Do bigger rims get better gas milage? 16in. vs 18in.

[jerryk]

A larger overall tire height (not just RIM diameter) will lower the RPM needed to maintain a speed, so it will improve your gas mileage. It will however make your speedometer and odometer read incorrectly, so those will have to be adjusted (this usually requires changing a small gear on the transmission). My 17” 235 x 45 x 17 (59 miles on odometer = 60 real miles)

Wider tires will reduce your gas mileage, because the added width adds friction between the tire & the road. Good for handling, bad for mileage.

Going up in rim size (like say from 15" to 17") will reduce MPG because the larger rims will be heavier, and will move the weight further out from the hub. In addition, lower-profile tires have to be built heftier to compensate for their shorter sidewall, which adds weight out near to the edge of the tire. Rotational weight (tires, rims, brakes rotors, etc.) count for approximately four times as much weight as vehicle weight. So adding 10 pounds on the wheels, is like adding 40 pounds in the car, and added weight reduces MPG.

[Sentientbydesign]

A larger overall tire height (not just RIM diameter) will lower the RPM needed to maintain a speed, so it will improve your gas mileage.

This is not always true. For a couple of reasons. Some engines will work much more efficiently at a higher RPM with light load, than a lower RPM with heavy load.

By increasing the overal tire diameter, you have increased the moment of inertia which increases the amount of torque needed to move it. You've also put the engine at a point that is probably less powerful and therefore will need more fuel to produce the same speed.

The 4:1 ratio is also debatable. It is extremely important to note where the weight is on th rotational body. If the weight is very close to the point of rotation than it's "weighted value" is nearly the same as chassis weight. On the other hand, if the weight is farther out on the torque arm, then it's "weighted value" is much higher.

The difference between rotating mass and static mass differ greatly depending upon the acceleration. The lower the acceleration, the less difference exists between the two.

[WizardBlack]

I like how everyone tries to get into the long calculations of rolling radius. :-)

Let me ask you; do you get tired faster running on concrete or in sand?

Assuming you have same type of tire construction, same tire pressure, same rolling radius and same weight, the larger rim makes for a "stiffer" wheel and therefore it's more like concrete than sand. Larger sidewalls, heavier wheels, lower tire pressure, softer road substrate all make for more "work" your car does in the form of spinning, flexing and squishing things; mostly the tire and rim itself. This = lower mpg.

[Sentientbydesign]

I like how everyone tries to get into the long calculations of rolling radius. :-)

Let me ask you; do you get tired faster running on concrete or in sand?

Assuming you have same type of tire construction, same tire pressure, same rolling radius and same weight, the larger rim makes for a "stiffer" wheel and therefore it's more like concrete than sand. Larger sidewalls, heavier wheels, lower tire pressure, softer road substrate all make for more "work" your car does in the form of spinning, flexing and squishing things; mostly the tire and rim itself. This = lower mpg.

So does an increase in the moment of inertia. I think everyone is missing that there are two areas to look at when addressing this question:

City style driving- Lots of accelerating and decelerating (shouldn't be a word)

Highway style driving- Mostly maintaining a constant speed.

For the latter, the larger rims may actually benefit the MPG because of what you mentioned regarding work. For the former, larger rims will cause a higher moment of inertia because the rims require more mass as they get "taller".

[slow40]

or you could do like me and mount tall tires (i.e. 235/70/16) onto an average sized rim (stock s14 wheels), and benefit from the lower rev's while cruising down the interstate. still have not calculated my new mpg's, but when I am cruising at 75, my speedo read's 65. not exactly jdm tyte, but for 30 bucks for 2 michelin tires with 90% tread, what could it hurt?

[Sentientbydesign]

It's possible for that to improve your fuel economy, if and only if the engine is still efficienct at producing the required power at the engine speed.

This is where CVTs pwn hehehe. Sorry I've been reading too many s-chassis threads lol.

[Red coupe]

I like how everyone tries to get into the long calculations of rolling radius. :-)

Let me ask you; do you get tired faster running on concrete or in sand?

Assuming you have same type of tire construction, same tire pressure, same rolling radius and same weight, the larger rim makes for a "stiffer" wheel and therefore it's more like concrete than sand. Larger sidewalls, heavier wheels, lower tire pressure, softer road substrate all make for more "work" your car does in the form of spinning, flexing and squishing things; mostly the tire and rim itself. This = lower mpg.

