Changing wheels/tires any guess on the mpg gains?
 

I am currently running stock del Sol Si wheels with 195/60/14 tires.
Average weight of a 195/60/14 tire is 18.3lb (calculated from tire rack tire weights). The del Sol wheel weighs 15lb.

Total unsprung rotating weight per wheel is 33.3lb.

I am hoping to snag a set of Civic VX wheels with 175/70/13 tires.
Average weight of a 175/70/13 tire is 14.8lb and the VX wheel weighs in at 9.7lb.

Total unsprung rotating weight per wheel would be 24.5lb (reduction of 26.4% or 8.8lb per wheel, 35.2 total)

The vehicle velocity would also change by reducing tire circumference by about 2.5% (my speedometer is currently 2.1% too slow because my tires are larger than stock and the vx tires would make it 0.4% too fast)

The frontal area would be reduced because the car would sit 0.3 inches lower and each tire is 0.8 inches narrower.

seems pretty dramatic to me when you combine the reduced area, reduced resistance to rotation and reduced total mass. the only drawback is the smaller circumference which would make the engine run at higher rpms (81 more rpms to maintain 70mph).

any educated guesses as to the mpg gains to be found?

thanks,
-tony

Those tires will need more pressure than the existing tires. Also, don't forget that the odometer will suffer from the same change in accuracy that the speedometer does, so calculate that in to your FE calculations.

I am skeptical about the rotating weight issue. Based on the science that I know about it and the words of experts in other contexts, I believe that it's a myth.

Decreasing rotating weight won't help much, unless you are drag racing where increasing rotational inertia sucks energy away from increasing your forward speed.

However reciprocating mass (pistons, valves, etc.) do "suck" energy to move.

-BC

I was thinking about lightweight spring retainers... i wonder what the improvement might be? I know they wouldn't pay for themselves with improvement but that doesn't matter when you go all out like I have a habit of doing.

Going to a smaller wheel and tire is going to hurt you because of the difference it will make in gearing. The engine will be turning more RPM's to do the same amount of work.

Why? I am running my existing tires over the recommended pressure anyway. i plan to do the same with the new ones.

I mentioned in my post that the vehicles velocity would change. I am currently adding 2.1% to my mileage because the OE tire for my car would have been a 185/60/14. For the new tires I will be subtracting 0.4%

From an engineers perspective, in a vacuum, it would take no extra energy to maintain the velocity of a larger mass than a smaller one. But accelerating the mass is where energy can be saved. It would consume less energy to accelerate to speed from every stop.

So if you did an A-B-A with a scangauge and started the measurements when you were allready at cruising speed then you would see no improvement. BUT, if you consider the full mileage covered by a full tank (like I do) then you should see more of a difference. That is why racers use lighter wheels, because less energy goes to accelerating the rotating mass and more energy goes into accelerating the vehicle.

If less energy is needed to drive the same way that I allready do, then less throttle would be applied and less fuel burned.

That seems to contradict itself and confirmmy argument. If less energy is lost to rotational inertia then less energy is required to accelerate the vehicle (assuming you are not trying to accelerate harder like a drag racer would be). So for stop and go driving, a vehicle with less rotational mass would be more efficient right?

I contend that if every other parameter was maintained equal, and a vehicles rotating parts were replaced with lighter weight materials (from the crank pulley, through the drivetrain, all the way to the tires) that the vehicle would be more efficient.

I am assuming of course that techniques like P&G are not used because that in fact requires rotational inertia to extract energy back out of on the glide. My assumption is for the standard American driver or for people like myself who accelerate normally through the gears and use cruise control often.

And Ford Man, I mentioned in my orginal post that to maintain the same vehicle speed (69.03mph, my car would change from 3200rpm now to 3281rpm with smaller tires) but I don't think 81rpm will consume significantly more fuel. i am considering a transmission change in the future however that would bring the rpms below stock at cruising speed.

Sorry for the long post, and I am very open to criticism or correction because that seems to be the best way to learn.

-Tony

The new tires are smaller and rated for less load at a given inflation. While I'm not suggesting that you'll be overloading them, your overinflation will be less with smaller tires at the same pressure.

