dang dude. crazy!

 

Lots of folks watching this thread. Please keep us updated on handling, fuel efficiency, braking, and rubbing issues. Thanks for thinking outside the box.

 

what's so special with the hydroedge that you are so unwillingly to look somewhere else for a more properly speced tires for the fit? sure you can live with effects but to the car's expense. I'm sure you already know what's been compromised...

 

Others here have tried to point out the size issue. OP wants to continue to justify his purchase (quite understandably) but has made some snide remarks towards people who are trying to be helpful. It makes me wonder why the thread even exists if he doesn't want to hear opinions.

I'm interested in the Hydroedge also but with 195/60R15. Those would fit my stock rims (Cdn LX) and match the original overall diameter.

 

I think he has looked elsewhere. But not everyone is working to the same specs. If you want a cheap tire that gives a rough ride, good gas mileage, and wears out quickly, then use the OEM Dunlops. Yes, 205/50/16 gives a match to OEM circumference and that may be necessary for "edge performance".

But the OP is specing his car for daily commuting. He wants a long lasting tire. The Hydroedge is the longest lasting Michelin. It's warranty is 90k miles. None of the 205/50/16 tires can meet that spec. The engine should run at lower RPM for the same road speed.... almost 4 % lower. And the tires should give a softer ride based on having .3 inch more thickness. The OP has already measured better gas mileage, although that has not been confirmed yet. All of these seem like good reasons.

 

it's not a question of performance... it's about going against the driving dynamics and design of the car but if it's all about getting more tire mmileage and life regardless to size and adverse effect... then there's no point in any disagreements and discussions.

 

Fuel increase so far above what I was seeing previously: 2.68%. Standard Deviation 1.75mpg. No other issues than what I have been seeing, took it on a road trip and though some hilly country last weekend. Acceleration for passing/hills wasn't any different as far as I could tell.

'Properly'.... according to you? They fit, and I haven't seen any extreme adverse affects as I've mentioned. That's proper fit in my book? I've already stated they probably cost some acceleration and handling near the limit, by the numbers, but seat of the pants (which is all I really care about in the performance of this car) hasn't bothered me any. As far as why I selected them, please read the thread above about reduced cruising RPM and reviews of Michelins.

No justification needed, thread is for others who may be considering these tires. If you're not interested in them, no need to read this thread. If you have some intelligent input, then please put some forth. If it's "zomg the tire is taller yous gonna dies!!!one!!!" please refrain.

 

'Driving dynamics'.... meaning handling? Acceleration? What exactly? How is the 205/55 size contrary to the entire design of the car? 'mmileage' is a huge element of what I'm seeking here. It's a commuter car. I've mentioned that dozens of times in this post. Long life is as well. It's not 'regardless to size and adverse effect', it's in balance with them. Each and every tire you can put on a car is a balance of plus and minuses. This one just borders to commuter side of the spectrum, while sacrificing my track prowess.

A 3.4% variation in OD is not in my book a large enough variation to warrant concern of overloading some element in this cars design. If you can actually put forth some sort of research or data that says otherwise, please do! It may help someone else who's considering this size make a more intelligent selection. But if you just 'feel' that its too large, please don't act like you're making some sort of informed opinion on the matter. Because at the end of the day, I've driven the car with the tires, and so far, I haven't seen any problems.

 

That's 2.68% more miles/gallon?

Did you do a comparison of cruising rpm before and after the tire change?

 

Yes, sorry, 2.68% increase in mpg. No... I didn't document actual cruising RPM before. There's always some variation between measured and calculated (calculated based off of the tire ratios) tire wall size but, assuming that the 205/55/16 and the 185/55/16 are both accurate it calculates out to a 3.59% decrease in RPM.

 

That's great. Have you placed any limits on your driving based on the increased angular momentum? For example, have you had the car up to 80 mph?

 

Yes I hit about 90 indicated (GPS'd 92,93 actual) and didn't die on last weekends road trip.

I've been pondering this at lunch, thinking about the actual effects. Here's what I've come up with to mathematically describe the situation:

There's some really great info here, half of which I had forgotten since school:
The Effects of Rotational Inertia on Automotive Acceleration

Using their calculator, a 205/55/16 tire has rotational inertia of .9464kg*m^2. A 185/55/16 tire, however, has equivalent rotational inertia of .693kg*m^2.

This is a pretty big difference, until you remember that a 205/55 has less angular velocity at a given vehicle speed because each rotation of the tire covers more ground.

To calculate the difference in energy at a given speed, I took the angular velocity in rads at 60mph, and squared it for both the 205 and the 185 tires. The math works out such that the 205/55 has about 3% more K.E. than the 185 does. So, what's this all mean?

Accelerating from 0-60mph in a car with a 205/55/16 tire is about equal to accelerating with that same car equipped with factory stock 185/55/16 and 18.9524lbs in the trunk.

