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Thread: How to lower the revs at highway speed?

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    How to lower the revs at highway speed?

    Hi, I have a 2015 Micra with 185/60/15's on it and my problem is the extremely high revs (4000rpm to 4500rpm
    ) I have to pull on the freeway. I put on a 65 profile which helped but was wondering if I could fit a 70 profile? And is there any other way to remedy this problem?



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    Slow down. The maximum speed limit in BC is 120 km/hr. At 4500 rpm you are going around 160 km/hr (100mph). $368 and you lose your car for a week and that is at least another $210.

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    Quote Originally Posted by tomcj2 View Post
    Slow down. The maximum speed limit in BC is 120 km/hr. At 4500 rpm you are going around 160 km/hr (100mph). $368 and you lose your car for a week and that is at least another $210.
    Obviously you don't commute from Abby to Vancouver, If you do less than 120km/hr. they will push you into the ditch. It's more like 120 to 130 and I wanted constructive help not criticism...if you can't answer the questions just stay silent please. I raised my snowies from 60/15 to 65/15 (which helped a little) and wanted to know if anyone had tried 70's. I would like to keep the rpm's to between 2600 and 3100. Thank you for your concern but it's difficult to apply to hwy 1 at 5 in the morning.

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    Senior Member Azmodon's Avatar
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    constructively... swap your transmission? Math is math... in top gear with a stock transmission, it must rotate (and thus your engine must turn) a certain number of times per revolution of your tires.

    since in top gear with stock sized rims, 4500RPM = 150km/h
    185/60/15 = 23.74"D = 74.582" Circ.
    You want your RPM to drop from 4500 to 3100 max
    4500/3100 = 1.45
    1.45 * 74.582 = 108.26" Circ = 34.46"D

    so also constructively, swap your rim and tire set up, throw 26's on your micra and grab a set of 305/35/26. Tada, you have a 34.4" overall diameter, and your RPM's will be down to 3100 at 150km/h

        __________________________________________

        click to view fuel log View my fuel log 2015 Micra SV automatic: 7.1 L/100 km ... 39.9 mpg (Imp) ... 14.1 km/L ... 33.3 mpg (US) ...


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    Also, there are numerous online calculators for viewing the effects of different tire sizes vs. engine RPM.

    EG:

    https://www.reillymotorsports.com/sp...alculator.html

    The Micra's gear ratios can be found here:

    Thread: Gear ratios: 2015 Micra transmission, 5-speed & automatic (speed vs RPM)

        __________________________________________

        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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    Senior Member Micra In The Mountains's Avatar
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    About 110 to 115 km/hr and 3000 rpm (auto trans) for us around the main highways in AB (speed limit 110 km/hr), unless you're on the Coquihalla in BC...
    Nice!

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    5 spd standard

    The 5 spd is much lower geared compared to the auto I guess...speed tested it at less than 95 km to 3000 rpm. My question was going from 60/15 to 65/15's helped and 70/15's would surely help more but do they fit the wheel wells?

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    Senior Member Azmodon's Avatar
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    assuming you're just putting higher aspect ratio tires on the stock rims, every bit you go up in aspect ratio should reduce your rpm by a matching amount (equivalent to the increase in overall diameter of the tire), in your case, about 3%

    185/60/15 (185X0.6) = 111
    111X2 = 222 / 25.4 = 8.74 + 15 = 23.74" diameter

    185/65/15 = 120.25
    120.25x2 = 240.5/25.4 = 24.46" (3% wider = 3% reduction in RPM, and 3% error on speedometer)

    185/70/15 = 25.19" (3% wider than '65')

    185/75/15 (if that's even a size) = 25.92" (2.9% wider than '70')

    So to drop 300rpm from 3000, you'd need to go from whatever you're at, up 15% on the aspect ratio.

    As far as fitting inside the wheel arch, you should be fine, prior to chopping up my car I had 205/45/17's (24.26"D) on, and the only reason they rubbed a bit was because instead of being on a 5.5" rim (stock), they were on an 8.5" rim.


        __________________________________________

        click to view fuel log View my fuel log 2015 Micra SV automatic: 7.1 L/100 km ... 39.9 mpg (Imp) ... 14.1 km/L ... 33.3 mpg (US) ...


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