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Thread: Taller tires = slightly better fuel economy* ... 195/65 R15

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    Taller tires = slightly better fuel economy* ... 195/65 R15

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    So I did a quick 'n' dirty test last week to compare MPG at the stock tire size vs. 12% larger (circumference) tires at the same, steady speed.


    Long story short: I saw a 3.4% increase in MPG *at a steady cruising speed*.

    I will be the first to admit this testing wasn't comprehensive. (A better test would have had many more runs.) So take this all with a grain of salt, but here are the details and the numbers I saw.


    The details:

    Last month, I bought a used set of Bridgestone Ecopia EP422 Plus low rolling resistance tires, half worn out for $80. These were OEM tires off a Prius. They're as tall a tire as will fit the Mirage - there's very slight occasional rubbing on the front inner fender liner.

    They decrease engine RPM by 12% vs. the stock tire size on the Mirage.

    I attached 100% smooth wheel discs on each set of wheels to minimize aerodynamic influences from the different wheel styles.


    (Pic actually shows 185/65 R15's on the front wheels only... old tires tested last year.)


    Testing conditions & route:

    - 26 C / 79 F
    - negligible wind
    - dry, 2-lane road, 1.6 km = 1 mile test section which is straight as an arrow and almost dead nuts level
    - NO other traffic in my lane, and rarely any opposing traffic, so no chance of "drafting effect" or buffeting skewing the numbers


    Speed:


    - 70 km/h = 44 mph cruising speed (5th gear), monitored by GPS (because the car's speedometer slightly OVER-reports actual speed on the stock tire size and UNDER-reports it on the taller tires.


    Measurement:

    - ScanGauge II, reset at the beginning of each run.


    Results:


    - 61.1 MPG (US) ... average of 2 runs in each direction on the oversize tire
    - 59.2 MPG (US) ... average of 2 runs in each direction on the stock tire size


    * Caveat/Disclaimer/Discussion:


    Assuming the numbers are accurate (that's a big assumption), it doesn't mean I should expect ~3% better MPG overall with these taller tires.

    For one thing, the EP422 wheel/tire combo are MUCH heavier than the stock combo I used (Miata wheels & Bridgestone RE92). TEN POUNDS per wheel heavier! That's a lot!

    I would expect city MPG could even suffer from the extra power needed to accelerate that much more rotating mass. On the other hand, I definitely notice the car coasts MUCH farther than it used to. So if you can make use of that increased coasting distance (eg. approaching turns/stops) it might even out in the end.

    But if you're burning up that rotational kinetic energy through harder braking, fuel economy is going to go down.

    But since I do ~90% highway driving, I'm not too concerned about the potential city MPG hit.

    Aero penalty of a taller tire


    The other factor to consider is the oversize tires raise the ride height of the car by 33 mm or just over 1 1/4 inches. That's enough to have a negative aerodynamic effect (generally a higher ride height = increased drag). So if I lower the car back to the stock height, the percent gain should go up even more.

    Also, a 30 mm wider tire presents a larger frontal area to the wind, increasing drag vs. the stock 165 tire.


    LRR vs. non-LRR

    Another point to consider: you probably won't see MPG gains by slapping any old "taller" tire on your car to replace your factory LRR tires.

    Rolling resistance is a critical part of efficiency, which is why I specifically sought out a tire known to have good LRR properties.


        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


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  3. #2
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    Are you not concerned at all about hitting a bump unevenly and bottoming out on one side with the wheel rubbing against the wheel well?
    please consider checking out my Mirage related youtube channel: https://www.youtube.com/channel/UC6c...IEViRFw/videos

  4. #3
    Senior Member Top_Fuel's Avatar
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    I'm only running 185-60-15s on my car, but definitely like the slightly taller tires. It's funny the number of times I have caught myself cruising around 40 mph in 4th gear. Around that speed it's easy to forget you've got another gear. Maybe that's what it's like with a Euro-spec 5-speed with the different gearing.

        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage ES 1.2 manual: 52.2 mpg (US) ... 22.2 km/L ... 4.5 L/100 km ... 62.6 mpg (Imp)


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  6. #4
    Quote Originally Posted by mohammad View Post
    Are you not concerned at all about hitting a bump unevenly and bottoming out on one side with the wheel rubbing against the wheel well?
    The minor occasional rubbing I've heard only happens on the sides of the wheel arch liner - near the 3 & 9 o'clock positions when the front wheels are turned.

    I jacked the opposite corner of the car as high as possible to check for clearance with the suspension compressed, and it wasn't close. I don't think I was able to fully compress it, so will report back if I experience anything.

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


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  8. #5
    Quote Originally Posted by Top_Fuel View Post
    It's funny the number of times I have caught myself cruising around 40 mph in 4th gear. Around that speed it's easy to forget you've got another gear.
    Ha. Should have included that in the disclaimers about another possible MPG downside.

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


  9. #6
    Quote Originally Posted by Top_Fuel View Post
    Maybe that's what it's like with a Euro-spec 5-speed with the different gearing.
    Basically, yeah! If you put the Euro 3.550 final drive in place of the everywhere else 4.055 f.d. and stuck with our top gearset it's about a 12.5% gearing change. (wait a minute... does a 12% difference in the f.d. ratio equate directly to a 12% difference in engine RPM?)

    Thread: Gear ratios: 2014 Mirage transmissions, 5-spd manual F5MDB & CVT (speed vs RPM chart)




    THEN do the 195/65/15 tire change on top of that.... and I'm sure we'd be looking at a 5 or 6% MPG improvement.

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


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  11. #7
    Think bigger Attachment 23353
    Mirage videos:

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)


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    Senior Member
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    may as well drop in a 1.0 engine and add in the cvt electric fluid pump to enable auto stop and go and enable regen braking
    please consider checking out my Mirage related youtube channel: https://www.youtube.com/channel/UC6c...IEViRFw/videos

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    I HATED the airplane tire Bigfoot soooo much as a kid...
    Resident Tire Engineer

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 44.4 mpg (US) ... 18.9 km/L ... 5.3 L/100 km ... 53.3 mpg (Imp)


  16. #10
    Are those airplane tires??


        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


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