Ever pushed a car on level ground at constant speed?

Ever spun a 25lb 18in metal disk from a stop to ~400rpm in a couple of seconds?

I'd have to do more math to give you an exact answer... but I can give you a fairly firm guarentee that the second is harder, if even doable then consider that X 4.

Rolling resistance is a lot less then it takes to accelerate a wheel.

[tehdude]

I will have to read all this 10 to 15 times more to actually understand half of it. However, until then i have a question, working on an actual example might simplify things up a bit: how and in what way exactly are my 205/40/17" tires are better for driving than the original 195/50/16" tires the car came with from factory? The car weighs ~2800 lbs, its a SVT Contour...(I want it gone (i hate it) and in with my S13 asap, thats why im here)

[FrancisCarey]

you can sometimes go one or two sizes bigger and it will help some on the road ,it wont always help in the city though,sometimes though it will improve some vehicles,and other it hurts them,the theory is it takes more gas to turn the wheels and less gas to turn smaller wheels but at certain speeds it takes less gas to maintain a certain speed with larger tires,so it can help some,good luck with it.__________________________car wax your car daily to protect its paint

[jerryk]

I can tell you my experience with 17” rims and tires. I had to reduce the cold tires pressure to 27 in the winter and 25.5 - 26 in the summer. The tires have very stiff sidewalls and do not flex enough to absorb road shock in the higher-pressure inflations. I try to keep a hot tire pressure of 28psi.

[SR20_S14]

This is honestly some interesting information based on equations i found on the internet the information here is just as useful to me.

[EMK]

The aftermarket industry DEPENDS on the typical buyer not understanding even high school physics.

No it's not required in HS at least not in NC. I can't believe I've neglected the engineering forums for so long. What was I thinking

So from one student of engineering that's really really ready to get rid of that whole student tag this is how I basically understand any and all questions now: For any question you ask there is no true answer that any one human has the mental capacity to fully understand from beginning to end. I do completely respect and understand that there are those out there that are smarter that me and that is taken into account...but for any question that is ever asked the answer given is simply giving enough information to achieve a desired result.

As far as these questions go... this is why engineers use computers to solve mutlivariable differential equations to answer a question like this and even then there are so very many was that the variables can be affected by one another that you can really only get close to a definite answer. Even then how close you get all depends on how much time and money you want to invest in the design process and at what point the math becomes a was and good old fashioned trial and error can tell you more faster and cheaper. So although I love and welcome the discussion...please just go and test it.

There's only so much that can be typed and even then there's so much to consider that its hard to be sure that enough detail is given and the right principles and rules are used to give the answer desired.

And having said that larger lightweight wheels with a design that favors weight kept near the studs and tires that are wider only if ones experiencing traction problems with a good tire now and even then don't go wild. The guys at the factory were at worse pretty close

Ever pushed a car on level ground at constant speed?

Ever spun a 25lb 18in metal disk from a stop to ~400rpm in a couple of seconds?

I'd have to do more math to give you an exact answer... but I can give you a fairly firm guarentee that the second is harder, if even doable then consider that X 4.

Rolling resistance is a lot less then it takes to accelerate a wheel.

Just a little note even though I know you know this already but any car wheel can't be modeled using an equation for a thin disc. Whether you are attempting to calculate the mass moment of inertia or the polar moment of area along any axis the fact that its not a thin circular disc and that it has a deformable body (tire) encasing it and that that's filled with air (a compressible fluid) means that you're already into Masters if not PHD level calculations and will probably need an applied mathematician on hand to help.

I know just when you think you get close to the answer it's like and that's just to solve one variable in the equation.

But no worries as long as the general idea is understood that should allow you to make a good choice or as they liked to call it back in school An Educated Guess.

[Red coupe]

Just a little note even though I know you know this already but any car wheel can't be modeled using an equation for a thin disc. Whether you are attempting to calculate the mass moment of inertia or the polar moment of area along any axis the fact that its not a thin circular disc and that it has a deformable body (tire) encasing it and that that's filled with air (a compressible fluid) means that you're already into Masters if not PHD level calculations and will probably need an applied mathematician on hand to help.

