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235 vs. 225

This is a discussion on 235 vs. 225 within the CLA Wheels and Tires forums, part of the Mercedes-Benz CLA Forum category; 2015 CLA 250. AMG 18 spoke wheels, black. Matches the black paint and black interior, had to go with black tinted windows too, considering the ...

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Thread: 235 vs. 225

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    Newbie amccue1@yahoo.com's Avatar
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    235 vs. 225

    2015 CLA 250. AMG 18 spoke wheels, black. Matches the black paint and black interior, had to go with black tinted windows too, considering the sun/heat in Central California.
    The Bay Area roads are in poor shape - the worst I've seen since the 1970's. My tire and wheel shop say two of my wheels are slightly bent and I remember what might have caused it - road construction on 580 in Livermore, CA, I dumped the run flats a few weeks after I bought the car new. I've been running Toyo Proxes4 225/40R/18, I've run two sets and I'm going for set number 3 this weekend. In an attempt at a taller sidewall we tried the 45 but the tire was too tall in the front and rubbed. I'm considering jumping up the width to 235. The tire is slightly taller and wider and looks like it might work. Curious if any of you have jumped to a wider tire and if the taller stance worked for you?

    (tried searching the posts here first but did not find an answer).

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    jmc
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    Wait ... 225/45 on stock rims “rubs” ?!?!
    Unless the suspension geometry has been changed theres a lot of room ... where was the rub?

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    Newbie amccue1@yahoo.com's Avatar
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    They said the tire rubbed on top when the suspension was compressed and came (very) close to the wheel-well when turned. I just installed the new (size) tires this morning: 235/40/18 and they do not rub and also fill out the wheel-well better. The tires look more comfortable on the wheels too. Very happy with the look but have not tried out the performance yet.

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    jmc
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    wow ... the stock 225/40 is about 90mm tire height ... then 235/40 is 94 ... and 225/45 is 101
    the 225/40 on 18x7.5-et52 and 235/40 on 18x8-48et are stock wheel/tire specs so those work
    granted the 225/45 is 11mm taller ... though still surprised about the rub as the suspension cave seems pretty roomy?

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    Newbie Shartek's Avatar
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    The difference between the first two options is really insignificant.

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    jmc
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    235 vs. 225

    Well actually for looks maybe not much difference but for function quite a bit
    I have been evaluating both 225/40 and 235/40 so 4 obvious things show up

    The 235 section is >10mm wider in physical space as in my case both are XL rating so the air scrub cross section is much larger ~8% per tire ... so despite the tire spoilers the air resistance is higher
    The 235 ride height is definitely taller at least +4mm
    The increased ride height changes the center of gravity and roll center so handling will be different
    The lower air mass volume is now much larger as well ~1044.973L vs ~1012.571L or 3.2% since the front spoiler has lifted 4mm so handling will be different at high speeds per aerodynamics

    So rolling resistance will go up despite tire compound formulations and tread:groove ratios and probably the larger parasitic air drag from the larger section will add to that to lower mpg, so more power is used to overcome parasitics instead of propulsion
    The total wheel diameter is at least +8mm so the angular momentum of the wheel will be different which usually means more power used just to turn the wheel though with lighter tires this can be mitigated somewhat
    I suppose for grandma grocery runs the handling physics and aerodynamics is not a big deal though for high speed cornering that few % could mean making the curve or hitting the guard rail

    Of course all this gibberish can be dealt with - at least to first order - with much lighter rims (say -6lbs each), slightly wider rim (say 8” from 7.5”) to match the wider tire section to maintain the sidewall performance design envelope so less squirm, and lowering springs to drop the ride height (just a tad to still have reasonable ground clearance to tackle roads and driveways) so as to match or improve on the original handling mechanics

    Good grief!
    Last edited by jmc; 04-09-2019 at 10:41 AM.

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