That is, remove the spare from my trunk and I'll have less brake wear, more acceleration, and similar handling compared to a 185/55/16 shoed car.

 

Thanks for looking so closely at the issue. I do understand what you have done, although I wouldn't trust myself to do the calculations. Our Fit is my wife's commuter car. She is an engineer working for an automotive company and she will definitely appreciate what you are saying.

So one other point that might affect acceleration. Let's assume you have removed the 19lb weight from the car. Still, at a given cruising speed, you have 3.59% lower rpm and therefore lower power. That would mean a change in your ability to accelerate at a given speed. The same could be said about the same car with 185/55/16 tires traveling at a lower speed and therefore lower rpm. I guess there is no reason to think that any particular velocity would require a certain level of power, except maybe at the low end. So if you are driving at, say 35 mph, does the lower rpm rate compromise your ability to avoid a collision? How big is the power effect?

 

You are claiming a 2.68% increase in mpg with a standard deviation of 1.75 mpg? This is over the period of about 1-1/2 months... as compared to? The lifetime of the previous tires? the same time period just prior to swapping out the previous tires? to the same time period and time of year average for the previous tires?

I get about 30 mpg when I do deliveries. 2.68% is too small to be significant in that case. The fact that your deviation is much larger means it's considered "within margin of error." Now, for 1.75 mpg to be equal or less, you'd have to have over 64 mpg. Do you get 64 mpg?

This tire is also 5 lbs heavier than stock 185 tires. Now, I don't know about you... but I'm pretty sure that having "LRR" cannot possibly make up for the wider contact patch and increase in weight (which should also include the weight of the increased volume of compressed air).

That being said... the difference in mpg, would appear to me to be simply weather related at this point. That you would've gotten more MPG, if you had stuck to stock.

How much "more" ground does the 205 cover compared to 185? 50% more? 25%? 10%? no... 3.35%. You're talking about over 35% inertia increase... for less than 4% of travel increase.

It has been a while since doing this much math computation... and I'll be honest, I can't be bothered to look up why... but I'm pretty confident that at 5 lbs per tire for a total of 20lbs all-around increase over stock, you'll need to remove more than "18.9524lbs in the trunk" to be equivalent to the stock tire.

Making quick use of your link... you'd have to double that number. The 205/55@23lbs have an "em" of 19.914 kg per tire. The 185/55@18lbs have 15.616 kg per tire. The difference multiplied by 4 (and converted to lbs), is 37.8224 lbs.

Since your stated goal is to reduce RPM... does it? I'm going to be generous... is 5% reduction in RPM that noticeable? 5% lower of 4k is...? 3.8k. Is it audibly different?

I'd say... no.

edit: I'll say this... if you want to use that tire size, fine. It doesn't "bother" me in the slightest. That fact that you like that brand/model of tire and it doesn't come in is size closer to stock is plenty of reason to use it. But please, enough of any other "justification."

 

At your 30mpg a Fit driven 90k miles (the warranty on the Hydroedge) would use 3000 gallons of gas. At $4 per gallon that would cost $12000 in gas. 2.68% of that would be $322 dollars, enough to buy 3 replacement Hydroedge tires for the car.

 

Actually, by "significant," I meant more along the lines of the difference possibly not being due to the tires. Like I said earlier, it's too small compared to the deviation he listed to say that it actually made a difference. I mean, from tank to tank, you can easily measure a change of 1 or two mpg (3-7%)... 5mpg even. I can make very subtle changes to my driving during deliveries to make more than 3% mpg difference per tank.

If his deviation had been much smaller, say, 1/2 mpg or better still, 1/4 of mpg. Then 3% would be more meaningful.

 

Completely agree, which is why I included the standard deviation. We're only 5 tanks in, this isn't statistically relevant yet. The improvement may just be noise.

Overall mpg since buying the car has averaged to about 35.1. The standard dev will decrease as I drive the car with these tires more, and the data become more relevant.

LRR is one impact. Revolutions per mile are another. You've decreased the angular velocity of the wheel at a given vehicle speed. Which in turn, at a set gear ratio, you've reduced the engine rpm.

Read: I'm too lazy to actually do any work. Instead I'll just make invalid assumptions and tell you what I think.

Good job. Here's the problem, you obviously don't understand rotational inertia. That em assumes a constant angular velocity. As explained, over and over and over again, this tire rotates less than a smaller OD tire. Which means the wheel also rotates less.

Here's the equation for the kinetic energy (K.E.) of a rotational body, stolen directly from wikipedia:



T here is K.E., omega is angular velocity. Notice that funky little 2 next to omega? That's raised to the power of two, for every doubling of omega the K.E. increases by a factor of 4. This K.E. reduction applies to not only the energy required to accelerate the tires, but also the wheels. So although you've increased tire weight, you're reducing the angular velocity of both the tire and the wheel, which is why your math doesn't check out for em. But again, why bother to actually think about this, just continue to jump to conclusions.

