Influence of speed, temperature, and weight on MPG

[Indyking]

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…

[a64pilot]

Interesting,
I've seen a drop from 62 to 55 average and haven't been able to find a cause for it. I wonder why temp has such an effect? There is increased drag from denser air, but surely that's very minor.
BTW, it's not very cold here either, we haven't seen a day below freezing yet.

[Indyking]

Originally Posted by a64pilot

Interesting,
I've seen a drop from 62 to 55 average and haven't been able to find a cause for it. I wonder why temp has such an effect? There is increased drag from denser air, but surely that's very minor.
BTW, it's not very cold here either, we haven't seen a day below freezing yet.

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.

[Codyroo]

Originally Posted by Indyking

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.

While I don't have any measurements (per se), the idea that cooler air increases air drag would match up with my observations.

When commuting home, there is a long downhill that I will....errrmmm....occasionally put my car into neutral and coast. In the summer (late in the day, very warm) the car can get upwards of 80 mph and I will coast it down to 55 mph. There is a landmark that I target to see how far I coasted and what speed I am going as I pass the landmark. On cooler days, I do not get as much speed going down the hill, and when I pass the landmark, my speed is slower than on warmer days. I've done this enough that I've discounted wind variations.

Very interesting table. I liked looking at the one part where you essentially got the same MPG three times (once with a medium load, twice with low loads), such that you could see the impact that temperature played (that a higher temperature could compensate for the extra weight in the car). I wonder if you blocked your engine on one of the trips (but not to the point of overheating it) if you would see a jump up in MPG's. Likely you would.....

[Indyking]

Originally Posted by Codyroo

Very interesting table. I liked looking at the one part where you essentially got the same MPG three times (once with a medium load, twice with low loads), such that you could see the impact that temperature played (that a higher temperature could compensate for the extra weight in the car). I wonder if you blocked your engine on one of the trips (but not to the point of overheating it) if you would see a jump up in MPG's. Likely you would.....

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...

[Jim05]

I just found similar temperature effects when we went from 40's to 70's late last week. I didn't record the data as you did, but my average during the 40's was ~55 (2 normal trips), and it jumped up to (same tank) ~57.5 over 2 additional normal round trips. Interpolation by way of "calibrated elbow" suggests about a 5 mpg difference just due to the temperature change.

[Indyking]

Originally Posted by Jim05

I just found similar temperature effects when we went from 40's to 70's late last week. I didn't record the data as you did, but my average during the 40's was ~55 (2 normal trips), and it jumped up to (same tank) ~57.5 over 2 additional normal round trips. Interpolation by way of "calibrated elbow" suggests about a 5 mpg difference just due to the temperature change.

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

[Jim05]

Originally Posted by Indyking

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

I'm not looking forward to it either. Time to get to work on a temp-controlled grille block.

[a64pilot]

I agree there seems to be a drop in mileage with temp, but surely the slight increase in air density can't account for much of it. Has to be other things.
I have the scan gauge and now in 60 deg OAT temps, it just doesn't seem to get above 180 engine water temp or so, but that's not highway driving.
Scan gauge and blend mount is about a 10 min. install, neat and about $200.
I don't have repeatable, regular trips like you do or I would gather data with grill blocking.

[a64pilot]

Originally Posted by Jim05

I'm not looking forward to it either. Time to get to work on a temp-controlled grille block.

How about some kind of cowl flap? Manually operated mechanism, not automatic, but effective.
On aircraft the strategy is to block the air from exiting, not entering the engine compartment.
I wonder if a full, insulated belly pan would be the way to go.