You make it up on the other end, unless your driving style is the polar opposite of basic efficient driving. Dead weight is ballast that stores kinetic energy reasonably well, and rotating weight is a flywheel, a concept that is often considered very effective (but not practical on a large scale). Basic efficient driving includes not accelerating all the way to your next stop, instead accelerating the minimum required amount and trying to roll through green lights and coast up to stops.

Drag racers use lighter wheels because they need to squeeze out every bit of acceleration possible and will continue accelerating until they stomp on the brakes/deploy parachutes. Circle/road racers use them because unsprung weight affects handling.

If it's going to make a measurable difference, that difference will be found in extreme stop-and-go traffic.

For that type of driving, it will have no measurable effect. The little bit of extra fuel would be smoothed out in the average.

I say go ahead, and we'll see what the long-term effects are...though there are too many variables to say for sure which are responsible for any measured effects. It's my prediction that there won't be much difference. There's one way to learn that's better than being corrected: Finding real world data.

I have found on my Hondas over the years that reducing wheel weight improves ride quality, nothing more.

However, switching to smaller wheels means switching to thinner tires, which does in fact improve mileage significantly. It's nearly impossible for me to predict how it would affect your fuel economy with so many real world variables to contend with, but I wouldn't expect more than a couple miles per gallon.

After having had four sets of tires of varying size on the CRX, I would say that any improvement would be so small as to be lost in the background noise. How long you idle at the Burger King drive-thru twice a week, or a small change in PSI would make as much of a change.

IMO.

We shall see
 

Well, the concensus on this forum seems to be that there will be negligible gains from the change.

But, I need tires anyway so I will be picking up and installing the VX wheels today after work. I start back to school next week (125 mile a day commute) so I will be updating my gaslog soon and often (fillup about once a week). I will post back here when I have results.

Although I believe you will see no impreovement from the weight of the wheels/tires, I do believe that diameter and width can have a (small) affect on MPG.

-Bob C.

I have tires that are .0635 percent larger than the car came with. With it being winter and all, along with more idling, there's no way to tell how much improvement there might be. Come summer I can only return to standard driving practices and compare last year to this year. It would still be difficult. You'd have to be a very consistant driver to get good numbers. That, or you could do the a-b-a thing if the old tires are still around.

Mathmatically, it has to be better. But how much may be difficult to tell.

As mentioned, this may help your ride comfort, but has very little bearing on mileage. I suppose those surprise stop-lights will penalize you a little less, but those should be fairly rare (if you are paying attention! :D )

Lowering ride height is not the same as reducing frontal area. Lowering the car will cause air passing under the car will be squeezed slightly more. This may increase or even decrease drag, but at 0.3 inches I'll bet the effect is marginal.

The narrower tires will probably reduce drag fractionally, but the other benefits seem pretty trivial, IMO. I have to agree with the mob, you probably won't see anything. (Or we could all be out to lunch. Please report what you find!)

Personally, I'm going to go the other way and install the tallest tires that will fit when my whale needs new rubber in a few months. Lowering the gearing is the main goal; the car has plenty of power and revs much more than necessary on the superslab. Stock is 195/60-15. 65 or even 70 (rare) profile tires will fit easily.

I don't think so. The amount of air going under the car is based on the aerodynamics of the front of the car. The space between the bottom lip and the ground is full of air that will go under -- this is what is reduced by lowering the car. Any air coming down off the front of the car will go under -- this is not reduced by lowering.

I agree, though; in this case we're talking about marginal amounts.

:thumbup:

You're talking about raising your gearing -- a low gear is 1st gear, a high gear is 5th. Gears that lower your RPM at a given speed are described as "high" or "tall".

Quick update of actual changes:

original wheels and tires weighed in at 32lb each and the VX's weighed 22lb each.

pictures here:
https://cheaptrixracing.com/delsol

FE update coming next week.

:confused: I agree with what you just said, and I thought I said much the same in the prior post. :)

Well, the collection area will lose 0.3 inches, as will the tunnel area between the ground and bottom of the car. Since the tunnel height is a smaller value, the 0.3" represents a greater % change. It seems to follow that the air will indeed be "squeezed" (Ok, not the best verbiage) or forced to travel slightly faster than before. OR, it might 'backup' somewhat, increasing lower leading edge pressure, possibly causing more overall drag. Who knows??

