will 20" cx9 with 245/50/20 (stock) fit?
- Thread starter Dragone
- Start date
The extra 1" in diameter will effectively gear the transmission taller. I would not do that since the CX-5 is already geared quite tall. That, coupled with the additional rotational inertia and tire width will make the car feel too sluggish. In stock form the CX-5 with 2.0L is on the lower end of power by modern standards but it still works well (assuming you are not too timid to put your foot into it or manually shift it) but this kind of modification takes the performance in the wrong direction.
it will work, it will be off by 4%, since OD is bigger., that is one heavy wheel, you will feel slower off the line but it should look pretty good.
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
225/65-17 5.8in 14.3in 28.5in 89.6in 707 0.0%
245/50-20 4.8in 14.8in 29.6in 93.1in 680 4.0%
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
225/65-17 5.8in 14.3in 28.5in 89.6in 707 0.0%
245/50-20 4.8in 14.8in 29.6in 93.1in 680 4.0%
Really not recommended. 4% is huge. You want to keep the difference under 2%, not to mention the weight. I have a car where I lost 92 lbs of unsprung weight by changing out the wheels and tires and that was a huge and noticeable difference in handling, acceleration, braking, although now, it feels normal. That situation was unusual, but highly illustrative to me of the weight differences wheels and tires can make!
I wouldn't do it, especially on the 2.0l
I can tell the difference just in going from stock 17s to my 255/18s. (which are heavier and larger than my 17s, but are pretty close to the stock 19s on both counts) A lot of the difference I'm seeing may be more in the tires... stickier, fatter tread = rolling resistance.
I think the 245/50/20s would have a big (negative) impact on MPG and performance on the highway.
I can tell the difference just in going from stock 17s to my 255/18s. (which are heavier and larger than my 17s, but are pretty close to the stock 19s on both counts) A lot of the difference I'm seeing may be more in the tires... stickier, fatter tread = rolling resistance.
I think the 245/50/20s would have a big (negative) impact on MPG and performance on the highway.
Are you seeing less MPG using the 18" wheels, having moved from the 17"?
If so I find it strange that Mazda would fit the 19" wheels, on a car sold in the UK as a green car, with low emissions,and high MPG.
If so I find it strange that Mazda would fit the 19" wheels, on a car sold in the UK as a green car, with low emissions,and high MPG.
I wouldn't either, having done that in the 70's.
The high gearing destroyed the performance, back then with a four speed box the revs had to be taken very high before changing gear, I very quickly took them off.
Going smaller would help acceleration, but unless you can recalibrate the speedo I wouldn't bother.
Yes, but the rim size itself isn't really the issue, except that larger rims are heavier than the same rim of a smaller size. So it does matter, a little. But the real issues are the overall diameter, weight, and width of the rim/tire. And maybe the tread pattern/rolling resistance of the specific tire. Like I mentioned, I think much of the difference I see is from the much wider tires. If it's actually the change in weight and size, then I would see a similar difference in swapping to the factory 19s, because the specs are very similar, expect for the 225 vs 255 tire. My 18s are nearly 10# heavier than the 17s, but again so are the factory 19s. Diameter is .4" taller (1.0% increase) than the 17s, but only .1" taller than the 19s, (0.3% increase) based on tire mfg info I can find. It's almost within the margin for error.... not every tire of a given size specs out exactly the same, going off mfg #s.
Again, the measuring tape and scale say there isn't much difference... that's why I think the smoking gun is the tire width, and perhaps a softer and more aggressive tread pattern. Skinny, smooth tires are more efficient, pretty simple. You don't see mountain bikes running the Tour de France
The impacts are any changes (between old/new wheels/tires) with regard to potential changes in four areas:
A) aerodynamic drag
B) rolling resistance.
C) rotational inertia + mass (not simply mass)
D) effective changes in gearing
I agree. The additional width affects the first three factors (above) quite strongly. But don't underestimate the effects of gearing on acceleration and efficiency (taller is not necessarily better).
Not really.
The diameter difference between tires is not proportional to the effective gearing change. It is necessary to multiply the difference by pi (3.14) to calculate the gearing change. And small changes in gearing can have a disproportionately large effect on performance and economy.
