$\rm CO_2$ Car Back Wheel Thickness For my physics class I'm designing a $\rm CO_2$ car from scratch, and I was wondering why most cars pictured online have wider back wheels than front wheels, and I'm confused at why that is. My understanding of friction is that wider wheels will cause more ground friction due to their increased contact with the ground as well as more air resistance due to their increased frontal area. In an article provided in my class, they cited "traction" as a reason for thicker wheels, but if a $\rm CO_2$ canister is pushing the car and not an engine spinning the wheels, that shouldn't matter at all. I don't see what the advantage is of thicker back wheels.
The car will race 65 feet guided by a taut fishing line through eyelets on the bottom of the car.
If anyone could explain this to me that would be great!
 A: 
I don't see what the advantage is of thicker back wheels.

Rolling resistance force is defined as :
$$ F_{rr} = \sqrt {\frac zd}~N $$
where $z$ is sinkage depth and $d$ - wheel diameter.
Sinkage depth is proportional to car pressure (and optionally depends on the ground material where car is standing-on,- obviously car will sink more in the sand than on asphalt, that's why they make roads from the latter.):
$$ z \propto P_{car} = \frac {W}{A}$$,
where $W$ is car weight and $A$,- wheel contact area with the ground. Obviously as wheel thickness increases,- so does the contact area with the ground. The outcome is that car exports less pressure to the ground in that case, and so does sinkage deph $z$ decreases too, due to smaller pressure. This affects final car rolling resistance force, which is minimized due to first equation.
A: You are correct that the increase in rear wheel width will not enhance the linear rolling performance of your vehicle since the vehicle is using thrust. It will add to the drag of your vehicle. I've attached a link to a photo of a jet powered vehicle attempting to break the land speed record. You will see that the rear tires are relatively narrow confirming your intuition. https://images.app.goo.gl/4HuKe5qvEeXu8CP3A Good luck.
