Many of us know the resizable wheels in the Landing Gear page. We use it for cars, and sometimes, custom landing gear. Here's some facts about these parts.

The first thing petrolheads notice from the thumbnail is the HUGE tire profile. Aircraft and heavy vehicles have it. In reality, this allows for better shock absorbtion and increased air movement area, reducing the chance of dreaded bounces (and tire explosions), but lowered responsiveness and stability, as the tires would deform under cornering forces. The large tires, generally with low tire pressure, allow aircraft to land at high downward vertical speeds, as large tires provide more space for the air to travel to. This is also why racecars have a small tire profile and higher tire pressure, to increase tire responsiveness.

Resizeable wheels, as the name suggests, can be resized. Bigger wheels are heavier, meaning lower acceleration rates. However, with the maxAngularVelocity tag in the XML file, bigger wheels can allow for greater top speeds. Angular Velocity is the rate of rotation, and so maxAngularVelocity is the maximum rotational speed of the wheel. It acts as a speed limiter. You can see this on the Llama (66 mph) and the RM-226 (52 mph).

Wheel width can also be changed. In reality, wider tires provide a larger contact area with the road, increasing friction (grip). This also increases the vehicle's weight, reducing acceleration. However, it seems as the grip produced in SimplePlanes by the wheels are the same.

Now to wheel design. "Normal" wheels are plane tires. "Street" wheels are mainly for road cars. "Offroad" and "Tractor" wheels are for offroad vehicles. In real life, the rounded shape of aircraft wheels absorb landing forces better. The flatter shape of street wheels increases contact with the road, and offroad wheels have special patterns that are deep and shaped for their functions. They 'dig' tiny holes in dirt which lock the tires into place. (Kind of like gears!) "Offroad" wheels are mainly for faster offroaders that require turning at high speeds, increasing both forward and sideways traction. "Tractor" wheels are mainly for slower offroaders like dump trucks (and tractors!) which would not need high speed turning. They focus mainly on forward traction but not as much on cornering traction as their race buggy counterparts. It is important to note that different designs have no changes in grip from each other, and are purely cosmetic.

Wheels allow for tweaking of suspension and traction. Suspension strength affects stiffness, and damper may affect the speed that the spring can contract. Tweaking the traction allows you to simulate different tire designs and compounds.

Now for the part you're waiting for.

When wheels are put together in skilful ways and connected with fuselage blocks and engines, you get a car. You may have noticed that front-wheel and rear-wheel drive designs slide around a lot less than all-wheel drive designs. This is because of the lack of a differential. Not even a simple one. This means that all the wheels do not variate in rotational speed or power, and as such are full throttle all the time. It means, SimplePlanes AWD is actually known as 4WD.

External link: Car Throttle - The Differences Between AWD and 4WD

"The differential allows the wheels of a car to turn (rotate) at different rates. This increases on-throttle grip." (Forza)
The differential limits the speed of wheels to increase the grip of the car on turns. This is because the wheels on the inside of a turn have to travel a shorter distance than those on the outside (too bad I have no GIFs to show this). SimplePlanes 4WD drivetrains will produce oversteer because of the unbalance.

And there you have it, an answer to all your wheel questions.