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Thread: 165/70 R14 VS 175/70 R14 (for gearing/fuel ecomony)

  1. #1
    Senior Member Ajijic's Avatar
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    165/70 R14 VS 175/70 R14 (for gearing/fuel ecomony)

    I'm thinking of changing the stock tires (165/70 R14) to 175/70 R14. This would result in a larger diameter and should reduce the rpm at highway speed (better fuel economy), however the wider tire will create more air and rolling resistance and should reduce fuel economy.

    What will the final result regarding fuel economy be?

    TIA


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        click to view fuel log View my fuel log 2016 Micra Active manual: 6.0 L/100 km ... 46.9 mpg (Imp) ... 16.6 km/L ... 39.0 mpg (US) ...


  2. #2
    Quote Originally Posted by Ajijic View Post
    I'm thinking of changing the stock tires (165/70 R14) to 175/70 R14. This would result in a larger diameter and should reduce the rpm at highway speed (better fuel economy), however the wider tire will create more air and rolling resistance and should reduce fuel economy.

    What will the final result regarding fuel economy be?

    TIA
    It's funny I have these 14" Honda rims that I was think of using 165/65R14 with.
    Our stock is tires are 185/55R16 & 185/60R15
    Final drive is 4.055

    https://tiresize.com/gear-ratio-calculator/

    Tire vs final drive

    http://www.willtheyfit.com/index.php...5-5&offset2=50

    Tire difference

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    Administrator MetroMPG's Avatar
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    Quote Originally Posted by Ajijic View Post
    What will the final result regarding fuel economy be?
    IMPUL has the right idea: have you calculated the change to engine RPM?

    FYI, speaking of changing gearing, I changed the final drive in my Firefly's transmission, which reduced my cruising RPM by 25%. That's a massive difference -- dramatically more than you'll see from the change you're considering. My cruising fuel economy improved on average by only about 5% (measured over a range of speeds). See: http://www.metrompg.com/posts/tranny-swap.htm

    But changing the tire makes it a bit more complicated. Going with a wider tire doesn't necessarily mean you're going to increase rolling resistance (though you're right - it will increase aerodynamic resistance a bit).

    You're also slightly raising the car's ride height, which is also usually bad for aerodynamics (though the change will be very small).

    Given all those variables, it's hard to say what will happen. But my guess is it will be hard to see any significant change in fuel consumption over and above the normal variation you see from fill-up to fill-up.

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    Senior Member Ajijic's Avatar
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    Quote Originally Posted by MetroMPG View Post
    IMPUL has the right idea: have you calculated the change to engine RPM?

    Given all those variables, it's hard to say what will happen. But my guess is it will be hard to see any significant change in fuel consumption over and above the normal variation you see from fill-up to fill-up.
    The problem is once I've commit to a size I'm stuck till I need new tires! All the info I have managed to get so far seems to favour a narrow tire for fuel economy...

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        click to view fuel log View my fuel log 2016 Micra Active manual: 6.0 L/100 km ... 46.9 mpg (Imp) ... 16.6 km/L ... 39.0 mpg (US) ...


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    Administrator MetroMPG's Avatar
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    Then again, how much of your driving is at high speeds where aero drag dominates, vs. lower speed where rolling resistance is more important?

    FYI, I'm not trying to discourage you. I recently installed narrower LRR tires on the front (non-driven) wheels on my Miata to improve efficiency. And I put larger diameter wheels/tires on the back (but same width as stock) to change the gearing, like you're considering. The size change lowered my RPM by 7.5%. Using my Firefly gearing change as an example, the gearing difference is maybe worth 1.25% better fuel economy in top gear.

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        click to view fuel log View my fuel log 2015 Micra S manual: 5.0 L/100 km ... 56.5 mpg (Imp) ... 20.0 km/L ... 47.0 mpg (US) ...


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    14" (fourteen inch) diameter alloy rims JUST clear the brake calipers, steel rims may give more clearance, so be careful !!

    Honda rims use an "acorn" nut not a 60 degree taper nut, so theoretically if they are true honda rims, you cant use stock lugnuts.

    (i own/ed a civic and a fit also, so unfortunately i know)
    2015 Micra S (manual)
    no A/C /no power options
    Feb 2015 build

  7. #7
    Quote Originally Posted by MrMicra2015 View Post
    14" (fourteen inch) diameter alloy rims JUST clear the brake calipers, steel rims may give more clearance, so be careful !!

    Honda rims use an "acorn" nut not a 60 degree taper nut, so theoretically if they are true honda rims, you cant use stock lugnuts.

