Everything with mass 'has gravity'.
Gravity and also the traction between the tires and the road. The ridges on the tires sort of grab the road so you don't slide, especially on rainy days.
Friction in cars are produced by the tires and the clutch. The friction caused by the tires allow the vehicle to stay on the road, while the friction produced by the clutch makes it possible for the car to get moving. While gravity, for example supplies the force that pulls the vehicle down the hill.
Nothing keeps them in contact with the road, besides gravity. If you hit a bump, the come off of it.
Gravity affects the speed of a toy car by pulling it downward, which can increase its acceleration when moving downhill and decrease its acceleration when moving uphill. The force of gravity can also affect the traction of the tires on different surfaces, influencing the overall speed and performance of the toy car.
Conduct and experiment to rest the trip of different tires
Centripetal force, which can be supplied by gravity, a string, friction (e.g. between tires and the road), etc.
Tires
SUV's are inherently unstable due to a high center of gravity. A wider tire creates more adhesion to the road surface, but you still have to be careful cornering an SUV regardless of other factors. The tires will in no way change the basic physics of the situation.
Some of the brands of tires that are manufactured today include Bridgestone Tires, Firestone Tires, Dunlop Tires, Michelin Tires, and also, Goodyear tires.
A bicycle with 20 tires has 2 tires.
Gravity itself does not slow things down (the Moon still keeps going round the Earth and the Earth still keeps going round the Sun). It's friction that slows things down. I have to say that there is likely to be more friction if there's more gravity but that's a secondary effect.
A bike with 28 tires would have 28 tires.