Also, you focus on discounting any weight differences but what matters more is rotational inertia because this factor is in addition to the effects of whatever the static weight difference is. It is nearly impossible to calculate rotational inertia of a wheel/tire assembly (and manufacturers very rarely publish this information) but the relative rotational inertia of two different setups can be roughly compared if you understand rotational inertia and the very negative effect it can have on performance (primarily acceleration and city MPG but also braking and handling).
The static weight difference between your wider 18" wheel/tire and the factory 19" setup may not be huge but the difference in rotational inertial could very well be huge because the tread of a tire is actually quite heavy (they don't call them "steel belted" for nothing ;-) and more width + more diameter could be devastating to keeping a low rotational inertia. I believe the effect of a gram on rotational inertia increases exponentially (not linearly) as that gram moves outward from the axle. This is why race cars rarely run oversize tires (generally they are as small as they can be and still dissipate the required heat) and why monster trucks need 800+ horsepower and still can't beat a stock go cart with 10 hp.
So, yes, the width is impacting your performance and MPG in a very significant way but it is all the factors coming together and an increase in rotational inertia and the higher effective gearing are very likely significant contributing factors, particularly in acceleration and mpg wherever speeding up and slowing down is involved. I'm not sure what inputs the CX-5 considers in it's shifting algorithms with the auto transmission but I imagine it includes vehicle speed sensors (which are impacted by changes in tire diameter) so it could very well be shifting at less than optimum points.
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OMG, you must be joking?!!!!
It's no surprise that you chime in to sound like a genius, to teach us all something. But wow, you underestimate the intelligence of others. I don't have a degree in any of this, but like so many others, I actually do know plenty about the basic physics involved. Even if a guy doesn't know the terms or formulas involved, he can still "get" how it works.
It's really damn simple, believe it or not. This is all basic fundamentals to anyone with much experience in some form of racing, for example. Or anyone who has mechanical inclination. Or played with tops as a kid. I think I used somewhere here is rollout.(if not, I was thinking it, which you already knew of course), which is a term in racing that refers to the effective total gear ratio that takes into account the tires also, measured in revolutions of the engine/drive train relative to the actual distance traveled. No pi calculations needed, professor... because that's already built in to the "rollout" figure. You do NOT have to be an engineer or have a degree to understand the basics involved here. Any "uneducated" motor head can figure gear ratios and rollout, and won't talk down to others as if it's beyond them. You tried so hard to sound like an expert this time, explaining really simple things as if it was complicated, it makes me think you actually don't get half what you are saying????
Seriously, why the need to step in and "educate" especially when it appears maybe you don't really understand it? BTW, tires of the same diameter and width, but different rim size (17 vs 19), will have a difference in rotational inertia... but main difference is in the rim, not the tire. The larger rim has more mass at the perimeter, so it's heavier and the center of mass is farther out. It's not the tire. It's pretty simple, if you take away the big words and arrogance.
But that 3.14 thing... yeah that's pretty advanced stuff there. Gee thanks, it's been a while since I was in 5th grade. Show of hands, how many here didn't know how to figure the circumference of a circle? anyone?
It's no surprise that you chime in to sound like a genius, to teach us all something. But wow, you underestimate the intelligence of others. I don't have a degree in any of this, but like so many others, I actually do know plenty about the basic physics involved. Even if a guy doesn't know the terms or formulas involved, he can still "get" how it works.
It's really damn simple, believe it or not. This is all basic fundamentals to anyone with much experience in some form of racing, for example. Or anyone who has mechanical inclination. Or played with tops as a kid. I think I used somewhere here is rollout.(if not, I was thinking it, which you already knew of course), which is a term in racing that refers to the effective total gear ratio that takes into account the tires also, measured in revolutions of the engine/drive train relative to the actual distance traveled. No pi calculations needed, professor... because that's already built in to the "rollout" figure. You do NOT have to be an engineer or have a degree to understand the basics involved here. Any "uneducated" motor head can figure gear ratios and rollout, and won't talk down to others as if it's beyond them. You tried so hard to sound like an expert this time, explaining really simple things as if it was complicated, it makes me think you actually don't get half what you are saying????
Seriously, why the need to step in and "educate" especially when it appears maybe you don't really understand it? BTW, tires of the same diameter and width, but different rim size (17 vs 19), will have a difference in rotational inertia... but main difference is in the rim, not the tire. The larger rim has more mass at the perimeter, so it's heavier and the center of mass is farther out. It's not the tire. It's pretty simple, if you take away the big words and arrogance.
