, does it? I'm not sure...

 

Like I said, I'm really looking forward to see what my MPG hit is going to be once the winter starts... Some of those trips will be bellow freezing all the time and potentially in the negative territory

 

Spiderman, I'm adding the heater data to the original post. Unless it is very cold inside the cabin, I don't usually turn the heater on, I just close the vents to avoid the cold air to come in. I tolerate cold a lot better than heat, although, during the winter, the heater will be on most of the time because my trips are from IN to WI and it can get quite cold along the way.

 

I should also have mentioned that my tire pressure was fairly uniform for all trips, 46/44 psi. Also, just once, I had significant rain; it was light but 70% of the time. That was in T3 (the trip with the lowest MPG overall), but that trip was poised for lower MPG because of the lowest temperatures among the 10 trips and heavy cargo weigh, and, of course, plus the rain…

I'm leaving for another trip today...

PS. Hey Austin, I lived in College Station - TX for 6 years... yes, I'm an Aggie! Do you know what I wish when I was there? I wish Texas A&M was in Austin, not in College Station...

 

I did not realize the heater can "steal" signifcant amounts of engine heat and result in a MPG drop! I'm adding the heat data to the original post...