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Wh/mi rate at startup -- calculation or consumption?

When I begin driving my Model 3, the display shows very high power use in Wh/mi -- typically in the range of 500 or so. Once I've been driving for a few minutes, it settles down to a more acceptable rate, under 300 (and under 250 if I'm not at freeway speeds). My question: is that initial figure reflecting actual power consumption, or a result of the computer not having enough data on the trip to calculate accurately? If the former, why would consumption be so high at the beginning of a trip?
 

jmaddr

Active Member
Mar 29, 2019
1,071
1,072
Florida
Take a look at your energy bar graph right below the speedometer right when you get in. Just like when you're driving, it's in the black range a little bit if the AC is blowing heavily. This shows how much energy the car is using...standing still...cooling down the cabin.
I've turned on pre-cool a little too early before and when I got in and took off, the Whr/mi was above 3000! It came down after a few miles, but you can see how much impact climate control can have on short trips. On longer trips, climate control becomes less of an issue.
 

camalaio

Active Member
May 28, 2019
1,483
2,197
Vernon, BC, Canada
It's kind of both.

The number is very real, and the same thing happens for ICE vehicles. When you first start moving, it requires a lot of power to accelerate. If you look at just the energy during acceleration time, it will indeed be a lot! But as you maintain speed, the energy required to accelerate gets overshadowed by how much energy it takes to maintain that speed.
 

electrongeek

Metrology Fanboy
Nov 1, 2019
69
74
Maine
I think a portion of that initial high consumption is related to cold tires and higher rolling resistance. As you drive, the rubber will get a bit warmer and more flexible, resulting in lower rolling resistance and contribute to a lower Wh/mile figure.
 

camalaio

Active Member
May 28, 2019
1,483
2,197
Vernon, BC, Canada
I think a portion of that initial high consumption is related to cold tires and higher rolling resistance. As you drive, the rubber will get a bit warmer and more flexible, resulting in lower rolling resistance and contribute to a lower Wh/mile figure.

More flexibility would mean greater rolling resistance. But warmer tires result in higher pressure and less flexibility as a result, which does help for rolling resistance. This would be hard to notice though, certainly not anywhere near as apparent as the OP's points.
 

Zoomit

Active Member
Sep 1, 2015
2,278
4,443
SoCal
More flexibility would mean greater rolling resistance. But warmer tires result in higher pressure and less flexibility as a result, which does help for rolling resistance. This would be hard to notice though, certainly not anywhere near as apparent as the OP's points.
The higher pressure reduces rolling resistance but higher temperature reduces energy dissipation by the elastomers which also contributes to lower rolling resistance, at least according to this Michelin slide package (page 86).

F22CC256-1212-42DF-918F-B036D2764836.jpeg


It appears the effects are about equal. According to the graphs in those slides, the increased pressure (~6 psi) of a fully warmed tire reduces Crr by about 10% while the higher temperature (~50 F) reduces Crr by another 10% or so.
 

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