Model 3

Question about efficiency

Sooo I went back and check my GA to NJ trip in June and our Efficiency was 249 on the way to NJ. Now our trip from Florida back to GA we did this week Efficiency never went below 305. Same amount of people, same or close to load (NJ trip probably had bigger load). Two things different. NJ trip we had hubs on, FL hubs off and maybe speed difference NJ mostly 75 with short burst to 85 and FL 80 with short burst to 85. I don't get why did my efficiency went to crap. Any ideas?

Comments

  • Headwinds? Weather? Changes in elevation? Tire pressure?
  • Weather about the same. That is why I said trip from FL. When we were leaving GA to Florida temps were in low 40. And warmer when we came back. Tire pressure was good. Elevation changes bigger in NJ trip since we went through Mountains. Headwinds is only thing I have no clue but possible.
  • You've actually answered your own questions. Faster you go, worse the mileage is going to get, no question. I may be off, but I think that wind resistance goes up as the cube of velocity.
    Next thing: Just like ICEs, Teslas get worse gas mileage in colder weather. When it's 10F out, a Tesla will pull something over 300 W-hr/mile, rather than the 225-250 W-hr/mile when it's 70F out.
    Note that ICEs have the same problems as Teslas at higher speeds, but, with them, it's typically worse. Efficiency varies with power output and engine RPM; there's typically a sweet spot that automotive engineers try to aim for with gearing, engine RPM, and velocity, but they have the same cube law issues with velocity that any vehicle has. Electric motor efficiency does go down with increasing RPM, but not to the same extent that ICEs do, so wind resistance issues tend to dominate.
    Electric motor efficiency is one thing, getting energy out of the battery is another. And that's the primary reason that Tesla tries to operate the traction battery at a fixed temperature.
  • A strong headwind could easily account for it
  • Thanks. Definitely wasn't expecting that much of the difference.
  • Aerodynamic drag increases with velocity squared, but the power needed to overcome that drag increases with velocity cubed, since P = F • v.

    The aero covers are good for about a 3% improvement, based on most tests, but speed is the biggest factor. Speed kills (your battery)! Cruise around 65mph and you'll see close to the rated efficiency, or better. Your numbers vs. your speeds look pretty normal to me.

    One Honda Insight driver had a sign in his back window: "88mpg @ 55mph, 55mpg @ 88mph" :-)
  • @WW_icefree: The Other Car in the household is a Prius. That car has an ICE in it; it's also a full hybrid. Further, as compared to a normal ICE, it's running a gas engine with an Atkinson cycle, rather than the Otto cycle most ICEs run. (Power stroke is over 90 degrees, which means, amongst other things, the exhaust and intake valves are both open at the same time.) Atkinsons have great efficiency, but lousy torque; but the Prius literally runs electric motor/generators in parallel with the ICE, and those motor/gens supply the torque that the ICE won't, so the car remains driveable. Finally, the planetary gearset and motor/generators constitute a continuously variable transmission. This gives an extra degree of freedom to the RPM of the ICE in a Prius; so, for a given steady-state power output of the ICE, the RPM can be adjusted, freely, to the most efficient spot in the gas-consumption/rpm/power-output 3d plot.
    The above are the reasons that a Prius can hit 50 mpg both in city and highway driving. Part of the city driving low MPG reasons is that the car will automagically shut down the ICE when at low speeds and low power outputs, but, still. Most ICE cars don't get close to these numbers and certainly don't hit those MPG numbers at _their_ sweet spots.
    Given all that: Drive the car at 65 mph on a warm sunny day and one will get 55 mpg. Drive at 70 and it'll be 50; drive at 75 and it's down to 40 to 45. Normal ICEs drop off a _lot_ faster. Some Prius maniac drove down the Autobahn at 120 mph; he got, if memory serves, something like 25 or 30 mpg. You can bet, however, that the Audis, BMWs, and Mercedes were getting in the single digits; they fall off a _lot_ faster.
    Drop the outside temperature to 15F; I would get in the 30's until the engine warmed up, thoroughly, and in the low 40's otherwise.
    The Prius zanies over at Prius Chat have got plots of all this stuff. They also have hypermilers who, given the right conditions and no other cars on the road, can get up to 100 mpg. Still not as good as a Tesla, but pretty blame good for a mass-market car, even if it is an econobox.
    Point I'm trying to make, though: Variation in MPG or the equivalent thereof is major and typically worse with ICEs than with a Tesla; and Teslas are affected by the Laws of Physics, just like everything else. They just happen to be better at it than an ICE-driven car, hybrid or not.
  • Oh one more thing that was different is that in June I used AP, and now I used EAP NoA since I got it on sale back in September. Wonder if that eats more energy as well.
  • I sometime wonder if the new software being loaded into dated hardware can cause strain and reduce efficiency to the battery.
  • A trip a couple of months ago, I couldn't figure why my efficiency was so poor. I think it was like 325 wh/mi or more at 70 - 75 mph for a DM with aero wheels. I checked the weather, and I was driving into a nearly direct headwind of like 20 kts. A headwind can make a BIG difference in efficiency.
  • Did not want to leave Florida and had to be dragged out? 😉
  • Haha yeap. Really loved Naples area. So tropical and wide hwys, palm 🌴,beautiful water, beautiful women )))
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