But that 3.14 thing... yeah that's pretty advanced stuff there. Gee thanks, it's been a while since I was in 5th grade. Show of hands, how many here didn't know how to figure the circumference of a circle? anyone?
In the specific example given we were comparing a tire width of 225 to 255. Since a wider tread is a heavier tread and since the tread weight is the furthest point from the axle, the additional weight will have a disproportionately large effect on the rotational inertia. Not to mention the fact that the tire with the wider tread also has a larger diameter, compounding this effect.
No arrogance intended and it's unclear which "big words" you are referring to.
In any case, the additional rotational inertia of the wheel is ADDED to the additional rotational inertia from the tire tread which it appears you missed.
I was responding to the difference in diameter being referred to as a percent (1%). I was simply pointing out that, for gearing purposes, it would be more accurate to think of it as a 3-4% difference which is worthy of consideration. I'm sorry if I was covering stuff that was too rudimentary for your brilliance.
You seem to have a very large chip on your shoulder (to put it gently).
Mike, you are trying way too hard to find something to nit-pick. You look past the concept of the discussion, in order to jump at anything you see to criticize... even when you agree? I'm not like that, so I don't get it. After a while it does get irritating, yes. lol
It appears you missed that my main point was that the width of the tire WAS the issue. One of the comments you tried to pick apart was where I made a comparison between stock 17 and 19s. Both are 225 in THAT case. So why are you continuing to talk about heavier tread etc...? You can't mix two different things in your effort to find fault with me. Funny thing is that we agree... (which makes sense, because it's simple really) but you are so critical that you look at each word, or even between the lines in this case, to see where you can correct me. You quote parts of whatever I post, and stretch and twist it to somehow be wrong, when it's not. You are stuck on the 1%,.... it's a correct statement. If you want to extrapolate it further, go ahead... but don't bother to pick it apart and assume I didn't understand the basic math. That wasn't needed then, or now.
I tend to use terms like "probably" "I would guess" "likely" etc, because I view the forums as a casual conversation, not a technical institution class and I'm not here to scan what other say just for opportunity to argue with or correct others. Maybe try to be more friendly and less critical? Nobody likes to participate in a discussion when the nannies, critics, and trolls are lying in the weeds.
It appears you missed that my main point was that the width of the tire WAS the issue. One of the comments you tried to pick apart was where I made a comparison between stock 17 and 19s. Both are 225 in THAT case. So why are you continuing to talk about heavier tread etc...? You can't mix two different things in your effort to find fault with me. Funny thing is that we agree... (which makes sense, because it's simple really) but you are so critical that you look at each word, or even between the lines in this case, to see where you can correct me. You quote parts of whatever I post, and stretch and twist it to somehow be wrong, when it's not. You are stuck on the 1%,.... it's a correct statement. If you want to extrapolate it further, go ahead... but don't bother to pick it apart and assume I didn't understand the basic math. That wasn't needed then, or now.
I tend to use terms like "probably" "I would guess" "likely" etc, because I view the forums as a casual conversation, not a technical institution class and I'm not here to scan what other say just for opportunity to argue with or correct others. Maybe try to be more friendly and less critical? Nobody likes to participate in a discussion when the nannies, critics, and trolls are lying in the weeds.
You appear to forget that I'm simply trying to have a technical discussion about the effects that would be attributable to various factors while you continually try to turn it personal and attempt to make it sound like I'm wrong (without having identified a single thing that I've presented that is misleading or incorrect).
In my opinion your participation is just noise.
And that's all the time I want to spend addressing your rude blather. Sorry if you think I'm being too blunt but it's the truth.
The general condescending nature , IMO, of your posts is what brings a more irritated response on my part. Normally I ignore that stuff, but I chose to address what I view as BS. That's how I see it and if you think that's making it personal, then so be it. pot and kettle. I can be as civil as anyone, but I'm not spineless.
I did explain why I took issue with your comments, and why your criticism is indeed wrong. But you actually have to pause and read the post for what it is, instead of focusing on the next point to criticize. It's there if you actually want to see it. I don't do the multi-quote thing because I don't need to nit pick and nag on what's already been written. Feel free to PM me if you really honestly missed why I had a problem with some of your comments.
