Pardon my ignorance if this has been discussed before but I have somewhat of a question regarding wheels, weight and it's effects.

A flywheel stores inertial energy as it spins, while it does take longer to spin up the heavier it is, it does help the engine keep momentum during speed transitions. Things I've noticed in past experiences, a 350z with a aluminum 14lb flywheel revs and accelerates faster then a stock 35lb flywheel model. But I did notice its harder to keep a steady speed on the aluminum flywheel model then the stock one and is harder to launch smoothly. As i understand this, this should apply to wheels as well I would think.

Well I've read in numerous sites regarding wheel weights and a couple things I've noticed is people complained their car got better mpg with the factory heavier wheel vs the lighter weight versions. A few things to consider is aerodynamics of the wheel and rolling resistance of the tire wasn't really mentioned so it's not entirely scientific. What they did note it got better city mpg but highway mpg suffered.

What I do understand since I'm a avid cyclist is lighter weight wheels do spin up faster, my carbon fiber wheelset do allow me to sprint faster however it does easily get effected by small hills where my heavier shimano rs20 alloys don't lose as much momentum as the carbon ones.

What I'd like to know is, a factory fit 16" sport wheel with tire weighs 37.4 lbs (I weighed mine a few days ago). If I reuse factory tire and install say a 12lb wheel of same size and get the wheel weight down to 30lb would the fit suffer from the same type of consequences as the scenario above? I'm just wondering if it's worth the investment to anyone looking to better the overall performance of their fit or does it just hurt its economy. I do drive a lot of highway so I'm a bit hesitant of installing lightweight wheels in fear of hurting my already good near 40mpg I'm getting. Anyone out there have a GE fit with lightweight wheels notice any mpg losses? Sorry for the long rant.... It's something that's been boggling me for days lol.

 

If you get lighter wheels of the same size and use the same sort of tires, you should not see any reduction in highway milage (aerodynamic changes aside). I suspect, without proof, that most cases of reduced highway milage are because many people also get different/larger tires when they get different wheels, tires with greater rolling resistance...and, of course, traction.

Much of the energy stored in the heavier wheel (due to flywheel action) is simply wasted by braking. The only time it would be recovered is if you coast down to a lower speed without touching the brakes.

 

Posted this before and I will again, read through:

Car guys: Why is unsprung weight so bad? - AnandTech Forums

I agree that loss of mileage with lighter wheels is more likely due to the addition of wider and heavier tires with softer compounds and higher rolling resistance than to the wheels themselves.

 

yeah stay away from eagle Gt there freakn heavy lol i got those at the moment lol handle well but mpg took a biiiigggg shittt ill probly end up going with the dws 205/4516 next then my mpg should be bak

 

That thread mainly covers effects of acceleration and handling in which i understand the benefits but what i don't fully understand is why a lot of people claim to have lost highway mpg due to lighter wheels. My hypothesis was the "flywheel effect" buffering the inertial energy helping the car cope with driving surface changes like small hills, bumps, and the likes. Things that are light tend to slow down more easily from hills, bumps, changes in road surfaces, etc so that's why the car may consume more fuel trying to hold steady pace, its a tricky subject to touch.

 

I guess what i can do, (not scientific) but i can swap my factory tires to the Kosei K4R which do weigh considerably less and see if it helps or hurts my overall mpg on a tank of gas. Since it will be on factory Dunlop 7000 the rolling resistance and tire weight wont be of any factor during this test. My weekday driving routes are pretty much routine and the same throughout the week. What has been impacting my mpg was this heavy rain this week, i lost like 2.5mpg overall, i guess the deep water on the freeway was really eating up my gas trying to maintain speed.

 

Theorize no more.. behold:

Angular Momentum
[L=(mvr)]

How m is distributed along r is important as well, You'll also note that v and r have an inverse relationship because the laws of the universe are trying to conserve L. not accounting for losses

And yes, water on the road would make for increased drag, windage, etc.

 

This is a good read/discussion. I agree with DrewE and Wanderer. The reason why some get lower highway mpg is because of the weight and size of the tires. Those two things heavily affect the outcome; weight = unsprung/rotational mass, so the heavier the more energy required to move, and size = overall diameter and circumference, which throws off mpg readings/calculations. e.g. A smaller overall diameter/circumference will require more energy to maintain the same speed, but a bigger overall diameter/circumference suffers from increased rotational mass, so you're still using more energy to move it.

