Prius performance at 65 miles per hour on the highway after warm-up, 20-30 minutes, tracks closely with outside air temperatures (and density):

• 15F - ~47.5 MPG
• 32F - ~49.3 MPG
• 68F - ~53.0 MPG
• 85F - ~54.4 MPG (w/o AC)

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Comments? Suggestions? Recommendations?

Bob Wilson

 

You probably should include the effects of running the AC at the higher temperatures, with the AC running fast enough to keep the cabin at about 75F.

When it's 85F or higher outside, no actual human being would want to be inside a car with the windows rolled up without running the AC. On a sunny day, it can be deadly within about 15 minutes.

 

Good point. I don't have the data, yet but with the warm weather coming back, I should be able to add a last one with AC. I will have to think about that one since a temperature setting that works for one person might be death to another. Certainly at 95F or 85F and high humidity, AC is no longer optional.

Bob Wilson

 

Hi Bob,

Good information. I wonder if you'd see such pronounced differences at lower speeds after warm-up? Harder to do, as finding a route you could run under the same conditions to simulate non-highway driving.

Living in a part of the U.S. that gets d*** cold (esp. this last winter where we went days wo/ seeing 0 F), the biggest hit is the time it takes to warm up. We only have a 5 minute commute, and our mileage goes into the crapper (I'm talking tanks in the mid-30 mpg when this is 75% of our travel) in the winter because we never get warm enough for the ICE to kick off or allow the electric engine to do more of the work.

Any FAQ on temperature affects has to discuss the problem of warm-ups and the much more detrimental affect of cold temperatures on short trips. There are a number of posts discussing this at length that could easily be adapted to serve the function.

 

Wayne Brown's simulator predicts the following for the NHW20:

• 15F: 41.3 MPG (w/o heat)
• 15F: 39.0 MPG (with heat set to 68F)
• 32F: 43.7 MPG (w/o heat)
• 32F: 42.0 MPG (with heat set to 68F)
• 68F: 49.7 MPG
• 85F: 53.1 MPG (w/o AC)
• 85F: 51.8 MPG (with AC set to 78F)

These assume level terrain, smooth dry pavement, no wind, steady speed, Michelin Hydroedges inflated to 50/48, and 300# for occupant and cargo.

I assume your numbers are for the NHW11, which I would think ought to be a tad lower. Are you assuming the same terrain, pavement, wind, and steady-state conditions?

Regardless, when I report expected fuel economy, I generally include statements about these assumptions so that one who reads them may be less likely to think they can expect comparable results in, say, a 10 miles per hour head wind or a downpour.

I recently plugged simulator numbers into a spreadsheet for several temperature ranges (and other variables) for purposes of making charts as needed. An example can be found via this PriusChat post. I'll be glad to generate a chart for you to include, if you think it helpful.

 

I will probably do more work next winter in this area. You'll find I have a tendency to include work that I can replicate in the FAQ. Sad to say, there hasn't been a whole lot of cold weather research other than:

• engine block heater
• radiator blocks

I've installed a transaxle oil pan heater but it doesn't show a dramatic improvement. Still I have some other areas I want to investigate when the cold weather returns:

• traction battery pre-heat and booster charge
• tire pre-heating

I notice winter and summer, there is an initial mileage hit in the first 20-35 minutes. This has bothered me because I don't see where anyone has taken a methodical look at Prius warm-up, including the wheel bearings. It has been my experience that anything not looked at by most folks often holds a greater promise of great discoveries.

Bob Wilson

 

Prius performance at 65 miles per hour on the highway after warm-up, 20-30 minutes, tracks closely with outside air temperatures (and density):

• 15F - ~47.5 MPG (with heat)
• 32F - ~49.3 MPG (with heat)
• 68F - ~53.0 MPG (w/o heat or AC)
• 85F - ~54.4 MPG (w/o AC)

Interesting difference. I'm running Sumitomo's at 50 psi and have gone to Type WS transaxle oil. I'm also using a 'water noodle' radiator block. Does Wayne's simulator include radiator blocking?

The greatest differences are at the coldest temperature ranges. I'm seeing a clue in that Wayne's numbers show a significant loss with the heater. This suggests the radiator block may be more effective than we think in improving cold weather mileage.

My 15F and 32F data came from fairly level terrain, bone dry, little to no wind, cruise control managed speed and Sumitomo T4s. However, I also had a radiator block, a water noodle, in the lower grill.

Good point.
I need to think about this.

I like Wayne's model but every model needs experimental verification. In this case, cold weather appears to be an area where I think we need more validation. Sad to say, I'm at least 9 months away from being able to repeat my cold weather tests. Worse, I don't have an easy way to get NHW20 data. Can you send Wayne a note about whether he has any correction factors for radiator blocking?

Bob Wilson

 

The radiator block certainly would explain at least some of the cold weather differences. Wayne makes no mention of one on his site, but I'll be glad to e-mail him to verify.