First Impressions: CT Fuel Economy

Discussion in 'Lexus Hybrids' started by SageBrush, Nov 27, 2011.

  1. SageBrush

    SageBrush Senior Member

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    I am driving a loaner while waiting for my car to arrive at the dealership. This is only relevant, in that I am not changing tyre pressures.

    So far I have driven about 400 miles of 65-75 mph highways, 100 miles of city, and 100 miles of 50 - 60 mph 'suburban' roads. These are roads I know well from my G2 Prius days, and my comments are best taken as comparison to my Prius. Do keep in mind that the CT has LRR 17 inch tyres that have less than 2k miles wear, compared to 15 inch, 195 mm hydroedges with 70k miles on my Prius.

    Where the Prius in my hands was good for 65 mpg at 55 mph, the CT is somewhere around 58-60 mpg at 55 mph. Surprising to me, the CT mpg falls off pretty rapidly as speed goes over 60 mph. I was hoping I could drive faster than my G2 Prius and not take a big hit, but alas this is not the case. I presume the reason is related to aero drag.

    Easy city driving (many lights timed, no driver rage around me) is 50 - 60 mpg, compared to 65 - 75 mpg in the G2 when warm. One nice aspect of the CT (which I presume the G3 also has) is quick warm-up to EV gliding and ICE shut-off at stop.

    All told, the CT has a narrow window where it comes close to a G2 Prius in fuel economy, but it has other common modes where a 15-20% drop in MPG is seen. I expect that in my own car, when I downsize the tyres to 205 mm width, inflate pressures, and grind off the new tyre stickiness, I'll end up 5-10% below my G2 Prius overall.
     
  2. F8L

    F8L Protecting Habitat & AG Lands

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    I noticed that mpg drops off significantly faster at higher speeds with my 17s than is the case with my 15s. We have the same 17" tires too. The tires may be a red herring but this is what I have observed.
     
  3. Tony420

    Tony420 New Member

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    From my performance days as a youth, larger wheels do decrease gas mileage in most cases.
     
  4. F8L

    F8L Protecting Habitat & AG Lands

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    That must have been the days before eCVT and HSD. :)

    Usually larger diameter wheels/tires would increase mpg due to reduced RPM at cruising speed unless the larger combo was significantly heavier or tread design changed drastically towards the performance side.

    With the eCVT RPM is unaffected by wheel/tire diameter but weight and weight increases further away from the bug can really decrease mpg in the Prius. :(
     
  5. SageBrush

    SageBrush Senior Member

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    I'll be the first to admit I do not understand the relative effect of wheel and tyre weight on fuel economy, but a tyre that has a larger width by x% and everything else equal, would have x% more contact area and therefore x% more rolling resistance, would it not ?
     
  6. F8L

    F8L Protecting Habitat & AG Lands

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    Correct. I wasn't even including the "wider tire variable". The larger diameter alone will reduce mpg because it requires more energy to rotate the wheel the further from the hub the weight moves. I.E. it takes less energy to spin a wheel whose main mass (tire) is 1.5' from the hub center vs. a wheel whose main weight (tire) is 2.5' from the hub center.

    More information on rotational inertia:

    The Science of Wheel Performance | HRE Performance Wheels

    Dumb videos but it really illustrates the point


     
  7. cyclopathic

    cyclopathic Senior Member

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    IMHO it is not the width of the tire what is the problem; and definitely not the weight. Bicyclists had been obsessed with reducing rotating mass, but school level physics show that rotating mass has x2 impact. So if you have 10lbs heavier tire, it would have an impact of 4 * 10 * 2 = 80lbs, not enough to explain MPG difference. Besides the outside diameter of 17" and 15" tire is the same.

    wider tire would have negative impact on MPG due to additional friction and tread deformation, but majority of losses come from sidewall flex. All efficient tires from different mfg (Michelin, Bridgestone, etc) utilize thinner sidewalls with special rubber compounds. And they are not lower profile tires either. Nokian states that per their tests majority of the losses are in sidewall.

    Lower profile tire = more sidewall flex = worse MPG.
     
  8. F8L

    F8L Protecting Habitat & AG Lands

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    This is a very confusing post. Are you stating that:


    • 80lbs of rotational unsprung mass is not enough to negatively affect mpg?
    • A wider tire does not increase friction?
    • Lower profile tires (less sidewall) flex more than a taller sidewall tire?
    • That the mass of the tire being pushed further away from the hub on a low profile tire has no effect on mpg?
    Remember that the larger wheel pushes weight further from the hub AND concentrates most of the tire weight further from the hub as well. This is especially important because larger tires are generally heavier so in the case of an OEM Integrity at 17lbs and a 215/45/17 MXM4 at 22lbs, you have effectively pushed at least 4lbs further from the hub center and this doesn't even account for the extra wheel weight. So while the overall diameter stays the same, more weight is pushed further from the hub.
     
  9. abcahc

    abcahc Junior Member

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    I have a 2012 CT200h and have noticed that above 60 mph the engine computer program for the CT200h is very different than the Prius program. Pulse and glide doesn't work with the CT200h the way it works with the Prius. When I glide, the engine remains on, and does not shut down (at or above 60 mph). Going up a hill, the CT200h engine goes up to 4,000 rpm to maintain a 65 mph speed. I use soft acceleration up a hill, and let off, and the engine goes back to a high rpm. The Lexus program is designed for responsive driving. I like the Prius programming better. This is one reason I believe the mileage rating is lower for the CT200h vs the Prius. Does anyone have any facts to back up my thoughts here? I can't find anything about the programming differences in the two cars that share the same engine and drive-train.
     
  10. El Dobro

    El Dobro A Member

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    That's in ECO mode?
     
  11. abcahc

    abcahc Junior Member

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    Yes, in all modes. The Lexus program is different than the Prius program. Lexus will not change the program, I just found out, today. My CT200h works just like the Prius at speeds below 45 mph. But higher speeds do not allow the engine to shut off. Plus as I go up hills, my rpm goes up to 4,000, with only a light acceleration on the pedal. The fuel consumption drops like a rock. It has been difficult for me to be happy with this operation. After two and a half weeks, I am ready to go back to the Prius.