I did explain why I took issue with your comments, and why your criticism is indeed wrong. But you actually have to pause and read the post for what it is, instead of focusing on the next point to criticize. It's there if you actually want to see it. I don't do the multi-quote thing because I don't need to nit pick and nag on what's already been written. Feel free to PM me if you really honestly missed why I had a problem with some of your comments.
Back to the original topic. I posted in the thread to give some basic real world feedback... might be able to add a couple things to clarify. The tire combo in question is a bad idea, if mpg and highway manners are important. I'm basing that on what I've seen from my tire change, and guessing that the added size of the 245/50s will make matters noticeably worse. I may notice the difference much more than some because I run 150-200 miles of the same roads most days, so I have a pretty clear basis to compare.... noting exactly how it handles certain hills, etc. I don't want to overstate the difference... it's subtle. You may not notice it at all, outside of a familiar commute. The power and gearing of the 2.0l is pretty much at the limit already, so it can't handle taller+heavier tire without at least some impact. With 245/50/20, I think the difference would be a lot worse. My guess... expect more sluggish acceleration and expect to run in 5th gear instead of 6th, quite often... unless you only drive on the salt flats. I would expect the 2.5l to fare better though, both in performance/feel and mpg. I would like to see what my tires would be like with the 2.5l or better yet, the diesel.
I had given some very basic comparisons of how my tire/rim combo compares to stock, partly because of the similarity between mine and the factory 19s. Basic point was that there are more factors involved that just the diameter, most importantly the width... which does impact the weight and rolling resistance. I don't have specific data #s or formulas to reference, but lets just say there is increased "drag" with a wider tire for a number of reasons. It's not just weight and rotating force alone. Tires don't run down the road perfectly straight and true. The alignment angles create additional rolling resistance, and a wider tire has more contact... more grip. So a wide tire will have increased "drag". And a softer tire or tire with more "grip" will also have increased resistance... so not all tires are equal, even of the same exact size. (some have a rubber compound designed for less resistance and thus claim to offer better mpg) It all adds up. My point isn't to specify or argue about which factor accounts for exactly how much of the total... Hopefully that's clear, and of some help to the OP.
I had given some very basic comparisons of how my tire/rim combo compares to stock, partly because of the similarity between mine and the factory 19s. Basic point was that there are more factors involved that just the diameter, most importantly the width... which does impact the weight and rolling resistance. I don't have specific data #s or formulas to reference, but lets just say there is increased "drag" with a wider tire for a number of reasons. It's not just weight and rotating force alone. Tires don't run down the road perfectly straight and true. The alignment angles create additional rolling resistance, and a wider tire has more contact... more grip. So a wide tire will have increased "drag". And a softer tire or tire with more "grip" will also have increased resistance... so not all tires are equal, even of the same exact size. (some have a rubber compound designed for less resistance and thus claim to offer better mpg) It all adds up. My point isn't to specify or argue about which factor accounts for exactly how much of the total... Hopefully that's clear, and of some help to the OP.
Performance impact aside, I just picked up a set but have not put them on. That front strut clearance is going to be tight. Is it documented which rims/tires have proven to rub? Lets assume everyones 245/50/20 are the same size, which I know is not true.
The 2012 SEMA cx5's had this same rim, but as far as I can tell they put 255/45/20's on them. No way of knowing if they needed spacers or not.
The 2012 SEMA cx5's had this same rim, but as far as I can tell they put 255/45/20's on them. No way of knowing if they needed spacers or not.
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- Mazdaspeed6
Don't forget - it will effect fuel efficiency, braking as well, in addition to suspension dynamics, and wear.
Chances are, unless you auto cross or race your CX-5, you'll never notice a difference.
But, what am I talking about? Look at the car in my sig. - I have clearly chosen form over function. haha!
That said, somehow the smallish CX-5 pulls off 20's very nicely. I think between 18 and 20 inch, 19's are the preferable compromise, IMO.
Chances are, unless you auto cross or race your CX-5, you'll never notice a difference.
But, what am I talking about? Look at the car in my sig. - I have clearly chosen form over function. haha!
That said, somehow the smallish CX-5 pulls off 20's very nicely. I think between 18 and 20 inch, 19's are the preferable compromise, IMO.