I suspect that undercar airflow is pretty dang turbulent, with chaotic pressure and velocity gradients. How all this will behave with a slight increase in velocity and a slight decrease in chamber height is anyone's guess.

www.google.com -> "Nitpick" "Flame Warrior" :D

This from the person who thinks pumping losses are highest when the throttle is closed.... (When there's nothing to pump!) :p

-Moo!

40 pounds is a pretty respectable dietary effort. I'm inclined to think this will have more effect than any other aspect of your mod.

That's not exactly what I think, but...why would there be nothing to pump?

The amount of air it's trying to pump is based on RPM; closing the throttle restricts the air, causing loss. Imagine trying to suck a thick milkshake (or frappe or whatever it's called where you live) through a jumbo straw and a coffee stirrer...which one will require you to use more energy to get an ounce into your mouth?

You have my 100% agreement there. ;)

if you drastically change the tire size, there will be a difference. I regeared my truck for 4.10s from 3.55s and my mileage went from 22 to 18. a very significant change. I could have done much the same by going from 29 inch tires to 25 or so inch tires.

that was actually suggested to me before changing out the gears. borrow some racing slicks to see if I like the feel of the gearing before changing the gears. it is much easier to change out the tires than change out the gears, especially if a friend would let you borrow them.

my take off was excellent and acceleration was much better but I was hitting 2400 rpms at 60 mph vs. 1900 rpms before the gear change.

I didn't do the tire trick because I knew I wanted more gear and didn't want to go to 4.56s because that was way too deep a gear for me on stock tires.

I know this is an extreme example but diameter will make some difference even if there is a small difference.

good point... I never thought about gearing change.

I wonder how many people actually do this. I assume the odometer will always be incorrect and it's just a matter of knowing the correcting conversion when it comes down to logging your miles... (and when you pass by a cop!:p )

it depends on where the sensor is as well. the gear change in my truck didn't change my mph at all because the speed sensor picks it up from the axle. since that is past the differential, it didn't care what gearing was in my rear end.

the tire size would have totally messed up my speedo. haven't done much with front wheel drive stuff so I am not sure how that works. I know some of the old cars had little plastic wheels that you could change out for gearing changes. I think that was still on a mustang though (still rear wheel drive)

you can buy programmers to fix that but they are expensive. I wouldn't have changed the gears if I would have had go that route.

my new tires are 0.4% different in circumference than the original equipment tires so i will not have my speedo recalibrated (.24mph error at 60mph). however when i report my mileage i will subtract 0.4% from the odometer reading before i divide by gallons. same as the way i have currently been adding 2.1% to my odometer for the larger than stock diameter tires i was running.

After putting on the new 13" tires, why not check how accurate the odometer reading is with a long highway drive, like 40 or 50 miles? You said you have a long commute to school.

It will help a lot
 

I have a drag race car and I can tell you we all use the smallest, lightest and narrowest tire we can get away with.
When I put the "Skinny's" up front the car was faster and even noticably easier to push around in the staging lanes. It car steered easier and did not need the power steering allowing me to save even more.
It is more than just saving the wieght it is reducing the rolling resistance by making it narrower and a taller tire helps to.
If it helps that much on a drag car I am sure a thinner, taller and lighter tire would help a lot in a car built for mileage.

first gaslog update
 

115.3 miles on 2.395 gallons. same pump at the same gas station on the same day. new personal best of 48.14mpg! more to come later this week.

I actually found the mileage loss from changing the original tires to Michelins to be dramatic. In the range of 6 MPG or close to 10%. This was on my VX when it had 37k original miles. That was the only thing I changed, and the difference was immediate and very obvious. Check my gas log and you can see where it happened.

therealtime, I have the left over original VX tires in my garage. The car is gone. It would be an interesting experiment to see how much difference they would make in your experiment.

regards
gary

Now that you bring it up i guess i should list the tires used. my 195/14s were Warrior fluent tires and the new 175/13s are general altimax rt's. I would be open to testing any other tires that someone gets to my house.

I know for a fact that oem tires that come on a car are made of special low rolling resistance compounds (only available to vehicle manufacturers) in order for the vehicle manufacturer to meet standards for fuel economy.

One of my professors used to work for michelin and he said that whenever a michelin employee would buy a new car they would immediately change to aftermarket tires (even if the same size and brand) for grip and performance boosts. This would be the opposite of what an ecomodder would try. You are suggesting reverting from higher rolling resistance tires back to oem lrr units.