I know just when you think you get close to the answer it's like and that's just to solve one variable in the equation.

But no worries as long as the general idea is understood that should allow you to make a good choice or as they liked to call it back in school An Educated Guess.

Uhhh thanks I guess, but did you even read what I was talking about? I wasn't talking about tire deformation, traction forces or anything like that I was talking about rolling resistance and rotational inertia. Even then I wasn't talking about actually calculating it. Sure the actual moment of inertia of a wheel can't be calculated as a simple disc, and yes the tire deforms with speed (a little bit, but really that only increases the moment of inertia amplifying what I am trying to point out)

But a disk is a perfectly fine demonstration to show how hard it would be to spin a wheel up to tire speed by hand. You don't need any complex computer modeling to show that it is harder to accelerate a spinning wheel with the mass and size of a wheel then to push a car on level ground at a constant speed.

But even IF what you were saying had anything to do with what I was talking about... Your still wrong. It WAS done by one man (Maurice Olly, granted with some shared test data though) as early as the 50's with out the luxury of computers. He was able to predict and match skid pad test data with in a reasonable margin of error... And the formulas and methods still exists and don't require more then some testing and mid Junior level mathematics and engineering to duplicate. Go Pick up "Chassis Design: Principals and Analysis" from SAE book, it describes the method and formulas. He does so starting his analysis of the wheel as a simple rubber disk, sandwiched between two concentric plates with a smaller diameter to for the "rim".

Gad to have another person in he engineering forum, and I realize that we will never get anything exact... But in a discussion like this what does a page long post about how no one knows really add?

[EMK]

Uhhh thanks I guess, but did you even read what I was talking about? I wasn't talking about tire deformation, traction forces or anything like that I was talking about rolling resistance and rotational inertia. Even then I wasn't talking about actually calculating it. Sure the actual moment of inertia of a wheel can't be calculated as a simple disc, and yes the tire deforms with speed (a little bit, but really that only increases the moment of inertia amplifying what I am trying to point out)

But a disk is a perfectly fine demonstration to show how hard it would be to spin a wheel up to tire speed by hand. You don't need any complex computer modeling to show that it is harder to accelerate a spinning wheel with the mass and size of a wheel then to push a car on level ground at a constant speed.

But even IF what you were saying had anything to do with what I was talking about... Your still wrong. It WAS done by one man (Maurice Olly, granted with some shared test data though) as early as the 50's with out the luxury of computers. He was able to predict and match skid pad test data with in a reasonable margin of error... And the formulas and methods still exists and don't require more then some testing and mid Junior level mathematics and engineering to duplicate. Go Pick up "Chassis Design: Principals and Analysis" from SAE book, it describes the method and formulas. He does so starting his analysis of the wheel as a simple rubber disk, sandwiched between two concentric plates with a smaller diameter to for the "rim".

Gad to have another person in he engineering forum, and I realize that we will never get anything exact... But in a discussion like this what does a page long post about how no one knows really add?

I did read what you were saying. I guess I didn't do a good job acknowledging that. I understood based on what you were saying not only that you knew what you were talking about but also that you were trying to give a general understanding of the idea of inertia of a rotational body. Yea I'm aware that the tangent I went off on didn't really help either. In retrospect I guess my post wasn't really helpful

I wasn't trying to say I knew everything or that the answer was impossible simply to say that the question is too general for a clear answer. As far as that book is that the whole title? I need to pick that up for my design project (gotta build a pseudo formula 1 car)

Thanks for the much needed correction. I expect to be wrong about some things at this point. That's why I'm still a student. Isn't that what testing and factors of saftey are for?

[jerryk]

Trying to disconnect from this thread

[Red coupe]

I wasn't trying to say I knew everything or that the answer was impossible simply to say that the question is too general for a clear answer. As far as that book is that the whole title? I need to pick that up for my design project (gotta build a pseudo formula 1 car)

Thanks for the much needed correction. I expect to be wrong about some things at this point. That's why I'm still a student. Isn't that what testing and factors of saftey are for?

The book is "CHASSIS DESIGN Principles and Analysis" by Milliken & Milliken. Its an SAE publication, so you can buy it off the SAE website and get a discount if your have a SAE membership.