    (i own/ed a civic and a fit also, so unfortunately i know)
    They are made by enkei and it seems they have a concave lug seat so acorn vs slant 60*
    12" inside spacing it won't clear my BBK but oem brakes should work.

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    Administrator MetroMPG's Avatar
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    A non-scientific anecdote to add to the discussion:

    I put 15 inch Civic rims on the back of my Miata to reduce RPM. New tire size is 185/65R15, the same width but larger diameter than the factory 185/60R14 tires.

    RPM difference @ 80 km/h:

    • OEM: 2587 RPM
    • NEW: 2404 RPM
    • 7.1% reduction in RPM


    Difference in rolling resistance? Improved (I did a comparison test to the previous tires). Difference in aero drag? Unknown! (It raised the rear of the car by about 3/4 inch). Difference in rotational weight? Unknown!

    So this is in NO WAY scientific, but the first full tank I had the taller tires on the car with similar driving patterns & speeds saw a ~10% improvement in fuel economy. BUT the tire change was done in addition to a number of other small efficiency modifications that were made at the same time.

    I'm just guesstimating, but I think it's reasonable to say the tire diameter change on its own was good for a difference of a few percent.

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        click to view fuel log View my fuel log 2015 Micra S manual: 5.0 L/100 km ... 56.5 mpg (Imp) ... 20.0 km/L ... 47.0 mpg (US) ...


  9. #9
    Member Azmodon's Avatar
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    The difference of 10mm thickness will lower your rpm (assuming you have the same gearing we do) by 23 per 1000 -> if 3000RPM is 120 km/h, it would drop to 2'931RPM - this is a 2.3% reduction in rpm. Lets look at the other variables:

    Assuming the tires are identical in formula (type of rubber, stiffness, speed rating etc.), then there will be a 5.8% increase in rolling resistance -> aerodynamic drag is moot given how poorly the cars are designed around the wheels.
    Assuming the tires are filled to the same PSI.. just in general if everything between the tires is literally 100% the same, then...

    Contact patch (at 100km/h) changes from 159 square inches to 172.3 (7.7% more), you'll also have 1mm more tire deflection (17.1 vs 16.1) - which is where the 5.8% added rolling resistance comes from.

    A higher contact patch and a higher deflection will very likely overwhelm the rpm reduction -> but...

    If your stock tire is set for 36psi... and your new tire was set to 40psi... then the contact patch becomes 159 for them both, and tire deflection becomes -0.8mm ... meaning you'd have lower rolling resistance, lower rpm, and negligible aero impact.

    Given that pressure impact tires so greatly - your better bet would be to simply increase the pressure in your current tires by 10%
    The contact patch would decrease to 146.8 (-12.2, 8.3% less) and deflection would become 14.4 (-1.7mm, 10.5% less).

    That said: 'A narrow tire for fuel economy' - it's completely dependent on the tire, what you have, and what you're going to. The formula of the rubber is huge, the tire pressure, also huge. My tires are 205/40R17 on 8.5" wide rims, because of their profile they run at 55psi -> which means that if made of the same rubber as used above, their contact patch is 152.3 (-4.2%), and their deflection is only 7.1mm (-56%). Which is why even with giant go-kart tires I'm getting "the same" fuel economy as our stock 185/60's.

    Edit: to clarify ^^ for a given pressure, a narrow tire will deflect more than a wide tire, both at rest and while rolling. This is because, like buoyancy, the volume of displaced air must be equivalent to support the same weight. The more the tire has to deflect, the more energy is wasted in to the rubber deforming around the road.

    Contact patch changes between tires of different widths are not linear - if tire A's patch is 1 unit long and 1 unit wide, and Tire B is 2 units wide, the patch will not be 0.5 units long, it'll be slightly more than that. More patch = more friction, though new tire compounds utilizing silica have found a way around that - silane silica rubber can be harder and more wear resistant (which also improves fuel economy just like a higher PSI would) but while rolling, the high frequency vibrations induced by the road surface allow for rapid deformation generating traction with no increase in rolling resistance.

    In other words, chemical wizards and making dino juice last longer by sprinkling sand on your tires... if you're considering new tires anyway, go high tech?

    For the daring, if you want to plug in more numbers and theory craft stuff :P give the calculator a try
    (Fill in the pink and orange boxes, then calculate)

    http://bndtechsource.ucoz.com/index/...alculator/0-20
    Last edited by Azmodon; 10-26-2017 at 02:58 AM.

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  11. #10
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    I just put on some snow tires for my Micra. My speedometer seemed to be bang on compared to a few gps speedometers that I've used, but now I've put 185 65 r15 tires and now when my speedometer reads 100, I'm actually doing 102. Slight difference, but it's something.
    Also, the ride quality is way better and the road noise is a lot lower.



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