Well, flywheels are best optimized when their mass has the highest moment of inertia. This means using a wheel with all of the mass concentrated on the outer edge. I don't have dimensions for your car, but I assume it is something like 20x35 centimeters in area, so the wheel(s) cant be any larger than that in diameter. The equation for energy in a flywheel is E = .5 I w2, and for a solid wheel I is .5 M R2.

This gives E = .25 M R2 w2.

I would also mention that you should be careful not to ruin the steering of the RC car by using just one wheel. If you try to turn with a giant wheel spinning in the vehicle, it will throw off the rotation of the vehicle and cause fun things like flips. You can prevent this by using to opposing flywheels, so that the net rotation of the car is zero.

Liam Plybon , the dimensions of the car are 15x7 in, and the tire diameter is 2.6 in.

Well I got to the Kinetic Rotational equation but since torque is variable through the entire system, how much minimum energy does the flywheel need to have to move the vehicle.

I am getting confused on which variable to make constant so that I can play around with the rest. I have the translation KE at peak speed, but that cannot be equated to rotational since both are different quantities, so I am getting confused on how to get the rotational energy which would be enough to give the minimum kinetic energy. If this somehow makes sense.....

It isnt how much energy the wheel has that determines if the car will start to move. You want to find the force required to exceed static friction. You could test this by putting a scale on the front of the car and pulling slowly until it starts to move; that force measured by the scale is the force you need to overcome to start motion. How much force you get out of the wheel depends on your transmission system. How are you translating the energy in the flywheel to the wheels of the car? Are you using a clutch?