Its a good book, but IMO the title is slightly missleading. I am not too far into it, but I flipped through the whole thing and its really on suspension design rather then the actual structural design of a chassis. I had bought it looking for more of a book on structures/frames, but they seem to be kinda hard to find... Most "Chassis" books seem to do the same, discussions of suspension... I guess the idea is that the chassis is basically just connecting suspension and other mounting points.

[EMK]

Thanks for that one I'll have to pick it up.

And on the structural chassis design... I'd have to agree. It seems most vehicles are built with the structural performance being an afterthought. More form over function. Except maybe flight vehicles.

[ravera]

Most vehicles are met with one standard in mind...

NVH.They realize that (unfortunately) only a very small percentage will be modifying, much less racing their vehicles, and the majority of the population wants a car that is quiet and comfortable. Building a car that is soft and flexes is not only safer when some idiot sues for not surviving hitting a brick wall at 80, but also can absorb road noise and assist the suspension in absorbing hard hits.

[Jacobus77]

At the risk of offending a lot of people, looking at one or two factors in theory is not going to answer the question for a specific vehicle being driven under specific conditions in the real world. I can't possibly cover everything involved and I'm not infallible even if I could. But from experience I offer the following to consider:

Short answer - it depends.

Not quite so short answer: They can, under certain conditions.

IF the material quality is higher so the wheel and tire combo is either lighter or at least not drastically heavier while maintaining strength, AND

You're making a substantial jump in diameter and width, AND

IF the engine has serious torque (the pounds-feet part of power) down low, AND

IF you do considerable highway driving, probably Yes.

Otherwise, probably not.

Practical considerations: The tire guy will be trained to maintain nearly the same diameter as stock by selling you a lower-profile tire. Make it clear what you want to do. And make sure the tire diameter will fit and move at full steering lock; some shops actually have a measuring device for this purpose. My B14 Sentra and my son's B13 can fit 17" wheels with low-pro wide racing tires with no problem. Later models, not so much.

The issue of rolling resistance was raised. I would counter that in the real world, tire slippage is a bigger problem. My brother in law bought a full-size pickup with a 6-cylinder engine a number of years ago. It got 20 mpg. He installed a loaded toolbox in the bed and promptly got 22 mpg from the change in weight distribution toward the drive wheels reducing tire slippage. Front-wheel drive is wonderfully stable, but tire slippage is often worse than RWD. When I bought my B14, the 13" x 5" stock rims would not hold a tire that could prevent wheelspin during a GENTLE takeoff (5-speed manual trans). A weight distribution issue partly, but I don't have a pickup bed over the front wheels. It now runs 14" x 6" alloy wheels with 195/60 tires and has drastically less wheel slippage and at least 2 more mpg, with a wider patch but not notably longer (roughly same tire diameter).

With larger-diameter tires, you only gain if you have torque for it, as you're effectively pulling a taller gear ratio. I'm considering that after I make a few more mods. The GA16DE engine is undersquare and delightfully torquey for its size. I stick with the best tuneup parts, Royal Purple engine and trans oil, and have a custom 2" catback to maximize that. (Yes, I can feel the difference from those. And it goes uphill a gear taller than it does with stock plugs or other oils.) People who put a too-oversize exhaust on generally give up torque for top-end horsepower. Also if you run an automatic trans and do a lot of stop-and-go, larger-diameter tires may cost more to get rolling than they save.

Brakes: You'll need at least a stronger performance grade front pad set (I'm thinking Yellowstuff), and drilled performance rotors to shed brake dust faster, and/or bigger brake conversion. If you already can stand the car on its nose, that might not be a problem. When my son tried 17" wheels and AAA traction tires on his B13, he enjoyed the results a lot... except he had to really ram down hard on the brakes for the same results. (Mods are planned.) Remember, Nissans usually are heavier built than most cars for their size so pay special attention to brakes!

And finally, larger diameter tires will measure less on the odo/speedo, so if you make the change, you'll need a good speedo shop to recalibrate it (kudos to the one peson who mentioned that), a minimum of $100 job around here. Besides staying at a legal speed, you will only know the result of the diameter change by doing the recalibration.

So... it depends.

Peas.

J