All this is to say, rotational inertia only impacts acceleration. Starting, stopping, turning. Cruising is basically unaffected by the weight of your tires/wheels. So if you were stop/go driving a ton, as you apparently are, this may not be a good solution.

Is this a serious question? Yes. A larger OD tire reduces engine RPM at a given speed.

 

Wow, you attack me for making invalid assumptions... How about you plug in your numbers and give us you results then. Tell us how much energy is consumed making that one exact rotation of the smaller tire and then how much energy is consumed by the larger tire making only 0.95 of a rotation. Prove that the larger tire (and wheel) is using equal or less energy. Or, if you want to be more precise, 0.9665 of a rotation, since that's how much your larger diameter tire is rotating to match the linear velocity of the smaller tire.

You're the one that brought this up, so I leave the burden of proof to you.

Is it a serious question? Well, not the way you quoted it. Granted, I could've left that little bit out, but tell me this: did you, or did you not say that the stock tires are too noisy at 4k rpm @ 80 mph? The full question, that you happily ignored is, is it audibly different? I can barely notice a 200 rpm difference... So, while it's true it's "different," is it worthwhile? I compare the difference to my radio being at level 22 switching to level 21... It's still loud.

I use 200 rpm as a nice round number... where as, the difference in tire size is actually closer to 150 rpm at the same linear velocity.

 

I did do this calculation, and I did give you my results. What exactly would you like to see? You basically integrate the equation I gave you, with respect to distance from the axis (variable L, from the axis of rotation to the rim for the wheel, and then from the bead to the tread for the tire) and angular velocity (variable omega, looks like 'w', 0 to whatever 60mph is in rads/sec for the tire combo you're calculating). You do this for the wheel and both tires, but you change the limits of integration to reflect the different maximum angular velocities. A ti89 can do this in about 30 seconds.

Ok. Basic Physics 101. No energy is 'consumed' while making 'exactly one rotation'. Neglecting friction no energy is consumed in a wheel's rotation. AS I'VE ALREADY STATED energy is either added or removed from the system during acceleration. This means, you only see the affects of a larger tire when you're accelerating the wheel. This happens when you speed up, slow down, or turn. When you speed up you need to add energy to the system, when you slow, remove it, when you turn, accelerate the system into a new direction.

A larger tire (... I really have no idea why you think my wheel size is changed, my wheel size is 16"...) requires more energy to change its velocity than a smaller one. This means accelerating my wheels and tires will consume more energy, and therefore more gasoline. I've said that from the onset. My point in the last post is that it's simply not as much as you stated, when you plugged your numbers into the calculator provided in my other post. It's less than that. I think the em is ~19lbs worth overall. You argued with me, incorrectly, and I backed my position. I'm not trying to be a douche, my point is that you obviously don't understand basic physics... why are you trying to prove me wrong by means of a tool you don't grasp yourself? Just say you don't like tall tires, you don't like my setup because you just don't like it, and move on. The numbers you're using don't back your case, because you're using the theory wrong.

The question is, is whether the savings at cruising speed will offset the losses from accelerating a greater mass. I think they will, and they have so far. Additionally my car shows ~3.5% less mileage on it than it used to for any distance traveled. Which is a small bonus but not an unwelcomed one.

If you're a delivery driver, as you stated, and you drive a ton in town at low speeds in stop and go, this will probably be a lose/lose tire setup for you. But this my friend is why they make different tire sizes. To fill different needs. And fit different cars.

Gotcha. Maybe I didn't explain my point well, the noise and the rpm are two separate issues. My stock tires were LOUD. They were loud at 20mph. They were loud at 80mph. They were tolerable at 0mph (). They were loud from the factory. I also always thought the ride was rough. After putting these on I have less road noise and less harshness. This has nothing to do with angular velocity of the tire and everything to do with the different compounds and thicker sidewall.

I still don't like 4krpm at 80mph. I don't like 3.8krpm at 80mph... but I dislike it slightly less than 4krpm. Ideally I'd like a 6th gear, but a custom transmission is much more expensive than a new set of tires. Since I was buying a set anyways, I tried to address the problem. My Fit has yet to blow up because of it.

 

I was wanting to order 205/55 but was concerned about clearance issues. Probably should have ordered the Michelin Hydroedge, but they were more money than I wanted to spend, plus my wife seemed not too please when I mentioned the 4% spedo error that would result. I ended up ordering Yokohama Avid EnVigor tires in the 195/55-16 size, which are about 2% larger (taller) than stock. No way was I going with the stock 185/55 size. I'll post some pics when mounted. Just placing the unmounted new tire next to the existing stock tires I can see a significant size improvement. My goal is to have more tire between the road and the car, so that the ride is improved.