I saw a truck this morning that had what looked like unusually small wheels on it, and this got me thinking about wheels. Why does a particular vehicle usually have wheels of a particular size on it – so consistently that, without my knowing anything about it, I was able to recognize that the wheels on that particular truck looked “small.” What are the pros and cons of going bigger or smaller with wheels?
Well, offhand the first thing that comes to mind is that the wheels have to contain enough pressurized air to support the weight of the vehicle (although this has more to do with the number of wheels than the size of them). Beyond that, it’s a mileage thing. You know how you have to recalibrate your speedometer when you reinflate your tires? That’s because when the tires are even that much larger, one revolution moves your car that much further. Large trucks, used for long distance deliveries, are naturally going to be designed with mileage in mind, and although it takes a bit more energy to turn a larger tire, they get more gas mileage out of them.
Sorry, I don’t understand – why does traveling further per revolution affect gas mileage? (Please note the “for dummies” in the thread title.)
Bigger wheels are better if you’re driving over uneven terrain. Also, bigger wheels travel a larger distance per revolution, but conversely, you need more power (or torque?) to turn them. Smaller wheels take up less space but need more revolutions/second to reach the same speed. That’s why really old bicycles like this one need no gear/chain contraption to speed up the pedal revolutions, but the big wheel makes them large and probably heavy compared to contemporary bikes.
Partly because bigger wheels have more surface on the ground, which gives you more grip. That probably also explains why it’s more efficient to have large wheels on heavy trucks - you need a more grip (friction) to move a bigger mass, which means (with similar tires) you need either more or bigger wheels to increase the friction.
The vehicle needs to use a certain amount of gas to turn the wheels one revolution. One revolution of the wheels equates to the vehicle traveling a distance equal to the circumference of the wheel. If the circumference of the wheel is larger, then one revolution means the vehicle moves further than it would with smaller wheels.
Back in the days when I would skate on the streets, someone pointed out that you really should have wheels with a major dimension somewhat larger than the average roughness of the road surface, which was certainly not true of my skates versus potholes.
If it were true we could get infinite gas mileage by using infinitely large tires.
The vehicle does not use a certain amount of gas to turn the wheels one revolution, it uses a certain number of engine revolutions to turns the wheels one revolution (I’ll go along with revolution here but I believe it’s rotation). The amount of gas used for a fixed engine speed/gear combination varies according to conditions.
The vehicle uses a certain amount of gas to move the vehicle forward by a given distance. The energy is used to overcome gravity, rolling friction, mechanical friction, and air resistance (and maybe a couple of other things). None of those factors are mitigated by larger tires.
Where to begin?
The size wheels chosen for a passenger car, light truck, or tractor/trailer are a trade-off between a number of competing factors, including cost, weight/inertia, packaging, efficiency, grip, ride quality, and aesthetics. Maybe by looking at how wheel/tire size affects each of these criteria, it may make more sense why a certain size is fitted as original equipment, and why an owner may choose to upsize/downsize their fitment. This is certainly not exhaustive, so maybe someone else can chime in on things that I’ve certainly missed.
Cost: smaller is almost always cheaper.
Packaging: smaller is easier to fit within the confines of the sheetmetal, and provides a tighter turning circle; bigger allows larger brake rotors and more clearance for calipers.
Weight/inertia: smaller is better, both weighing less and requiring less energy to spin up and then bring to a stop again.
Efficiency: mileage is generally helped by high inflation pressures (less deformation of the tire carcass), and aerodynamics are aided by narrow cross-sections and low height (to allow a lower hood line).
Grip: Cornering is aided by a wide contact patch, and often by lower inflation pressures which allow more surface contact area between tire and road (of course the biggest gains come from softer tire compounds, tire construction, and tread patterns); straight-line acceleration likes a longer contact patch, but this only applies (for most cars) at launch or during panic braking.
Ride quality: more air volume helps here, which means tall cross-sections.
