Scioly.org

Since he is talking about a 3 wheel vehicle, as he mentioned using servo city, it is not difficult to come up with the hub attachment that will fit the wheel and the axle of whatever rpm motor you think will work best for you. It avoids gears very nicely. Using their structural material and attachments , it is not terribly difficult to get the buggy to go rather straight. It will take quite a bit of testing to get it accurate enough to medal in many states.

What size of axle should I put the gears on?

I doubt it really matters...
I think ours are 1/4 inch. Just make sure its compatible with your parts.
Technically, a wider axle means more rotational inertia-1/2*m*r^2 (or moment of inertia whatever you guys refer to it as), so theoretically, it would be harder to start and stop the car from moving lol. But I really don't think it matters...

I'm using a 1/2 inch thick threaded axle for my battery buggy and I had a hard time finding spur gears that work with my pinion gear for my motor. Also, the pitch of the spur gear and pinion gear must be the same to work, right? If anyone knows a good website for gears, please share it.

yup same pitch is needed. I suggest finding a local hobby store, it will always beat out some random online service.

In my opinion, a 1/2 inch shaft is excessive and as you see finding gears to fit can bee a problem, as require much heavier gears.
Both of these add weight which will cause both a slower car and a longer stopping distance.
1/4" is more than enough if you do not abuse the car. I am using 4 mm for a prototype and it works fine other than doing a header down a stair well. To do over, I would probably go with 1/8" .
Most people will go with banebot wheels as they have a lot of options. They are relatively more expensive tho. Servo city wheels will work quite well when matched up with their motors, gears and mounts.

Yea, I agree. The main thing with finding parts is just making sure everything is compatible.

I know some people mentioned traxxas gears, but I looked on the website and found nothing compatible with my 1/4" threaded rod axle. I am thinking of 3d printing the gears, but that is probably not the best option out there. Ideas?

OOF dont 3d print your gears the PLA will get rekt

My suggestion: find a local hobby store and just pay them a visit. Im sure you will find something there. Thats what we did, and we are using a 1/4 inch drive/front axle as well. As windu said, plastic is better as it is much lighter (trust the OP experienced dudes lol). I didn't realize until I tried it.

Thx so much, you just saved me a ton of time by telling me this. I was ready to spend a whole day on learning how to make thread in Tinkercad.