To put things into perspective, I sort of did an experiment a year ago when I had several sets of wheels/tires. I stuck with the same kind of tire (Kumho Ecsta 4x) and two sets of 16s and one set of 17s. I got the corresponding tire sizes to match the stock size's overall diameter and circumference, two in 205/50/16 @19lbs (0.2% bigger), and one in 205/45/17 @19lbs (1.6% bigger). The wheels were 16x7 14lbs, and 16x7 16lbs paired with 205/50/16, and 17x7 14.5lbs paired with 205/45/17.

Cut to the chase, I drove 20 hwy miles on each set at 70mph. I started with the heavier 16s first, and then the lighter 16s. With the lighter set the average mpg for that 20mi trip went up by 2 mpg, almost 3. The 17s was actually right about the same as the heavier 16s in terms of avg hwy mpg for the same trip, but I did not like the acceleration and responsiveness.

With the lighter 16s, the car felt smooth, and the motor had less of a load so it's more efficient. So after reading all this lol, the weight and size (diameter/circumference) are a factor. I have 205/50/16 tires @18lbs each paired with 11lbs wheels now and I've gotten even better overall mpg. In fact, you can roll 15s with light weight tire sizes close to OEM and get better mpg, but it won't be good in the handling department because of sidewall flex.

I can go on some more about counter arguments with things like gear ratios; did you know that auto Fits have taller gear ratios than MT?

 

Yeah, what DSM said while I was typing my post lol.

 

Ahhh DSM <3 back

 

Good to be back.. you took my star! >.< haha

 

Glad your back as well! .... What is that star/# thing any hoo?

 

Lighter wheels do not decrease MPG, wider contact patch and sportier tires do.

Lighter wheels make suspension work less to combat bumps and settle the car. In itself, I don't think this work will effect MPG much if at all.

Heavier wheels WILL help the car coast further if enough of the wheel weight is on the outside of the wheel, this is the flywheel effect you're thinking about. I don't think the gas you save during acceleration with lighter wheels will be canceled out from this effect.

I do say go ahead and swap the tires, i'm always game for an experiment!

 

It's a participation award of some sort lol

When I logged back in this morning, I was #1 in participation after my little vacation

 

Rewarding non participation now are we?

Still glad to see your comments back!

On topic,

My race wheel/tire setup are near OEM combined weights as opposed to my daily set which runs around 7.5 pounds lighter than OEM. I have been running the WIDE race rubber on the "heavy" wheels on the street for a few weeks, and yea, the acceleration and free way manners are not supremely different, HOWEVER, pushing the MUCH wider and sticker 225's combined with the rain has been making me see near V8 levels of poor MPG's

 

How's that combo been working out for you? Scary?

I haven't lived anywhere it's rained this much and this heavy in over 10 years. I like how there's random standing water on the freeways in the carpool lane sometimes.

 

Actually, I am using this as an opportunity and a learning experience

I took a recent driving course and at the start of the day it was dumping BUCKETS. So, I was able to push the Fit too it's limits on the "race rubber" in the wet. Let me tell you, it was and EXCELLENT learning experience.

My recent winter "slush" racing season was surprisingly not that wet, so I've been rolling these during the recent fricking monsoon season to get more "wet" experience. I was VERY nervous at first, but the tires let me know exactly whats going on and when I'm pushing past the grip limit... it is HILARIOUS though seeing that my avg MPG is in the low to mid 20's though

 

This thread has gotten interesting replies, i thank you guys for the informative replies. Im going to install my new wheels and let you know on the outcome.

 

Rolling resistance and air pressure would have to be included in the keep all things the same. When I first drove Fit I thought it was a jackrabbit, quick and needed a light touch on the gas, after reading the good posts on this forum learned about drive by wire and resistance of pedal. I added the spring and raised the seat track and it is a different animal, hard to prove but consensus of group is verified.
RPM's and mph in 5th gear comparisons couldn't be made with stock setup, in my opinion most of what you trying to measure would have to be what you feel.