Aesthetics: in general, this seems all about proportion, and what the designer/owner is trying to achieve with their “look”. A huge vehicle like a Suburban/Excursion would look foolish with the same size wheels as a Honda Civic, and visa versa. Of course, some cars of the same size will have markedly different wheel sizes, to emphasize their different missions. A stock economy sub-compact might have 14" wheels, but a performance-oriented sub-compact, such as a Mazda3, might come with 17" wheels. Likewise, an off-road oriented SUV or light truck might have 15-16" wheels with tall mudder/off-road tires, whereas a street SUV like a Porsche Cayenne might have 19-20" wheels with low-profile tires.
So, the original equipment wheels/tires on a vehicle make sense as a carefully thought-out compromise between all the conflicting demands facing the designer, bean-counter, marketer, etc. What you see on a vehicle in the aftermarket might be carefully chosen to optimize a particular facet of the vehicle performance envelope (e.g., handling, cost, off-road abilty, etc.), or it might be a desperate cry for help (20” ‘dubs’ on a ghetto-Civic? please).
Check this article: http://www.yokohamatire.com/utplus.asp
The reason most trucks look the same is that a lot of trucks back up to loading docks that are a specific height. If they put different size wheels on the trailers, they wouldn’t line up correctly. A noticeable exception is a moving van. The tires on the trailer are substantially smaller (lower) than most of the other van trailers because they usually unload with a ramp or by hand and lower is much easier.
On passenger cars, as was pointed out above, there are a variety of factors that enter into the size tire that comes as original equipment. The chosen size is a compromise. Aftermarket, an individual can emphasize a certain feature to satisfy his/her taste - a wider tire for better cornering, a softer tire for more comfort, a taller tire so the car rides higher, etc.
If a certain model car changes from 16" tire to 17" tires the next year, it won’t get better gas mileage because of that change. A change like that is to make the car look different so they can sell more cars.
Thanks for your answers, everyone – esp. Schuyler, since you somehow managed to figure out what I was trying to ask and answer it very clearly. And oboelady, the loading height thing makes a lot of sense.
A pneumatic tire is always climbing a small hill due to inflation pressure and the specific tire construction, rolling friction if you will.
A friend bought a new construction quality wheelbarrow for a job and ran a cut nail into the pneumatic tire the first day on the job. He replaced it with a new permanent foam filled tire that shrugs off cut nails, etc.
Thanks so much for that feedback. As a newbie, it’s kind of cool.
I did forget to include load-carrying capacity. This is a function of the inflation pressure and the allowable carcass deformation - if you can tolerate a lot of deformation, then low pressures are OK (drag racing tires), if you can tolerate high pressures, then you can carry lots of load (semi tires have inflation pressures >100 psi in many cases [this can really tear up a road]).
It’s been commented that the tires are among the most complicated optimization structures in an automobile, yet they have no moving parts!
Back to the OP: You asked about “wheels”.
Did you really mean wheels, ie the rotating metal part on a car/truck, or tires, the rubber part that fits around the wheel, or did you mean both as a unit?
For a given tire outside diameter you can have small wheels and a tall tire sidewall, or large wheels and a short tire sidewall. There are limits to how far one can go in either direction, but the available variation is still +/- 20%.
Heavy truck tires are generally towards the former end of the envelope, while serious sports car tires tend to the latter end. The “in” style for generic passenger car tires these days seems to have drifted pretty far to the low-profile end of the spectrum.
What I was originally thinking about was about the circumfrence (sp?) of the round object that hits the road, that there might be some connection between the number of revolutions/rotations and amount of distance traveled, such that certain circumfrences/proportions might have advantages or disadvantages. That’s also why I put “physics” in the thread title. Is there information about that that hasn’t been addressed?
OTOH, I had no firm agenda on the question, and am finding all of the answers interesting, since I’ve never really thought about wheels or tires before (other than when I’ve gotten a flat, of course). Who’da thunk the issue was so complicated?
(BTW – welcome to the boards, Schuyler! Come over to MPSIMS and we’ll greet you with warm welcomes and oodles of inside jokes. 
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