I have another question
 

if changing wheels to lighter weight aluminum units is #10 on the list of gas saving tips here: https://www.gassavers.org/showthread.php?t=2584 and included on ecomodders list here: https://ecomodder.com/forum/fuel-econ...cations.php#17 how come everyone said that they expected little or no change? if this had indeed been busted wouldn't it be left off of the lists? no one has said anything about my first tank increase. if I did indeed go from 40 to 48mpg, then the wheels and tires should pay for themselves in less than 34,100 miles (probably the life of the tires). this is at $1.69/gal where I am saving around $.007/mile. If gas prices rise again then the time will be shorter (little over 14k miles if gas was $4.00/gal again).

Everyone didn't write those lists...individuals did. Every individual would write a very different list based on their experiences, knowledge, and ideas. I also disagree with much of the list that you get if you click the "201 Tips To Save Gas" tab in this site's navigation header.

Your jump in FE is definitely interesting and (assuming it stays higher in the long run) could be because of the weight of the wheels. My first guess would be that your new tires are designed/constructed with better rolling resistance than your old ones. It's tough to prove, of course, and I doubt we'll have any way to prove it; and so, this is one more data point, this one showing that wheel weight could be worth more than I think.

yea, but that "BEST TANK EVER" was on an extremely short run. I could get my best tank to about 43 (which is great for me) If I only used 2 gallons and chose the route.

before saying it made that much of a difference, run an entire tank through.

I can't say why anything is on the gas savings tip list. some of the items on there is just plain out there and crazy to me but obviously someone has seen gains from whatever is on there or it wouldn't be there.

I'd also like to see more data. And, if you truly believe there was a difference due to wheels & tires, put the original ones back on and you should return to your previous MPG.

I will bet my left knee on the fact that you did not pick up 20% MPG due to wheels and tires, there were other factors involved. I think if you managed to magically levitate the car off the ground with zero rolling resistance you could not gain 20%. I'm sure wheels & tires help, but not 20%.

-Bob C.

There must be a measurable improvement to lightweight rims, or the VX, HX, Insight 1 and Civic hybrid wouldn't be using them. I trust Honda's (and other makers, too) engineering departments to only make changes like this if there is an improvement.

That's some good rational thinking right there. Same thing goes for the VX's aluminum engine brackets. It's probably something that will fall into the "margin of error" from A-B to B-A testing. But in the long run you'll see differences.

If you drive on flat and smooth highways at steady speeds, I wouldn't think lightweight wheel and tire combos would do much. Dream roads aside though, what will help is the reduction in unsprung weight, the car will lose less forward momentum to bumps....

Although, the usage of them in the mentioned cars could be a consequence of keeping the overall vehicle weight low, it's the unsprung/sprung weight ratio that matters, hence ride and handling may have got unacceptably bad with normal weight wheels. Considering that they were planning on using thin low energy tires also, which are not known for their grip, it may have been an attempt to get back some lateral grip, because lighter wheels wouldn't jounce and break contact so easily. So it may have been a way to make other compromises less "awful".

Then I guess anything gained in less energy to accelerate in city traffic after that is a bonus, and not necessarily the whole object of their use.

My road conditions are as follows:

Lots of hills (I live in the "foothills" of north carolina). About 30% of my commute is 45-55mph on rural highways and the other 70% is at 70mph on Interstate 85. I will wait a few more days to refill (I will have about 140 miles today), but I want to move on to my next mod this weekend. I understand that this is not a thoroughly controlled test with A-B-A conditions as preferred, but I also don't have the time for that. I don't intend to publish a 17% increase in fuel economy based solely on wheel weight. I know that that is not the case. And if I had a set of 13" steel wheels with identical tires I could give a comparison on that variable alone. But since I don't my results are a combination of tire width reduction, tire weight reduction, wheel weight reduction, unsprung weight reduction, rotating weight reduction, total vehicle weight reduction, tire tread pattern and compund change, lowered ride height, and probably a few more.

Regardless, my original question included all of those variables and only asked for guesses as to the change. At the time, it seemed logical to me that all of those changes combined should net some kind of increase, though the actual change due to each might not be definable. Maybe it is not as logical as I thought...